CERN: Feature https://home.cern/ en The Higgs boson and the rise of the Standard Model of Particle Physics in the 1970s https://home.cern/news/series/higgs10/higgs-boson-and-rise-standard-model-particle-physics-1970s <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2809116" data-filename="Screenshot%202022-05-10%20at%2016.08.01" id="CERN-HOMEWEB-PHO-2022-081-1"> <a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-081-1" title="View on CDS"> <img alt="At Gargamelle with Paul Musset" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-081-1/file?size=large"/> </a> <figcaption> At Gargamelle with Paul Musset <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--2" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>The Higgs boson and the rise of the Standard Model of Particle Physics in the 1970s</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/john-ellis"><span class="cern-tag">John Ellis</span></a></p> <p class="text-align-center">10 May, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs-boson-and-rise-standard-model-particle-physics-1970s">français</a></i></p></span> </div> </div> </div> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__centered section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-medium" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>At the dawn of the 1970s, the idea of a massive scalar boson as the keystone of a unified theoretical model of the weak and electromagnetic interactions had yet to become anchored in a field that was still learning to live with what we now know as the <a href="/science/physics/standard-model">Standard Model of Particle Physics</a>. As the various breakthroughs of the decade gradually consolidated this theoretical framework, the Brout–Englert–Higgs (BEH) field and its <a href="/science/physics/higgs-boson">boson</a> emerged as the most promising theoretical model to explain the origin of mass.</p> <p>In the 1960s, there were remarkably few citations of the papers by Sheldon Glashow, Abdus Salam and Steven Weinberg on the theory of unified weak and electromagnetic interactions. All that changed, however, in 1971 and 1972 when, in Utrecht, Gerard ’t Hooft and Martinus Veltman (a former CERN staff member) proved that gauge theories employing the Brout-Englert-Higgs mechanism to generate masses for gauge bosons are renormalisable, and hence are mathematically consistent and can be used to make reliable, precise calculations for the weak interactions. This breakthrough was given broad publicity in an influential talk by Benjamin Lee of Fermilab during the ICHEP conference held there in 1972, in which he talked at length about “Higgs fields”.</p> <p>Encouraged, in particular, by the CERN theorists Jacques Prentki and Bruno Zumino, the <a href="/science/experiments/gargamelle">Gargamelle</a> collaboration prioritised the search for weak neutral current interactions in the CERN neutrino beam, and their representative Paul Musset presented the first direct evidence for them in a seminar at CERN on 19 July 1973. This first experimental support for the unification of the electromagnetic and weak interactions attracted great interest and close scrutiny, but was generally accepted within a few months. The <a href="/news/news/physics/forty-years-neutral-currents">neutral-current discovery</a> convinced physicists that the nascent Standard Model was on the right track. Former CERN Director-General <a href="/about/who-we-are/our-people/biographies/luciano-maiani">Luciano Maiani</a>, quoted in a <a href="https://cerncourier.com/a/neutral-currents-a-perfect-experimental-discovery/">2013 CERN Courier article</a>, puts it this way:</p> <blockquote>"At the start of the decade, people did not generally believe in a standard theory, even though theory had done everything. The neutral-current signals changed that. From then on, particle physics had to test the standard theory." –<strong> Luciano Maiani</strong></blockquote> <p class="text-align-right"> </p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-083-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-083-1" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-083-1/file?size=large" /></a> <figcaption>Mary K. Gaillard (center), her granddaughter Cleo (left), and John Ellis (right), in 2019, during the celebration of Mary’s 80th birthday.<span>(Image: Berkeley Science Review)</span></figcaption></figure> <p>The next breakthrough came in 1974, when two experimental groups working in the United States, led by Sam Ting at Brookhaven and Burt Richter at SLAC, discovered a narrow vector resonance, the J/psi, with prominent decays into lepton–antilepton pairs. Many theoretical interpretations were proposed, which we at CERN discussed over the phone in excited midnight seminars with Fred Gilman at SLAC (almost 40 years before Zoom!). The winning interpretation was that the J/psi was a bound state of the charm quark and its antiquark. The existence of this fourth quark had been proposed by James Bjorken and Sheldon Glashow in 1964, and its use to suppress flavour-changing neutral weak interactions had been proposed by Glashow, John Iliopoulos and Maiani in 1970. Mary K. Gaillard (a long-term visiting scientist at CERN), Jon Rosner and Lee wrote an influential paper on the phenomenology of charm in 1974, and experiments gradually fell into line with their predictions, with final confirmation coming in 1976.</p> <p>The attention of most of both the theoretical and experimental communities was then drawn towards the search for the massive <a href="/science/physics/w-boson-sunshine-and-stardust">W</a> and <a href="/science/physics/z-boson">Z</a> vector bosons responsible for the weak interactions. This motivated the construction of high-energy hadron colliders and led to the discovery of the W and Z bosons at CERN.</p> <p>However, it seemed to Mary K. Gaillard, Dimitri Nanopoulos and myself at CERN that the key question was not the existence of the massive weak vector bosons, but rather that of the scalar Higgs boson that enabled the Standard Model to be physically consistent and mathematically calculable. At the time, the number of papers on the phenomenology of the Higgs boson could be counted on the fingers of one hand, so we set out to describe its phenomenological profile in some detail, covering a wide range of possible masses. Among the production mechanisms we considered was the possible production of the Higgs boson in association with the Z boson, which generated considerable interest in the days of <a href="/science/accelerators/large-electron-positron-collider">LEP</a> 2. Among the Higgs decay modes we calculated was that into a pair of photons. This distinctive channel is particularly interesting because it is generated by quantum effects (loop diagrams) in the Standard Model.</p> <p>Despite our conviction that something like the Higgs boson had to exist, our <a href="https://cds.cern.ch/record/874049/files/CM-P00061607.pdf">paper</a> ended on a cautionary note that was somewhat tongue-in-cheek:</p> <blockquote>"We apologise to experimentalists for having no idea what is the mass of the Higgs boson … and for not being sure of its couplings to other particles, except that they are probably all very small. For these reasons we do not want to encourage big experimental searches for the Higgs boson, but we do feel that people performing experiments vulnerable to the Higgs boson should know how it may turn up."<br /> –<strong> John Ellis, Mary K. Gaillard and Dimitri Nanopoulos</strong></blockquote> <p>This caution was in part because the senior physicists of the day (Dimitri and I were under 30 at the time) regarded the ideas surrounding electroweak symmetry breaking and the Higgs boson with rather jaundiced eyes. Nevertheless, as time went on, the massive W and Z were discovered, the existence or otherwise of the Higgs boson rose up the experimental agenda, and no plausible alternative theoretical suggestions to the existence of something like the Higgs boson emerged. Experimentalists, first at LEP and later at the Tevatron and the <a href="/science/accelerators/large-hadron-collider">LHC</a>, focused increasingly on searches for the Higgs boson as the final building block of the Standard Model, culminating in the <a href="/news/press-release/cern/cern-experiments-observe-particle-consistent-long-sought-higgs-boson">discovery on 4 July 2012</a>.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> As the various breakthroughs of the 1970s gradually consolidated the Standard Model, the Brout–Englert–Higgs field and its boson emerged as the most promising theoretical model to explain the origin of mass </div> </div> </div> Mon, 16 May 2022 12:46:46 +0000 katebrad 182562 at https://home.cern A boson is born https://home.cern/news/series/higgs10/boson-born <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="1606851" data-filename="Higgsfield" id="OPEN-PHO-ACCEL-2013-052-1"> <a href="//cds.cern.ch/images/OPEN-PHO-ACCEL-2013-052-1" title="View on CDS"> <img alt="Artistic view of the Higgs field" src="//cds.cern.ch/images/OPEN-PHO-ACCEL-2013-052-1/file?size=large"/> </a> <figcaption> A graphic artistic view of the Brout-Englert-Higgs Field <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--4" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>A boson is born</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/matthew-chalmers"><span class="cern-tag">Matthew Chalmers</span></a></p> <p class="text-align-center">28 April, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/boson-born">français</a></i></p> <hr /> <p class="text-align-center">In the first part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, we look at the how the Higgs boson came to be</p> </span> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__centered section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-medium" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p><strong><em>On 4 July 2012, half a century’s wait came to an end as the <a href="/science/experiments/atlas">ATLAS</a> and <a href="/science/experiments/cms">CMS</a> experiments announced the discovery of the <a href="/science/physics/higgs-boson">Higgs boson</a>. </em></strong><a href="https://home.cern/news/news/cern/higgs10-save-date"><strong><em>Celebrate 10 years</em></strong></a><strong><em> since this extraordinary achievement by learning more about the history that led up to it, the next steps in understanding the mysterious particle, and CERN’s role in this endeavour. The “Higgs10” series will walk you through this journey, starting with an account by CERN Courier editor, Matthew Chalmers, of the theorisation of the Higgs boson in the 1960s.</em></strong></p> <p><strong><em>_______________</em></strong></p> <figure class="cds-image align-right" id="CERN-HI-1207136-101"><a href="//cds.cern.ch/images/CERN-HI-1207136-101" title="View on CDS"><img alt="higgsjuly4,seminar,Milestones,Higgs Boson Discovery,360" src="//cds.cern.ch/images/CERN-HI-1207136-101/file?size=large" /></a> <figcaption>Theoretical physicists François Englert (left) and Peter Higgs at CERN on 4 July 2012, at the announcement of the discovery of a Higgs boson by the ATLAS and CMS experiments. <span> (Image: M. Brice/CERN)</span></figcaption></figure> <p>It’s every theoretical physicist’s dream to conjure a new particle from mathematics and have it observed by an experiment. Few have scaled such heights, let alone had a particle named after them. In the CERN auditorium on 4 July 2012, Peter Higgs wiped a tear from his eye when the ATLAS and CMS results came in. The Higgs boson holds the record (48 years) among elementary particles for the time between prediction and discovery, going from an esoteric technicality to commanding the global spotlight at the world’s most powerful collider.</p> <p>Revealing that the universe is pervaded by a stark “scalar” field responsible for generating the masses of elementary particles was never something Robert Brout and François Englert, and independently Peter Higgs, set out to do. Their short 1964 papers – one by Brout and Englert, two others by Higgs – concerned an important but niche problem of the day. “Of no obvious relevance to physics” was how an editor of <em>Physics Letters</em> is said to have remarked on rejecting one of Higgs’ manuscripts. The papers went from fewer than 50 citations by the turn of the decade to around 18 000 today.</p> <p>At the time the “BEH” mechanism was being dreamt up independently in Brussels and Edinburgh – and in London by Gerald Guralnik, Carl Hagen and Tom Kibble – the Standard Model of particle physics was years away. Physicists were still trying to understand the menagerie of hadrons seen in cosmic-ray and early accelerator experiments, and the nature of the weak force. The success of quantum electrodynamics (QED) in describing electromagnetism drove theorists to seek similar “gauge-invariant” quantum field theories to describe the weak and strong interactions. But the equations ran into a problem: how to make the carriers of these short-range forces massive, and keep the photon of electromagnetism massless, without spoiling the all-important gauge symmetry underpinning QED.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-071-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-071-1" title="View on CDS"><img alt="home.cern,Miscellaneous" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-071-1/file?size=large" /></a> <figcaption>The 1964 Peter Higgs paper that first predicted the existence of what would come to be known as the Higgs boson. (Image: paper, APS; logo, CERN)</figcaption></figure> <p>It took a phenomenon called spontaneous symmetry breaking, inherent in superconductivity and superfluidity, to break the impasse. In 1960, Yoichiro Nambu showed how the “BCS” theory of superconductivity developed three years earlier by John Bardeen, Leon Cooper and John R. Schrieffer could be used to create masses for elementary particles, and Jeffrey Goldstone brought elementary scalar fields to the party, picturing the vacuum of the universe as a “Mexican hat” in which the lowest-energy state is not at the most symmetrical point at the peak of the hat but in its rim. It was an abstraction too far for soon-to-be CERN Director-General Viki Weisskopf, who is said by Brout to have quipped: “Particle physicists are so desperate these days that they have to borrow from the new things coming up in many-body theory like BCS. Perhaps something will come of it.”</p> <p>Four years later, Brout, Englert and Higgs added the final piece of the puzzle by showing that a mathematical block called the Goldstone theorem, which had beset initial applications of spontaneous symmetry breaking to particle physics by implying the existence of unobserved massless particles, does not apply to gauge theories such as QED. Unaware that others were on the trail, Higgs sent a short paper on the idea to <em>Physics Letters</em> in July 1964 where it was accepted by Jacques Prentki, the editor based at CERN. In a second paper sent one week later, Higgs demonstrated the mathematics – but it was rejected. Shocked, Higgs sent it to <em>Physical Review Letters</em>, and added crucial material, in particular : “it is worth noting that an essential feature of this type of theory is the prediction of incomplete multiplets of scalar and vector bosons” – a reference to the Higgs boson that was almost never published. In a further twist of fate, <a href="https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.13.508">Higgs’ second paper</a> was received and accepted the same day (31 August 1964) that <em>Physical Review Letters</em> published <a href="https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.13.321">Brout and Englert’s similarly titled work</a>. Today, the scalar field that switched on a fraction of a nanosecond after the Big Bang, giving the universe a non-zero “vacuum expectation value”, is generally referred to as the BEH field, while the particle representing the quantum excitation of this field is popularly known as the Higgs boson.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-071-2"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-071-2" title="View on CDS"><img alt="home.cern,Miscellaneous" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-071-2/file?size=large" /></a> <figcaption>The 1964 Brout-Englert paper<span> (Image: APS)</span></figcaption></figure> <p>In further Nobel-calibre feats, Steven Weinberg incorporated the BEH mechanism into electroweak theory developed also by Abdus Salam and Sheldon Glashow, which predicted the W and Z bosons, and Gerard ‘t Hooft and Martinus Veltman put the unified theory on solid mathematical foundations. The discovery of neutral currents in 1973 in Gargamelle at CERN and of the charm quark at Brookhaven and SLAC in 1974 gave rise to the elecroweak Standard Model. Flushing out and measuring its bosons took three major projects at CERN spanning three decades – the SPS proton-antiproton collider, LEP and the LHC. In the mid-1970s, John Ellis, Mary Gaillard and Dimitri Nanopoulos described how the Higgs boson might reveal itself, and experimentalists accepted the challenge.</p> <p><a href="https://home.cern/news/press-release/cern/cern-experiments-observe-particle-consistent-long-sought-higgs-boson">The discovery of the Higgs boson at the LHC in 2012</a> ended one journey, but opened another fascinating adventure. Understanding this unique particle will take every last drop of LHC data, in addition to that of a “Higgs factory” that may follow. Is it elementary or composite? Is it alone, or does it have siblings? And what are the roles of the mysterious BEH field in the beginning and the fate of the universe?</p> <p>“We’ve scratched the surface,” said Peter Higgs in 2019. “But we have clearly much more to discover.”</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> The Higgs boson holds the record (48 years) among elementary particles for the time between prediction and discovery, going from an esoteric technicality to commanding the global spotlight at the world’s most powerful collider </div> </div> </div> Mon, 09 May 2022 11:38:08 +0000 katebrad 182526 at https://home.cern Searching for the unknown https://home.cern/news/series/lhc-physics-ten/searching-unknown <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--6" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>Searching for the unknown</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/achintya-rao"><span class="cern-tag">Achintya Rao</span></a></p> <p class="text-align-center">5 MARCH, 2021</p> <hr /> <p class="text-align-center">In the final part of the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series, we look at the searches that go beyond our current understanding of the universe</p> </span> </div> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2752987" data-filename="SUS19004_EventDisplay" id="CMS-PHO-EVENTS-2021-004-2"> <a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2021-004-2" title="View on CDS"> <img alt="Display of candidate event with a lepton and high jet multiplicity, as used in (for example) searches for RPV top squarks" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2021-004-2/file?size=large"/> </a> <figcaption> An event recorded during 2016 with the CMS detector that contains 10 jets (orange cones) and a muon (red line). <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>Count all the known kinds of particles in the universe. Now double it. This is the promise of a family of theoretical models known as Supersymmetry, or SUSY for short.</p> <p>The notion of theories predicting a doubling of observed particles may not be as bizarre as it seems. In fact, it has historical precedent with the story of antimatter.</p> <p>“The first hints of antimatter came from Paul Dirac trying to solve problems in relativistic quantum mechanics,” says Laura Jeanty, who co-leads the Supersymmetry (SUSY) group on the ATLAS experiment at the Large Hadron Collider. “He came up with equations that essentially had four solutions instead of two, and the symmetries of the maths allow positive as well as negative values.” In 1928, Dirac concluded that if the negative values represented electrons, the positive values must represent an equivalent positively charged particle. The positron, or antielectron, was eventually discovered by Carl Anderson in 1932.</p> <p>“At the time of Dirac’s theoretical work, however,” Jeanty adds, “it was a mathematical quirk that didn’t have any known physical reality.” Today, we have discovered antiparticles for all the charged particles in the “Standard Model” of particle physics – the best description we have of our universe at the quantum scale. The Standard Model, however, has important limitations, and SUSY provides a theoretical extension to it by introducing new mathematical symmetries.</p> <h2>Inexplicable hierarchies</h2> <figure class="cds-image breakout-right" id="CMS-PHO-EVENTS-2021-004-2"><a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2021-004-2" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,CMS" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2021-004-2/file?size=large" /></a> <figcaption>A CMS event display from 2016 containing 10 jets (orange cones) and a muon (red line), which is representative for the signatures that certain supersymmetry models would leave in the CMS detector. (Image: CMS/CERN)</figcaption></figure> <p>As a scientific theory, the Standard Model is incredibly robust. Frustratingly so for physicists, because they are aware that this theory does not explain everything about the infinitesimal world of particles and quantum forces. Nevertheless, experimentalists have found no chinks in its armour, no deviations from its very accurate predictions, despite its limitations.</p> <p>One such limitation is that the Standard Model accounts for only three of the known quantum forces in the universe: the strong, electromagnetic and weak forces; gravity is not in the mix. A symptom of this is known as the hierarchy problem, pertaining to the vast difference between the strengths of the strong, electromagnetic and weak forces on one hand and gravity on the other. Despite its name, the weak force is around 24 orders of magnitude (10<sup>24</sup>) stronger than gravity. But why does this matter?</p> <p>“The hierarchy problem,” remarks Pieter Everaerts, Laura’s counterpart on the CMS experiment, “tells us that there have to be corrections to our current knowledge of physics.” The problem affects, for example, the mass of the Higgs boson that was discovered by ATLAS and CMS in 2012. According to quantum mechanics, the Higgs boson should have a mass several orders of magnitude higher than what was observed, because of its interactions with ephemeral virtual particles that pop in and out of existence.</p> <p>SUSY provides an elegant theoretical solution to this problem. It does this by proposing the following: fermions – particles that make up matter – have force-carrying super-partners known as “sfermions”, while bosons – force-carriers in the Standard Model – are paired up with fermionic “bosinos”.</p> <p>“With SUSY, the Higgs boson has twice as many particles to interact with,” adds Everaerts. Neatly, this allows the excess values of its expected mass coming from its interactions with ordinary particles to cancel out with the values of its interactions with supersymmetric particles. You are left with a predicted mass for the Higgs boson that is close to the observed mass of 125 GeV.</p> <p>All that remains is the one small detail of finding at least one of the predicted SUSY particles.</p> <h2>Optimism at a new frontier</h2> <p>Before the LHC began colliding protons together, there was a buzz of expectation among experimental and theoretical particle physicists.</p> <blockquote>“It seems strange to say this now,” continues Everaerts, “but when the LHC began colliding protons in 2010, some were expecting us to discover six or seven SUSY particles immediately.”</blockquote> <figure class="cds-image breakout-left" id="ATLAS-PHO-Event-2015-047-1"><a href="//cds.cern.ch/images/ATLAS-PHO-Event-2015-047-1" title="View on CDS"><img alt="Density of allowed supersymmetric models before and after the ATLAS Run-1 searches" src="//cds.cern.ch/images/ATLAS-PHO-Event-2015-047-1/file?size=large" /></a> <figcaption>The density of allowed supersymmetric models before and after ATLAS had searched through Run-1 data (data gathered up to February 2013)(Image: ATLAS/CERN)</figcaption></figure> <p>There was even concern that too many SUSY particles (or “sparticles”) at a low enough mass would add to the “background”, or noise, and make it harder to study Standard Model processes at the LHC. This optimism had to quickly face reality when no sparticles manifested. Indeed, no deviations from the Standard Model have been observed in the nearly 15 million billion (15 000 000 000 000 000) proton–proton collisions that have taken place inside each of ATLAS and CMS.</p> <p>Over the years, data collected by ATLAS and CMS enabled the collaborations to discard several of the simpler SUSY models, ruling out sparticles with masses up to around a teraelectronvolt (or TeV). All this showed, though, was that the most rudimentary interpretations of the theories were inadequate. “When we rule things out to a certain energy, we are aware that these are not realistic models, they are benchmarks,” says Federico Meloni of ATLAS. “And when you look at the same data through a less simplified interpretation, what we call an analysis in multidimensional parameter space,” he continues, “a limit of 2 TeV can become 500 GeV [gigaelectronvolt] or maybe we don’t have a limit at all. When looking at the big picture, it is only after ten years of operations that we are starting to be able to make interesting statements about the key issues.”</p> <p>For the moment, in the absence of a discovery, SUSY remains firmly in the realm of theory alone. “Supersymmetry in the way we were thinking has been ruled out and we have to now look for it in a different way,” says Gian Giudice, head of CERN’s Theoretical Physics department. “We continue to advance techniques to search for supersymmetric particles,” Jeanty adds. “More LHC data will help us to look further into the challenging corners of phase space, where new physics could still be lurking.”</p> <p>The LHC however is after more than just SUSY. Indeed, extensions of the Standard Model come in many forms, and on ATLAS and CMS the several teams performing searches for physics beyond the Standard Model are grouped together under the name “Exotics”. (The name “Miscellaneous” is quite obviously less exciting.) Some of these searches seem to come right out of science fiction…</p> <h2>Extra dimensions and micro black holes</h2> <p>Elementary particle physics concerns itself with the very small: tiny particles interacting through quantum forces at an unimaginably minuscule scale. Gravity, on the other hand, applies to the very large – think planets, stars and galaxies – and has sat apart from the quantum domain in our understanding of the universe. A quantum theory of gravity has remained the holy grail of high-energy physics for decades.</p> <blockquote>“If there is a quantum description of gravity, is there a particle that is responsible for mediating gravity?” asks Carl Gwilliam, a former coordinator of ATLAS exotic physics.</blockquote> <p>Discovery of such a particle, known as a graviton, would help settle the debate on the many theoretical models that attempt to unify gravity with the other three forces.</p> <figure class="cds-image breakout-right" id="CMS-PHO-EVENTS-2015-009-2"><a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2015-009-2" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,CMS" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2015-009-2/file?size=large" /></a> <figcaption>A multi-jet event display observed by the CMS detector in 2015 in the search for microscopic black holes. (Image: CMS/CERN)</figcaption></figure> <p>ATLAS and CMS are searching for gravitons directly, as for any other new particle, by looking for a bump in a smoothly falling distribution in the data. But, because the theories that predict the existence of a graviton also predict the existence of more than four dimensions of spacetime, the physicists are also looking for particles that are produced in collisions before they disappear into the extra dimensions. You cannot detect these disappearing particles directly; indeed, you cannot even ask the detectors to take snapshots of such collision events because there is nothing to “trigger” the detectors. You <em>can</em>, however, infer their presence by detecting an accompanying jet of particles produced from the same collision and observing simultaneously a lot of missing energy from the interaction itself. Niki Saoulidou, who co-leads the CMS Exotics team, points out that before the LHC was switched on, it was thought that these kinds of mono-jet searches, which also look for dark matter or supersymmetric particles, were too challenging for hadron-collider environments. “But we have evolved our tools, our techniques, our detector and our physics understanding so much that we now consider these as standard searches,” she says.</p> <p>Another way of detecting extra dimensions made headlines before the LHC began operations: micro black holes. If produced in high-energy proton–proton collisions at the LHC, these tiny black holes would instantly evaporate, leaving behind multiple jets of particles. “The thing with black-hole searches is that they would be very spectacular!” remarks Gwilliam. “You would expect to see a black-hole event very early on in a new energy regime.” Since it began operations, the LHC has explored three new high-energy regimes: 7 TeV, 8 TeV and 13 TeV. ATLAS and CMS also searched at the lower energy of 900 GeV. “Unfortunately, these have not showed up in the data,” adds Gwilliam, “so we have set very strong limits on their existence.”</p> <p>What does this mean for unifying gravity with the quantum forces? Saoulidou is philosophical: “It could be that we don’t have a quantum theory of gravity for a very good reason, which nature knows but we don’t.”</p> <p>Nevertheless, those 15 million billion collisions that ATLAS and CMS have analysed make up only 5% of the total data volume the LHC will deliver over the course of its lifetime. The graviton could still be out there.</p> <h2>Mysterious missing pieces and uncanny coincidences</h2> <p>“Results from searches for exotic physics were at the forefront of work done at the start of the LHC era,” says Adish Vartak, former co-leader of the CMS Exotics team. There was a lot of potential for finding new physics, given that the LHC was operating at an energy of around four times higher than the previous highest-energy collider, the Tevatron at Fermilab.</p> <blockquote>“When the LHC started,” Vartak continues, “we wanted to see whether there was a new resonance – a new particle – at a few TeV or so, at energies that the Tevatron could not probe.”</blockquote> <p>It is not only the spectacular that particle physicists are after. Many of the searches conducted by the Exotics groups of ATLAS and CMS look for answers to particularly puzzling questions. For example, the data have so far shown that quarks are elementary particles; that is, they are themselves not made up of any particles. But we don’t know for sure if that is the case. Finding quarks in excited states at high energies would demonstrate that they have inner substructure. Another puzzle is that the two families of fermions – leptons and quarks – curiously come in three generations each. There is no particular reason for this, unless they are somehow related to one another. ATLAS and CMS are therefore looking for leptoquarks, particles predicted to be hybrids of both kinds of fermions.</p> <p>Physicists are also looking for previously unobserved quantum forces, which would manifest in the form of heavier versions of the electroweak-force-carrying W and Z bosons, called W′ (“W-prime”) and Z′ (“Z-prime”). In the case of neutrinos, the reason for their extremely light but non-zero mass could be explained by discovering heavier exotic neutrinos, which balance the lighter regular neutrinos through a “see-saw” mechanism.</p> <p>Other searches are for heavier Higgs bosons, charged Higgs bosons and even composite (non-elementary) Higgs bosons. Yet more focus on hypothesised magnetic monopoles (a single north or south pole) that, rather than bending in the high magnetic fields of ATLAS and CMS, would get accelerated through them.</p> <p>Of course, experimentalists are also looking for any new particles and phenomena, even ones not explicitly predicted by theory. Giudice, a theorist, adds: “Experimentalists can make progress without a theorist telling them, ‘Oh, this comes from this model.’ Before the LHC, much of the analysis was done in terms of models. Now they try to present the data without relying on a specific model but rather on a broader language.”</p> <h2>The 750-GeV bump-that-was-not</h2> <figure class="cds-image breakout-right" id="CERN-HOMEWEB-PHO-2016-002-1"><a href="https://commons.wikimedia.org/wiki/File:ArXiv_submissions_citing_750_GeV_diphoton_CMS_and_ATLAS_data.png" title="View on CDS"><img alt="Graph of axXiv submissions after December 2015" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2016-002-1/file?size=large" /></a> <figcaption>Graph showing the sharp rise in arXiv paper submissions after December 2015, as theorists attempted to explain the 750-GeV bump in the data (Image: André David)</figcaption></figure> <p>This model-independent approach caused much excitement in 2015. Over the course of the first year of the LHC’s second run, both ATLAS and CMS began to notice something peculiar in their data. There appeared to be a slight excess of events in the two-photon channel at a mass of 750 GeV/c² in both their data sets. Initially the excess was of very low statistical significance, far from the 5-sigma threshold for claiming a discovery. Nevertheless it intrigued experimentalists and theorists alike. “As data began to be collected,” Vartak recounts, “there was a lot of hope that a new kind of physics – one that had evaded us previously – would show up.”</p> <p>In December, at the annual end-of-year seminar from the LHC experiments, excitement reached fever pitch. ATLAS <em>and</em> CMS presented data showing the significance of the excess was around 3 sigma. In the following three months, around 300 papers were submitted to arXiv by theorists seeking to explain the inexplicable. By the time all of the data from Run 2 (2015–2018) were studied, the excess had evaporated. There is a reason physicists wait until the 5-sigma threshold is breached: smaller excesses are not unusual and are usually statistical fluctuations and low-significance flukes.</p> <p>For the Exotics teams, though, it was the closest they came to something from beyond the Standard Model.</p> <h2>Changes in strategy and the road ahead</h2> <p>The lessons learnt so far are helping shape the search strategies of the future. For one, the triggers that select the collision data worth storing for further analysis are being recalibrated to handle so-called “long-lived particles”, which might transform into lighter particles outside of the typical timeframe when the triggers take their snapshots of collisions. Efforts are also underway to reanalyse the data recorded so far using novel techniques.</p> <p>The challenges provide more than adequate motivation and inspiration. “I’ve always had a lot of fun with my research,” Everaerts says with a smile. “Collaborating with people from different backgrounds to work on a common goal: I find that amazing!”</p> <p>So what is the legacy of the LHC after its first decade of operation? Giudice is emphatic: “The LHC has changed radically the way we view the world of particle physics today.” It might not have shown what theorists hoped it would, but it has helped make important strides in both theory and experiment. “When I start with an idea and then it turns out to be wrong, it’s not a question of failure; it is the scientific method,” Giudice continues. “You make a hypothesis, you check it with experiment, if it is right you keep on going but if it is wrong, you explore a different hypothesis.”</p> <p>“As experimentalists,” Meloni adds, “when we search beyond the Standard Model, our job is to look for everything. We know that we look for something, we look for it everywhere, and chances are it’s not going to be there. Still, our job is to understand our measurement, our search, and get to the result. And then the results are up for interpretation.”</p> <p>After all, looking and not finding is not the same as not looking.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> In the final part of the “LHC Physics at Ten” series, we look at the searches that go beyond our current understanding of the universe </div> </div> </div> Mon, 01 Mar 2021 13:15:32 +0000 katebrad 156590 at https://home.cern Standard Model surprises at high energies https://home.cern/news/series/lhc-physics-ten/standard-model-surprises-high-energies <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--8" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>The surprises of the Standard Model at high energies</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/corinne-pralavorio"><span class="cern-tag">Corinne Pralavorio</span></a></p> <p class="text-align-center">16 DECEMBER, 2020 · <i>Voir en <a href="/fr/news/series/lhc-physics-ten/standard-model-surprises-high-energies">français</a></i></p> <hr /> <p class="text-align-center">In the seventh part of the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series, we look at the surprising phenomena of the Standard Model at high energies</p></span> </div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2747736" data-filename="ATLAS_Experiment_dijet_event" id="OPEN-PHO-ACCEL-2020-007-1"> <a href="//cds.cern.ch/images/OPEN-PHO-ACCEL-2020-007-1" title="View on CDS"> <img alt="ATLAS Event DIsplay 2016: Highest-mass dijet event" src="//cds.cern.ch/images/OPEN-PHO-ACCEL-2020-007-1/file?size=large"/> </a> <figcaption> An event display of the highest-mass dijet event, (Event 4144227629, Run 305777) recorded in 2016 by the ATLAS Experiment. Without the signature of the experiment <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>"Robust” is what scientists working on the <a href="https://home.cern/science/accelerators/large-hadron-collider">Large Hadron Collider</a> (LHC) like to use to describe the <a href="https://home.cern/science/physics/standard-model">Standard Model</a>. By stubbornly probing it for weaknesses over the past 10 years, they have run up against the extreme solidity of this theory, which describes particles and forces. However, particle physicists are well aware that this model, finalised in the 1970s, has a few shortcomings. They are therefore searching for a wider theory that could resolve certain mysteries, and are banking on the LHC to help them find it. But apart from the triumphant discovery of the Higgs boson, no other new fundamental particle has been discovered, nor any extraordinary phenomenon that might lead to a more comprehensive theory.</p> <p>Has all this deterred them from their quest? “Quite the opposite,” smiles Nadjieh Jafari, co-leader of the top-quark group at the <a href="https://home.cern/science/experiments/cms">CMS</a> experiment. “There are many different territories for us to explore with the LHC: it’s an exciting period.” By venturing to the highest energies ever reached, physicists are observing many phenomena that were previously out of their reach.</p> <div class="blockquote">“We are measuring the behaviour of nature at new energies,” says Jonathan Butterworth, a physicist with the ATLAS experiment. “Even though they fit with the Standard Model, these phenomena are totally new to us.” </div> <h2><strong>New energies bring new phenomena</strong></h2> <p>The physicists at <a href="https://home.cern/science/experiments/atlas">ATLAS</a> are interested, for example, in the high-energy transverse jets of quarks and gluons. These jets can contain massive particles such as the W and Z bosons, the messenger particles of the weak force. “These new observations open up fields of research on the structure of such jets, to help us understand the strong interaction, as well as the electroweak interaction when a W or Z boson is emitted,” says Butterworth. The image above shows an ATLAS experiment event with two such jets (yellow and green cones).</p> <p>The experiments are therefore examining every square centimetre of this new territory, looking for processes that have been predicted but are either extremely rare, have never been observed before, or even better, are completely unexpected. These experiments include ATLAS and CMS, which observe the fusion and diffusion of electroweak bosons – very rare interactions. These events produce W and Z bosons, which either fuse together to produce another particle (fusion) or bounce away from each other (diffusion). “It’s as if the LHC had become a <a href="https://cms.cern/news/probing-standard-model-boson-boson-collider">collider of weak bosons</a>; these phenomena are completely new at these energies,” says Paolo Azzurri, co-leader of the Standard Model group at CMS.</p> <figure class="cds-image" id="CERN-HOMEWEB-PHO-2020-065-2"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2020-065-2" title="View on CDS"><img alt="home.cern" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2020-065-2/file?size=large" /></a> <figcaption>On the left, a CMS event display of a candidate event in which two W bosons and one Z boson are produced. On the right, an ATLAS event display of a candidate event in which two Z bosons are produced.<span> (Image: CMS and ATLAS, CERN)</span></figcaption></figure> <p>Another observation in this region, which is around 50 times rarer than the production of the Higgs boson, is the <a href="https://cerncourier.com/a/lhc-physics-shines-amid-covid-19-crisis/">simultaneous production of three weak bosons</a>. This phenomenon is seen only once in approximately every 100 billion proton collisions. These interactions also provide a new tool with which to probe the Standard Model and the weak interaction carried by the W and Z bosons. “The programme of boson fusion and diffusion started recently,” says Andrew Pilkington, a physicist with the ATLAS experiment. “There is still a long way to go before we can move from observation to the precision measurements that could allow us to detect deviations.”<em> </em></p> <h2><strong>The promise of virtual particles </strong></h2> <p>Physicists measure the frequency of these phenomena (their cross section) as precisely as possible and compare it with theoretical predictions. Any difference could indicate the presence of new particles. If unknown particles exist, they may be too massive to be produced at the LHC, but their quantum behaviour could help spot them.“In quantum field theory, anything that isn’t forbidden can happen,” explains Claude Duhr, a theoretical physicist at CERN. “Particles that are too massive to be produced in reality may appear and disappear fleetingly during an interaction.” These particles are known as virtual particles: they are involved in the interaction, but they are not directly detected. “We can deduce their presence because they have an impact on the interaction. For example, we could observe an excess of events during an interaction, which would indicate the presence of virtual particles,” continues Duhr. This is why it is necessary to measure interactions very precisely, in order to be able to compare the results with the theoretical predictions.</p> <p>However, one big difficulty is obtaining precise theoretical predictions. Due to the virtual particles, there are not just one but many ways in which the particles can be produced during a proton collision. Physicists have to take into account not only the direct processes (leading order or LO), in which these particles are directly produced without any contribution from virtual particles, but also the processes that result from the appearance of a virtual particle (next-to-leading order or NLO) or even two virtual particles (next-to-next-to-leading order or NNLO) and so on.</p> <p>These processes with the appearance of virtual particles occur more frequently when the strong interaction is involved (which is the case for proton collisions) and when the energy level is high. It is crucial to take them into account for certain interactions, such as the production of the Higgs boson. However, these <a href="https://cerncourier.com/a/the-two-loop-explosion/">“perturbative” theoretical calculations are very complex</a> and have required physicists to develop new mathematical tools, spurred on by the results from the LHC experiments. “It took four people four years to calculate the production of the Higgs boson at the next-to-next-to-leading order NNLO,” explains Duhr, a specialist in this field. And physicists are studying numerous interactions at the LHC, which pushes theorists to carry out many perturbative calculations to allow a comparison with the theory.</p> <p>To make things even more challenging, the theoretical predictions also rely on <a href="https://cerncourier.com/a/the-proton-laid-bare/">solid knowledge of the proton</a>. Paradoxically, the proton, which makes up all the matter around us, is a complex system and its structure is poorly understood. Its three quarks are bound by the strong force, which acts through the exchange of gluons, the messenger particles of the strong interaction. Determining the distribution of a given proton energy among the components of the proton (which we also refer to as partons) is anything but simple. This information is important to understand the initial conditions or, in other words, the energy available during the collision. “The huge amounts of data from the LHC have allowed us to considerably improve our understanding of the structure of the proton,” says Giorgio Passaleva, a physicist with the LHCb experiment.</p> <h2><strong>The top quark: a massive effect</strong></h2> <figure id="CMS-PHO-EVENTS-2019-004-3"><a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2019-004-3" title="View on CDS"><img alt="Real Events,For Press, collision, event display, CMS event display,top quark,top quarks,four top quarks" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2019-004-3/file?size=large" /></a> <figcaption>Event recorded by the CMS experiment in 2016 in which four top quarks were produced simultaneously <span> (Image: CMS/CERN)</span></figcaption></figure> <p>Among the many studies of the Standard Model, those relating to the top quark are particularly special. The top quark is the heaviest of the quarks and is almost 90 000 heavier than the lightest, the up quark. It has a very strong coupling with the Higgs boson, which is to be expected since the mechanism associated with this boson that gives elementary particles their mass. As the top quark is also sensitive to the strong, weak and electromagnetic forces, it can be produced by a myriad of processes. It is therefore an ideal candidate for exploring the new energy territories made accessible by the LHC. Florencia Canelli, co-leader of the top-quark physics group at the CMS experiment, started working on the topic at Fermilab in the US in 1998, three years after the laboratory discovered the quark. Pioneering studies were carried out at the Tevatron to define the top quark’s characteristics but, for the past 10 years, the LHC has provided an excellent observation ground for this particle. In the space of just a few years, ATLAS and CMS have been able to measure the <a href="https://cms.cern/news/cms-presents-new-precise-measurement-top-quark-mass">mass of the top quark with excellent precision</a>.</p> <div class="blockquote">“With the LHC, we have access to unexplored regions and huge amounts of data, which allow us to gain a more complete and precise understanding of the top quark. These measurements also allow us to <a href="https://cms.cern/news/a-toy-Universe-to-study-top-quarks-and-search-for-new-physics">constrain new physics processes</a>,” explains Florencia Canelli, CMS physicist.</div> <figure class="cds-image breakout-right" id="ATLAS-PHOTO-2020-013-1"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-013-1" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-013-1/file?size=large" /></a> <figcaption>Event recorded in 2018 by the ATLAS experiment in which <a href="https://atlas-public.web.cern.ch/updates/briefing/evidence-four-top-quark-production">four top quarks are produced</a>. (Image: ATLAS/CERN)</figcaption></figure> <p>The high energies at the LHC also provide an opportunity to study the production of top quarks with massive particles such as the W and Z bosons. “Or the simultaneous production of four top quarks, a quite extraordinary phenomenon,” confirms her colleague Nadjieh Jafari, who has been working on the subject since 2008 and is now co-leader of the CMS top-quark analysis group.</p> <p>The study of the top quark is one of the main focuses of the search for physics beyond the Standard Model. It is thought that unknown particles with a higher mass could decay into top quarks. “The top quark opens a door to theories beyond the Standard Model. Many predict new particles would decay into top quarks or into the same final states as those of the top quark,” confirms Francesco Spano, co-leader of the ATLAS top-quark analysis group.</p> <p>The study of the interactions involving this special particle is far from complete. Wolfgang Wagner, a physicist with the ATLAS experiment, displays a table indicating the different processes producing the top quark at the LHC and the analyses carried out for each of them. 19 of the 48 boxes in his table are marked with a cross, indicating that the process in question has been studied. “Ten years ago, we were just starting the study of the production of top-antitop pairs, the most accessible of the processes. Today, we have exceeded the precision of the theory for this process, but we still have many other processes to examine,” he explains.</p> <h2><strong>Strange assemblies</strong></h2> <figure class="cds-image breakout-left" id="OPEN-PHO-EXP-2015-009-3"><a href="//cds.cern.ch/images/OPEN-PHO-EXP-2015-009-3" title="View on CDS"><img alt="pentaquark,LHCb" src="//cds.cern.ch/images/OPEN-PHO-EXP-2015-009-3/file?size=large" /></a> <figcaption>Illustration of the possible layout of quarks in a pentaquark particle, such as those discovered at LHCb. (Image: Daniel Dominguez/CERN)</figcaption></figure> <p>In its exploration of these new energy territories, <a href="https://home.cern/science/experiments/lhcb">LHCb</a> has unearthed exotic assemblies of quarks in which four or even five quarks are bound by the strong interaction. According to the model of hadrons, there are two categories of composite particles: mesons, composed of pairs containing a quark and an antiquark, and baryons, such as protons, containing three quarks. In the quark model proposed in 1964, Murray Gell-Mann and George Zweig also predicted the possible existence of exotic hadrons such as tetraquarks and pentaquarks.</p> <p>In 2010, LHCb spotted its first tetraquark, followed by several others over the course of the last 10 years. In 2015, the experiment created a stir by announcing <a href="https://home.cern/news/press-release/cern/cerns-lhcb-experiment-reports-observation-exotic-pentaquark-particles">the first discovery of a pentaquark</a>. In 2019, a <a href="https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark">second pentaquark was identified</a>. “These exotic systems are so extreme and strange that they have aroused the interest of theoretical physicists,” explains Giovanni Passaleva, a physicist and former LHCb spokesperson. In fact, the appearance of these exotic hadrons has inspired new research in order to understand their internal mechanisms.</p> <p>“The study of these exotic assemblies is another tool for testing the hadron model and quantum chromodynamics, the theory of the strong interaction,” adds Tatsuya Nakada, the first spokesperson of LHCb.The experimental data on exotic hadrons will allow physicists to improve their understanding of quantum chromodynamics at low energies, which describes the bound states of quarks.</p> <p>LHCb physicists are pursuing their examination of this small corner of the Standard Model, just like all the other thousands of LHC scientists studying the new areas opened up by the LHC. Even though the number of events produced by the LHC is already phenomenal, large quantities of data are still required to understand these new phenomena in detail. The Standard Model is robust, so scientists need patience and precision to find its limits.</p> <div class="blockquote">“We explore nature by getting close to the conditions at the very beginning of time, on the smallest scales ever achieved, and we look for deviations from our expectations. It’s in these minuscule regions of space and time that we will be able to detect the limits of the Standard Model,” concludes Francesco Spano, ATLAS physicist.</div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> In the seventh part of the “LHC Physics at Ten” series, we look at the surprising phenomena of the Standard Model at high energies </div> </div> </div> Tue, 15 Dec 2020 14:37:04 +0000 cmenard 156231 at https://home.cern Welcome to the precision era https://home.cern/news/series/lhc-physics-ten/welcome-precision-era <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--10" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>Welcome to the precision era</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/corinne-pralavorio"><span class="cern-tag">Corinne Pralavorio</span></a></p> <p class="text-align-center">8 DECEMBER, 2020 · <em>Voir en <a href="/fr/news/series/lhc-physics-ten/welcome-precision-era">français</a></em></p> <hr /> <p class="text-align-center">Our sixth story in the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series looks at the precision measurements of the Standard Model made at the Large Hadron Collider</p></span> </div> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2746561" data-filename="LHCb-B0s-mumu-2016" id="OPEN-PHO-ACCEL-2020-005-3"> <a href="//cds.cern.ch/images/OPEN-PHO-ACCEL-2020-005-3" title="View on CDS"> <img alt="LHCb event display B0s decaying into a μ+ and μ- pair." src="//cds.cern.ch/images/OPEN-PHO-ACCEL-2020-005-3/file?size=large"/> </a> <figcaption> Bs0→ μ+μ- decay candidate event recorded in 2016 <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>At the start of 2010, the particle physics community was abuzz with hopes and excitement. Just a few weeks later, the experiments at the <a href="https://home.cern/science/accelerators/large-hadron-collider">Large Hadron Collider</a> (LHC) would <a href="https://home.cern/news/series/lhc-physics-ten/lhc-physics-ten-entering-uncharted-waters">venture beyond the energy frontier</a>, where physicists hoped to find exotic particles that would pave the way for a more complete theory of the infinitely small: to physics beyond the Standard Model.</p> <p>The Standard Model of particles and forces was developed in the second half of the 20th century to explain the discovery of a host of new particles, and to describe – within the framework of a single theory – their behaviour and the forces that link them. This model has been hugely successful and accurately summarises the various phenomena that have been observed. However, it leaves a number of questions unanswered, about subjects such as the <a href="https://home.cern/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter">nature of dark matter</a> or the <a href="https://home.cern/science/antimatter/matter-antimatter-asymmetry-problem">absence of antimatter</a> in the universe.</p> <p>Indeed, when the huge collider was on the verge of starting up, theories about “Beyond the Standard Model” physics were igniting passionate debates and the acronym BSM was cropping up in everyone’s presentations. Claude Duhr had just defended his thesis at the Catholic University of Leuven (Belgium) and was wondering which direction to take. “I had a choice of focusing on precision calculations of the Standard Model or studying physics beyond the Standard Model. Lots of my colleagues couldn’t see any future in precision calculations and advised me to pursue research into BSM theories,” he recalls, ten years later. But Claude Duhr’s hunch was right.</p> <figure class="cds-image" id="OPEN-PHO-CHART-2015-001-1"><a href="//cds.cern.ch/images/OPEN-PHO-CHART-2015-001-1" title="View on CDS"><img alt="Standard Model,Higgs boson,Diagrams and Charts" src="//cds.cern.ch/images/OPEN-PHO-CHART-2015-001-1/file?size=large" /></a> <figcaption>The Standard Model of particles and forces describes three of the four forces of nature that act on 12 particles of matter through the exchange of messenger particles. (Image: Daniel Dominguez/CERN) <p> </p> </figcaption></figure> <p>Two years later, <a href="https://home.cern/science/experiments/atlas">ATLAS</a> and <a href="https://home.cern/science/experiments/cms">CMS</a> <a href="https://home.cern/news/series/lhc-physics-ten/higgs-boson-what-makes-it-special">discovered the Higgs boson</a>, confirming the validity of the Brout-Englert-Higgs mechanism. A fabulous discovery that has left physicists hungry for more. But the Higgs was a giant tree hiding a meadow full of well-known flowers. No exotic plants were to be found in these high-energy plains. No unknown particle has made its presence felt by producing a bump on the physicists’ charts since. Month after month, the Standard Model has revealed itself to be more solid than ever.</p> <h2><strong>On the road to precision</strong></h2> <p>But our explorers weren’t discouraged. As no hitherto unknown particle had emerged, they would study, with ever more precision, known phenomena at all new energies. They scrutinised every blade of grass and every flower in this beautiful meadow, looking for a curiosity, an anomaly that would lead them to something new.</p> <div class="blockquote">“To begin with, scientists were looking for spectacular phenomena that have now mostly been ruled out. The approach now is to carry out precision measurements,” explains Paolo Azzurri, co-leader of the Standard Model group in the CMS experiment.</div> <p>Over the years, the LHC has therefore been used for increasingly precise studies, which represent a real challenge for a hadron collider (we’ll see later why this is the case). This was the road that Claude Duhr, who today is a theorist at CERN, ultimately decided to take. “More and more of the work of theorists focused on precision calculations to test the Standard Model as thoroughly as possible,” he explains. In the <a href="https://cerncourier.com/a/lhc-at-10-the-physics-legacy/"><em>CERN Courier</em> article in March 2020 </a><a href="https://cerncourier.com/a/lhc-at-10-the-physics-legacy/">o</a>n the same theme as this series of features, Michelangelo Mangano, a theorist at CERN, reminded readers that 1600 of the 2700 articles on the LHC in peer-reviewed publications report measurements of Standard Model particles.</p> <p>The <a href="https://home.cern/science/physics/standard-model">Standard Model of particles and forces</a> describes three of the four forces of nature that act through the exchange of messenger particles, known as bosons. The strong force, which binds quarks in protons and neutrons, is carried by gluons. The electromagnetic force is transmitted by photons, and the weak force, which is responsible for radioactive decay,  is carried by the W and Z bosons. There are also 12 particles of matter, grouped into two families: quarks, like those that form protons and neutrons, which feel the strong and weak forces; and leptons, such as electrons, on which the electromagnetic and weak forces act. Each of the two families comprises six particles (see table above).</p> <p>In reality, the Standard Model is built on two quantum theories : the <a href="https://home.cern/science/physics/unified-forces">electroweak</a> theory, which describes the electromagnetic and the weak forces, and  quantum chromodynamics, which describes the strong force. So, here we have the basics.</p> <h2><strong>Determining the free parameters</strong></h2> <figure class="cds-image breakout-left" id="ATLAS-PHOTO-2017-020-1"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2017-020-1" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,ATLAS,W bosons,7 TeV,muons,neutrinos" src="//cds.cern.ch/images/ATLAS-PHOTO-2017-020-1/file?size=large" /></a> <figcaption>A candidate event for a W boson decaying into one muon and one neutrino recorded by the ATLAS experiment in 2011. Such events were used for the measurement of the W boson’s mass.<span> (Image: ATLAS/CERN)</span><span> (Image: CERN)</span></figcaption></figure> <p>One advantage of the Standard Model is that it is predictive: it predicts all possible interactions between particles with a precise probability (which physicists call the “cross section”). However, it doesn’t predict the masses of the fundamental particles: these are among the parameters measured by the experiments. Moreover, these masses vary greatly: for example, the mass of the heaviest quark, the top quark, is almost 90 000 greater than the up quark, the lightest.</p> <p>In total, there are 19 free parameters (aside from the parameters relating to neutrinos). Measuring them precisely is crucial to be able to calculate the interaction cross sections and test the consistency of the Standard Model. Although the Standard Model doesn’t predict their values, it ties some parameters together. And, as the measurements still have a degree of uncertainty, “if the measured mass of the W boson changes while the measured mass of the top quark remains unchanged, then the predicted mass of the Higgs should also change,” explains Andrew Pilkington, a physicist with the ATLAS experiment. “By measuring all of these parameters independently, we test the relationships predicted by the Standard Model and impose constraints on physics beyond the Standard Model.</p> <p>One of the success stories of the LHC is how it has improved the measurements of these free parameters, starting, of course, by determining the <a href="https://home.cern/news/series/lhc-physics-ten/higgs-boson-revealing-natures-secrets">mass of the Higgs boson. ATLAS</a> has also increased the precision of the <a href="https://home.cern/news/news/first-high-precision-lhc-measurement-w-boson-mass">mass of the W boson</a>. “This was a remarkable achievement that no one had anticipated,” says Jonathan Butterworth, a physicist with the ATLAS experiment who was co-leader of the Standard Model group in 2010.</p> <p>Using a hadron collider to make precise measurements is far from easy. The LHC collides composite particles, protons, formed of three quarks that interact via gluons. The starting energy is not known – we don’t know which components of the proton are colliding – and the background (all the simultaneous minor interactions that interfere with the result we’re looking for) is very significant. But, armed with ten years’ worth of simulations of their proposed detector and drawing on the outstanding work of hundreds of physicists to understand and reconstruct the events (physics jargon for the collisions and the particles emerging from them), the LHC experiments managed to deliver precise results after just two years.</p> <figure class="cds-image breakout-right" id="CMS-PHO-EVENTS-2018-011-4"><a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2018-011-4" title="View on CDS"><img alt="Real Events,For Press,CMS ,Top quark,Single Top,Event Display,collision,CMS event display,tZq" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2018-011-4/file?size=large" /></a> <figcaption>A collision event from 2016 in which a top quark is produced in association with a Z boson at CMS. <span>(Image: CMS/CERN)</span></figcaption></figure> <p>The LHC has also chalked up <a href="https://cms.cern/news/cms-presents-new-precise-measurement-top-quark-mass">one of the best measurements of the mass of the top quark</a>, which was discovered in 1995 at the Tevatron collider in the United States. “The value combining the results of the Tevatron was already very precise,”<em> </em>remarks Nadjieh Jafari, co-leader of the top-quark physics group of the CMS experiment. “But at the LHC we are able to measure the mass of the top quark using additional channels of production of top quarks, and for some we got equal or better accuracy.”</p> <p>Other free parameters enter into the calculation of interactions. The precise measurement of the electroweak mixing angle is one of the key results from the LHC experiments. This result serves to constrain the masses of the W and Z bosons.</p> <h2><strong>Beauty particles and flavour physics</strong></h2> <p>The <a href="https://home.cern/science/experiments/lhcb">LHCb</a> experiment specialises in the study of B hadrons, particles that contain a bottom quark or its antiparticle, and has developed expertise in measuring the parameters that can be used to determine the probability that a quark will transform into another via the weak interaction. These transformation processes were first described by Nicola Cabibbo, Makoto Kobayashi and Toshihide Maskawa, and can be calculated using a matrix that bears their initials. The CKM matrix is made up of four free parameters – like the masses of particles – that are measured in experiments. Measurements via different processes can be used to test the robustness of the Standard Model. The structure of the CKM matrix can be represented graphically by triangles, with the parameters represented by the lengths of the sides and the angles. <a href="https://lhcb-public.web.cern.ch/Welcome.html#gamma">For example, LHCb has obtained the best measurement of one of these angles, γ</a>. This work is linked to work on the phenomenon of charge-parity (CP) violation, which is at the origin of a difference in behaviour between matter and antimatter. The experiment has also obtained excellent results relating to CP violation, including <a href="https://home.cern/news/press-release/physics/lhcb-sees-new-flavour-matter-antimatter-asymmetry">proof of the phenomenon occurring with particles containing a charm quark</a>, whereas before it had been observed only with particles containing a strange or a bottom quark. </p> <p>But B mesons have opened up an even wider field of study for LHCb.</p> <div class="blockquote">“The LHCb programme has evolved not only to confirm CP violation with B mesons, but also to understand the phenomena of flavour physics in general,” explains Tatsuya Nakada, a pioneer of LHCb and its first spokesperson. “The study of these phenomena is an extremely useful way of measuring the coherence of the Standard Model.”</div> <p>The smallest of the LHC’s main experiments has become a gold standard in the field of flavour physics. A great success considering that its capabilities were considered to be limited to begin with. “The start-up of LHCb wasn’t easy,” recalls Giovanni Passaleva, another former spokesperson of the experiment. “Our objectives were considered far too ambitious for a hadron collider with so much background. The B factories (the BaBar experiment in the United States and the Belle experiment in Japan – Ed.) already covered the research we were planning. We were worried, but today we are proud and happy.”</p> <figure class="cds-image" id="CERN-PHOTO-201609-209-2"><a href="//cds.cern.ch/images/CERN-PHOTO-201609-209-2" title="View on CDS"><img alt="LHCb,Cavern,Caverne" src="//cds.cern.ch/images/CERN-PHOTO-201609-209-2/file?size=large" /></a> <figcaption>LHCb experiment has become a gold standard in the field of flavour physics, achieving crucial results studies of the weak interaction, and in the field of CP violation.<span> (Image: Maximilien Brice/CERN)</span></figcaption></figure> <p>Among other phenomena, the experiment is interested in decays that the Standard Model predicts to be very rare. The comparison of their measurements with the predictions allows it to test the robustness of the Standard Model. “If you discover a deviation, you might have come across a sign of new physics,” explains Tatsuya Nakada. LHCb and CMS have thus measured the cross section of the <a href="https://home.cern/news/news/detectors/cms-and-lhcb-experiments-reveal-new-rare-particle-decay">decay of the B<sup>0</sup><sub>s</sub> meson into two muons</a>, a process that, according to the theory, is produced in only three of every billion decays of this meson (the image at the top shows such an event, recorded by LHCb in 2016. The two muon tracks from the B<sup>0</sup><sub>s</sub> decay are seen as two green tracks running through the whole detector). LHCb has studied other very rare interactions of B mesons. The results are in agreement with the Standard Model but the possibilities for new measurements are far from being exhausted, covering many other phenomena.</p> <p>“Precision is a fantastic tool for understanding the world of particles,” says Gian Giudice, head of CERN’s Theory department.<em> </em>“The LHC has moved from discovery to precision and there is lots to learn.”</p> <p><strong><em>Next up: The surprises of the Standard Model at high energies</em></strong></p> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> <div id="vidyowebrtcscreenshare_is_installed"> </div> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Our sixth story in the LHC Physics at Ten series looks at the precision measurements of the Standard Model made at the Large Hadron Collider </div> </div> </div> Sun, 06 Dec 2020 17:26:58 +0000 cmenard 156181 at https://home.cern Recreating Big Bang matter on Earth https://home.cern/news/series/lhc-physics-ten/recreating-big-bang-matter-earth <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--12" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>Recreating Big Bang matter on Earth</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/ana-lopes"><span class="cern-tag">Ana Lopes</span></a></p> <p class="text-align-center">13 NOVEMBER, 2020 · <em>Voir en <a href="/fr/news/series/lhc-physics-ten/recreating-big-bang-matter-earth">français</a></em></p> <hr /> <p class="text-align-center">Our fifth story in the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series looks at how the LHC has recreated and greatly advanced our knowledge of the state of matter that is believed to have existed shortly after the Big Bang</p></span> </div> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2158419" data-filename="2011-Sep-30-run137161_1.20_ev96_1_3D" id="ALICE-EVENTDISPLAY-2016-006-1"> <a href="//cds.cern.ch/images/ALICE-EVENTDISPLAY-2016-006-1" title="View on CDS"> <img alt="Run 137161 (Golden Run), 2010-11-09, Global Tracks" src="//cds.cern.ch/images/ALICE-EVENTDISPLAY-2016-006-1/file?size=large"/> </a> <figcaption> First collisions of Pb+Pb seen by the ALICE experiment on 09.11.2010. <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>The Large Hadron Collider (LHC) at CERN usually collides protons together. It is these proton–proton collisions that led to the discovery of the Higgs boson in 2012. But the world’s biggest accelerator was also designed to smash together heavy ions, primarily the nuclei of lead atoms, and it does so every year for about one month. And for at least two good reasons. First, heavy-ion collisions at the LHC recreate in laboratory conditions the plasma of quarks and gluons that is thought to have existed shortly after the Big Bang. Second, the collisions can be used to test and study, at the highest manmade temperatures and densities, fundamental predictions of quantum chromodynamics, the theory of the strong force that binds quarks and gluons together into protons and neutrons and ultimately all atomic nuclei.</p> <p>The LHC wasn’t the first machine to recreate Big Bang matter: back in 2000, experiments at the Super Proton Synchrotron at CERN found compelling evidence of the quark–gluon plasma. About five years later, experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the US started an era of detailed investigation of the quark–gluon plasma. However, in the 10 years since it achieved collisions <a href="https://home.cern/news/press-release/cern/lhc-research-programme-gets-underway">at higher energies than its predecessors</a>, the LHC has taken studies of the quark–gluon plasma to incredible new heights. By producing a hotter, denser and longer-lived quark–gluon plasma as well as a larger number and assortment of particles with which to probe its properties and effects, the LHC has allowed physicists to study the quark–gluon plasma with an unprecedented level of detail. What’s more, the machine has delivered some surprising results along the way, stimulating new theoretical studies of this state of matter.</p> <blockquote>“In the ultimate textbook about the theory of the strong interaction, the chapter on the quark–gluon plasma will be filled with figures of LHC data,” says ALICE experiment spokesperson Luciano Musa.</blockquote> <p>“These figures excel in data precision and kinematic reach, and they are the first to inform us about how quark–gluon plasma-like properties emerge gradually as one transitions from proton–proton to heavy-ion collisions.”</p> <figure role="group" class="align-center"> <img alt="Illustration of the history of the universe, from the Big Bang to today" data-entity-type="file" data-entity-uuid="c504150e-70c0-4eb4-ac05-81f5050ccbaf" src="/sites/default/files/inline-images/abelchio/The_History_of_the_Universe.jpg" width="1280" height="629" loading="lazy" /> <figcaption>Illustration of the history of the universe. About one microsecond (μs) from the Big Bang, protons formed from the quark–gluon plasma. (Image: BICEP2 Collaboration/CERN/NASA)</figcaption> </figure> <h2>Heavy collision course</h2> <p>When heavy nuclei smash into one another in the LHC, the hundreds of protons and neutrons that make up the nuclei release a large fraction of their energy into a tiny volume, creating a fireball of quarks and gluons. These tiny bits of quark–gluon plasma only exist for fleeting moments, with the individual quarks and gluons, collectively known as partons, quickly forming composite particles and antiparticles that fly out in all directions. By studying the zoo of particles produced in the collisions – before, during and after the plasma is created – researchers can study the plasma from the moment it is produced to the moment it cools down and gives way to a state in which composite particles called hadrons can form. However, the plasma cannot be observed directly. Its presence and properties are deduced from the experimental signatures it leaves on the particles that are produced in the collisions and their comparison with theoretical models.</p> <p>Such studies can be divided into two distinct categories. The first kind of study investigates the thousands of particles that emerge from a heavy-ion collision collectively, providing information about the global, macroscopic properties of the quark-gluon plasma. The second kind focuses on various types of particle with large mass or momentum, which are produced more rarely and offer a window into the inner, microscopic workings of the medium.</p> <p>At the LHC, these studies are conducted by the collaborations behind all four main LHC experiments: ALICE, ATLAS, CMS and LHCb. Although ALICE was initially specifically designed to investigate the quark–gluon plasma, the other three experiments have also since joined this investigation.</p> <h2>Global properties</h2> <p>The LHC has delivered data that has enabled researchers to derive with higher precision than previously achieved several global properties of the medium.</p> <blockquote>“The LHC can “hear” much more precisely the quark–gluon plasma,” says CERN theorist and quark–gluon plasma specialist Urs Wiedemann.</blockquote> <p>“If we listen to two different musical instruments with closed eyes, we can distinguish between the instruments even when they are playing the same note. The reason is that a note comes with a set of overtones that give the instrument a unique distinct sound. This is but one example of how simple but powerful overtones are in identifying material properties. Heavy-ion physicists have learnt how to make use of “overtones” in their study of the quark–gluon plasma. The initial stage of a heavy-ion collision produces ripples in the plasma that travel through the medium and excite overtones. Such overtones can be measured by analysing the collective flow of particles that fly out of the plasma and reach the detectors. While previous measurements had revealed only first indications of these overtones, the LHC experiments have mapped them out in detail. Combined with other strides in precision, these data have been used by theorists to characterise the plasma’s properties, such as its temperature, energy density and frictional resistance, which is smaller than that of any other known fluid,” explains Wiedemann.</p> <p>These findings have then been supported in multiple ways. For instance, the ALICE collaboration <a href="https://arxiv.org/abs/1702.07233">estimated</a> the temperature of the plasma by studying photons that are emitted by the hot fireball. The estimated temperature, about 300 MeV (1 MeV is about 10<sup>10</sup> kelvin), is above the predicted temperature necessary for the plasma to be created (about 160 MeV), and is about 40% higher than the one obtained by the RHIC collider.</p> <p>Another example is the estimation of the energy density of the plasma in the initial stage of the collisions. ALICE and CMS <a href="https://arxiv.org/abs/1702.07233">obtained</a> a value in the range 12–14 GeV per cubic femtometre (1 femtometre is 10<sup>-15</sup> metres), about 2–3 times higher than that determined by RHIC, and again above the predicted energy density needed for the plasma to form (about 1 GeV/fm<sup>3</sup>).</p> <figure class="cds-image " id="ALICE-EVENTDISPLAY-2018-004-1"><a href="//cds.cern.ch/images/ALICE-EVENTDISPLAY-2018-004-1" title="View on CDS"><img alt="" src="//cds.cern.ch/images/ALICE-EVENTDISPLAY-2018-004-1/file?size=large" /></a> <figcaption>Particle trajectories and energy deposition in the ALICE detector during the last lead–lead collisions of the second LHC run. <span> (Image: CERN)</span></figcaption></figure> <h2>Inner workings</h2> <p>The LHC has supplied not just more particles but also more varied types of particle with which to probe the quark–gluon plasma.</p> <blockquote>“The LHC has given us access to a very broad palette of probes,” says ALICE physics coordinator Andrea Dainese.</blockquote> <p>“Together with state-of-the-art particle detectors that cover more area around the collision points as well as sophisticated methods of identifying and tracking particles, this broad palette has offered unprecedented insight into the inner workings and effects of the quark–gluon plasma.”</p> <p>To give a few examples, soon after the LHC started, ATLAS and CMS made the <a href="https://home.cern/news/press-release/cern/lhc-experiments-bring-new-insight-primordial-universe">first direct observation</a> of the phenomenon of jet quenching, in which jets of particles formed in the collisions lose energy as they cross the quark–gluon plasma medium. The collaborations found a striking imbalance in the energies of pairs of jets, with one jet almost completely absorbed by the medium.</p> <p>Another example concerns heavy quarks. Such particles are excellent probes of the quark–gluon plasma because they are produced in the initial stages of a heavy-ion collision and therefore experience the entire evolution of the plasma. The ALICE collaboration has more recently <a href="https://home.cern/news/news/physics/cern-collaborations-present-new-results-particles-charm-quarks">shown</a> that heavy quarks “feel” the shape and size of the quark–gluon plasma, indicating that even the heaviest quarks move with the medium, which is mostly made of light quarks and gluons.</p> <p>The LHC experiments, in particular ALICE and CMS, have also significantly improved our understanding of the hierarchical “melting” in the plasma of bound states of a heavy quark and its antiquark, called quarkonia. The more weakly bound the states are, the more easily they will melt, and as a result the less abundant they will be. CMS was the first to observe this so-called hierarchical suppression for bottomonium states, which consist of a bottom quark and its antiquark. And ALICE revealed that, while the most common form of charmonium states, which are composed of a charm quark and its antiquark, is highly suppressed due to the effect of the plasma, it is also regenerated by the recombination of charm quarks and antiquarks. This recombination phenomenon, observed for the first time at the LHC, provides an important testing ground for theoretical models and phenomenology, which forms a link between the theoretical models and experimental data.</p> <h2 class="no-column">Surprises in smaller systems</h2> <p class="no-column">The LHC data have also revealed unexpected results. For example, the ALICE collaboration showed that the enhanced production of strange hadrons (particles containing at least one strange quark), which is traditionally viewed as a signature of the quark-gluon plasma, arises gradually in <a href="https://home.cern/news/news/experiments/new-alice-results-show-novel-phenomena-proton-collisions">proton–proton</a> and proton–lead collisions as the number of particles produced in the collisions, or “multiplicity”, increases.</p> <p>Another case in point is the gradual onset of a flow-like feature with the shape of a ridge with increasing multiplicity, which was first observed by CMS in proton–proton and <a href="https://cerncourier.com/a/mysterious-long-range-correlations-seen-in-ppb-collisions/">proton–lead</a> collisions. This result was further supported by ALICE and ATLAS <a href="https://cerncourier.com/a/alice-and-atlas-find-intriguing-double-ridge-in-proton-lead-collisions/">observations</a> of the emergence of double-ridge features in proton–lead collisions.</p> <figure class="cds-image" id="OPEN-PHO-EXP-2017-003-2"><a href="//cds.cern.ch/images/OPEN-PHO-EXP-2017-003-2" title="View on CDS"><img alt="Experiments and Tracks" src="//cds.cern.ch/images/OPEN-PHO-EXP-2017-003-2/file?size=large" /></a> <figcaption>As the number of particles produced in proton–proton collisions increases (blue lines), the more particles containing at least one strange quark are measured (orange to red squares in the graph). <span> (Image: CERN)</span></figcaption></figure> <div class="blockquote">“The discovery of heavy-ion-like behaviour in proton–proton and proton–nucleus collisions at the LHC is a game-changer,” says Wiedemann.</div> <p class="no-column">“The LHC data have killed the long-held view that proton–proton collisions produce free-streaming sets of particles while heavy-ion collisions produce a fully developed quark–gluon plasma. And they tell us that in the small proton–proton collision systems there are more physical mechanisms at work than traditionally thought. The new challenge is to understand, within the theory of the strong force, how quark–gluon plasma-like properties emerge gradually with the size of the collision system.”</p> <p class="no-column">These are just examples of how 10 years of the LHC have greatly advanced physicists’ knowledge of the quark–gluon plasma and thus of the early universe. And with data from the machine’s second run still being analysed and more data to come from the next run and the <a href="https://home.cern/science/accelerators/high-luminosity-lhc">High-Luminosity LHC</a>, the LHC’s successor, an even more detailed understanding of this unique state of matter is bound to emerge, perhaps with new surprises in the mix.</p> <p class="no-column">“The coming decade at the LHC offers many opportunities for further exploration of the quark–gluon plasma,” says Musa. “The expected tenfold increase in the number of lead–lead collisions should both increase the precision of measurements of known probes of the medium and give us access to new probes. In addition, we plan to explore collisions between lighter nuclei, which could cast further light on the nature of the medium.”</p> <p><strong>Further reading:</strong><br /> <a href="https://cerncourier.com/a/lhc-at-10-the-physics-legacy/">LHC at 10: the physics legacy</a><br /> <a href="https://inspirehep.net/literature/1089148">First Results from Pb+Pb collisions at the LHC</a><br /> <a href="https://inspirehep.net/literature/1300425">Heavy-ion collisions at the LHC</a><br /> <a href="https://inspirehep.net/literature/1403170">Heavy-ion collisions at the Large Hadron Collider: a review of the results from Run 1</a><br /> <a href="https://arxiv.org/abs/1702.07233/">An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: bulk properties and dynamical evolution</a><br /> <a href="https://arxiv.org/abs/1702.07231">An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: hard probes</a></p> <hr /> <p><em>Don't miss the next articles of our series, which will cover the Standard Model and more.</em></p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Our fifth story in the LHC Physics at Ten series looks at how the LHC has recreated and greatly advanced our knowledge of the state of matter that is believed to have existed shortly after the Big Bang </div> </div> </div> Thu, 12 Nov 2020 10:12:51 +0000 abelchio 156056 at https://home.cern Breaking new ground in the search for dark matter https://home.cern/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--2" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>Breaking new ground in the search for dark matter</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/ana-lopes"><span class="cern-tag">Ana Lopes</span></a></p> <p class="text-align-center">7 AUGUST, 2020 · <em>Voir en <a href="/fr/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter">français</a></em></p> <hr /> <p class="text-align-center">Our fourth story in the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series discusses the LHC’s hunt for the hypothetical particle that may make up dark matter</p> </span> </div> <div class="background__veil"></div> <div class="background-component background__image" style="background:url(&#039;/sites/default/files/2020-07/600px-NASA-HS201427a-HubbleUltraDeepField2014-20140603%20copy.jpg&#039;) no-repeat center top / cover; height: 100%;"></div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p>The Large Hadron Collider (LHC) is renowned for the hunt for and discovery of the Higgs boson, but in the 10 years since the machine collided protons <a href="/news/press-release/cern/lhc-research-programme-gets-underway">at an energy higher than previously achieved at a particle accelerator</a>, researchers have been using it to try to hunt down an equally exciting particle: the hypothetical particle that may make up an invisible form of matter called <a href="/science/physics/dark-matter">dark matter</a>, which is five times more prevalent than ordinary matter and without which there would be no universe as we know it. The LHC dark-matter searches have so far come up empty handed, as have non-collider searches, but the incredible work and skill put by the LHC researchers into finding it has led them to narrow down many of the regions where the particle may lie hidden – necessary milestones on the path to a discovery.</p> <blockquote>“Before the LHC, the space of possibilities for dark matter was much wider than it is today,” says dark-matter theorist Tim Tait of UC Irvine and theory co-convener of the LHC Dark Matter Working Group.</blockquote> <p>“The LHC has really broken new ground in the search for dark matter in the form of weakly interacting massive particles, by covering a wide array of potential signals predicted by either production of dark matter, or production of the particles mediating its interactions with ordinary matter. All of the observed results have been consistent with models that don’t include dark matter, and give us important information as to what kinds of particles can no longer explain it. The results have both pointed experimentalists in new directions for how to search for dark matter, and prompted theorists to rethink existing ideas for what dark matter could be – and in some cases to come up with new ones.”</p> <figure role="group" class="align-center"> <img alt="Simulation of the dark-matter distribution in the universe" data-entity-type="file" data-entity-uuid="ef8a396d-bd4a-4e23-a7b9-ac64d0f5f76f" src="/sites/default/files/inline-images/abelchio/seqD_063%20copy%202_2.jpg" width="1200" height="900" loading="lazy" /> <figcaption>Simulation of the dark-matter distribution in the universe. (V. Springel et al. 2005)</figcaption> </figure> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="">Make it, break it and shake it</h2> <p>To look for dark matter, experiments essentially “make it, break it or shake it”. The LHC has been trying to make it by colliding beams of protons. Some experiments are using telescopes in space and on the ground to look for indirect signals of dark-matter particles as they collide and break themselves out in space. Others still are chasing these elusive particles directly by searching for the kicks, or “shakes”, they give to atomic nuclei in underground detectors.</p> <p>The make-it approach is complementary to the break-it and shake-it experiments, and if the LHC detects a potential dark-matter particle, it will require confirmation from the other experiments to prove that it is indeed a dark-matter particle. By contrast, if the direct and indirect experiments detect a signal from a dark-matter particle interaction, experiments at the LHC could be designed to study the details of such an interaction.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2>Missing-momentum signal and bump hunting</h2> <figure class="cds-image breakout-left" id="ATLAS-PHOTO-2019-012-1"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2019-012-1" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2019-012-1/file?size=large" /></a> <figcaption>An ATLAS detector event with missing transverse momentum. A photon with transverse momentum of 265 GeV (yellow bar) is balanced by 268 GeV of missing transverse momentum (red dashed line on the opposite side of the detector). <span> (Image: ATLAS/CERN)</span></figcaption></figure> <p>So how has the LHC been looking for signs of dark-matter production in proton collisions? The main signature of the presence of a dark-matter particle in such collisions is the so-called missing transverse momentum. To look for this signature, researchers add up the momenta of the particles that the LHC detectors can see – more precisely the momenta at right angles to the colliding beams of protons – and identify any missing momentum needed to reach the total momentum before the collision. The total momentum should be zero because the protons travel along the direction of the beams before they collide. But if the total momentum after the collision is not zero, the missing momentum needed to make it zero could have been carried away by an undetected dark-matter particle.</p> <p>Missing momentum is the basis for two main types of search at the LHC. One type is guided by so-called complete new physics models, such as supersymmetry (SUSY) models. In SUSY models, the known particles described by the <a href="/science/physics/standard-model">Standard Model</a> of particle physics have a supersymmetric partner particle with a quantum property called spin that differs from that of its counterpart by half of a unit. In addition, in many SUSY models, the lightest supersymmetric particle is a weakly interacting massive particle (WIMP). WIMPs are one of the most captivating candidates for a dark-matter particle because they could generate the current abundance of dark matter in the cosmos. Searches targeting SUSY WIMPs look for missing momentum from a pair of dark-matter particles plus a spray, or “jet”, of particles and/or particles called leptons.</p> <p>Another type of search involving the missing-momentum signature is guided by simplified models that include a WIMP-like dark-matter particle and a mediator particle that would interact with the known ordinary particles. The mediator can be either a known particle, such as the Z boson or the Higgs boson, or an unknown particle. These models have gained significant traction in recent years because they are very simple yet general in nature (complete models are specific and thus narrower in scope) and they can be used as benchmarks for comparisons between results from the LHC and from non-collider dark-matter experiments. In addition to missing momentum from a pair of dark-matter particles, this second type of search looks for at least one highly energetic object such as a jet of particles or a photon.</p> <p>In the context of simplified models, there’s an alternative to missing-momentum searches, which is to look not for the dark-matter particle but for the mediator particle through its transformation, or “decay”, into ordinary particles. This approach looks for a bump over a smooth background of events in the collision data, such as a bump in the mass distribution of events with two jets or two leptons.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="">Narrowing down the WIMP territory</h2> <p>What results have the LHC experiments achieved from these WIMP searches? The short answer is that they haven’t yet found signs of WIMP dark matter. The longer answer is that they have ruled out large chunks of the theoretical WIMP territory and put strong limits on the allowed values of the properties of both the dark-matter particle and the mediator particle, such as their masses and interaction strengths with other particles. Summarising the results from the LHC experiments, ATLAS experiment collaboration member Caterina Doglioni says “We have completed a large number of dedicated searches for invisible particles and visible particles that would occur in processes involving dark matter, and we have interpreted the results of these searches in terms of many different WIMP dark-matter scenarios, from simplified models to SUSY models. This work benefitted from the collaboration between experimentalists and theorists, for example on discussion platforms such as the <a href="https://lpcc.web.cern.ch/content/lhc-dm-wg-dark-matter-searches-lhc">LHC Dark Matter Working Group</a>, which includes theorists and representatives from the ATLAS, CMS and LHCb collaborations. Placing the LHC results in the context of the global WIMP search that includes direct- and indirect-detection experiments has also been a focus of discussion in the dark-matter community, and the discussion continues to date on how to best exploit synergies between different experiments that have the same scientific goal of finding dark matter.”</p> <p>Giving a specific example of a result obtained with data from the ATLAS experiment, Priscilla Pani, ATLAS experiment co-convener of the LHC Dark Matter Working Group, highlights how the collaboration has recently searched the full LHC dataset from the machine’s second run (Run 2), collected between 2015 and 2018, to <a href="https://atlas.cern/updates/physics-briefing/probing-dark-matter-higgs-boson">look for instances in which the Higgs boson might decay into dark-matter particles</a>. “We found no instances of this decay but we were able to set the strongest limits to date on the likelihood that it occurs,” says Pani.</p> <p>Phil Harris, CMS experiment co-convener of the LHC Dark Matter Working Group, highlights searches for a dark-matter mediator decaying into two jets, such as a <a href="https://cms.cern/news/new-ways-approach-most-classical-search-new-particles">recent CMS search</a> based on Run 2 data.</p> <blockquote>“These so-called dijet searches are very powerful because they can probe a large range of mediator masses and interaction strengths,” says Harris.</blockquote> <p>Xabier Cid Vidal, LHCb experiment co-convener of the LHC Dark Matter Working Group, in turn notes how data from Run 1 and Run 2 on the decay of a particle known as the B<sub>s</sub> meson has allowed the LHCb collaboration to place <a href="https://lhcb-public.web.cern.ch/Welcome.html#CMSSM">strong limits on SUSY models</a> that include WIMPs. “The decay of the B<sub>s</sub> meson into two muons is very sensitive to SUSY particles, such as SUSY WIMPs, because the frequency with which the decay occurs can be very different from that predicted by the Standard Model if SUSY particles, even if their masses are too high to be directly detected at the LHC, interfere with the decay,” says Cid Vidal.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="">Casting a wider net</h2> <p>“10 years ago, experiments (at the LHC and beyond) were searching for dark-matter particles with masses above the proton mass (1 GeV) and below a few TeV.  That is, they were targeting classical WIMPs such as those predicted by SUSY. Fast forward 10 years and dark-matter experiments are now searching for WIMP-like particles with masses as low as around 1 MeV and as high as 100 TeV,” says Tait. “And the null results from searches, such as at the LHC, have inspired many other possible explanations for the nature of dark matter, from fuzzy dark matter made of particles with masses as low as 10<sup>−22</sup> eV to primordial black holes with masses equivalent to several suns. In light of this, the dark-matter community has begun to cast a wider net to explore a larger landscape of possibilities.”</p> <figure role="group" class="align-center"> <img alt="Diagram showing the possible explanations for the nature of dark matter" data-entity-type="file" data-entity-uuid="b68a8057-6856-4853-bde7-8fee49f78395" height="“auto”" src="/sites/default/files/inline-images/abelchio/Map-1_Plan%20de%20travail%201.png" width="3792" loading="lazy" /> <figcaption>The possible explanations for the nature of dark matter. (Image: G. Bertone and T. M. P. Tait)</figcaption> </figure> <p>On the collider front, the LHC researchers have begun to investigate some of these new possibilities. For example, they have started looking at the hypothesis that dark matter is part of a larger dark sector with several new types of dark particles. These dark-sector particles could include a dark-matter equivalent of the photon, the dark photon, which would interact with the other dark-sector particles as well as the known particles, and long-lived particles, which are also predicted by SUSY models.</p> <blockquote>“Dark-sector scenarios provide a new set of experimental signatures, and this is a new playground for LHC physicists,” says Doglioni.</blockquote> <p>“We are now expanding upon the experimental methods that we are familiar with, so we can try to catch rare and unusual signals buried in large backgrounds. Moreover, many other current and planned experiments are also targeting dark sectors and particles interacting more feebly than WIMPs. Some of these experiments, such as the <a href="/news/news/experiments/faser-cern-approves-new-experiment-look-long-lived-exotic-particles">newly approved FASER experiment</a>, are sharing knowledge, technology and even accelerator complex with the main LHC experiments, and they will complement the reach of LHC searches for non-WIMP dark matter, as shown by the <a href="http://pbc.web.cern.ch/">CERN Physics Beyond Colliders initiative</a>.”</p> <p>Finally, the LHC researchers are still working on data from Run 2, and the data gathered so far, from Run 1 and Run 2, is only about 5% of the total that the experiments will record. Given this, as well as the immense knowledge gained from the many LHC analyses thus far conducted, there’s perhaps a fighting chance that the LHC will discover a dark-matter particle in the next 10 years. “It’s the fact we haven’t found it yet and the possibility that we may find it in the not-so-distant future that keeps me excited about my job,” says Harris. “The last 10 years have shown us that dark matter might be different from what we had initially thought, but that doesn’t mean it is not there for us to find,” says Cid Vidal.</p> <blockquote>“We will leave no stone unturned, no matter how big or small and how long it will take us,” says Pani.</blockquote> <p><strong>Further reading:</strong><br /> <a href="https://www.nature.com/articles/s41586-018-0542-z">A new era in the search for dark matter</a><br /> <a href="https://atlas.cern/updates/atlas-feature/dark-matter">Searching for Dark Matter with the ATLAS detector</a></p> <p>Hero header image: NASA, ESA, H.Teplitz and M.Rafelski (IPAC/Caltech), 
A. Koekemoer (STScI), R. Windhorst (ASU), Z. Levay (STScI)</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <hr /> <p><em>Don't miss the next articles of our series, which will cover the Standard Model, the early universe and more.</em></p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Our fourth story in the LHC Physics at Ten series discusses the LHC’s hunt for the hypothetical particle that may make up dark matter </div> </div> </div> Thu, 23 Jul 2020 09:45:26 +0000 abelchio 155441 at https://home.cern The Higgs boson: Revealing nature’s secrets https://home.cern/news/series/lhc-physics-ten/higgs-boson-revealing-natures-secrets <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--4" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>The Higgs boson: Revealing nature’s secrets</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/achintya-rao"><span class="cern-tag">Achintya Rao</span></a></p> <p class="text-align-center">4 JULY, 2020 · <i>Voir en <a href="/fr/news/series/lhc-physics-ten/higgs-boson-revealing-natures-secrets">français</a></i></p> <hr /> <p class="text-align-center">Our third story in the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series takes us on a deeper dive into the Higgs boson</p> </span> </div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="1102948" data-filename="oreach-2008-001" id="ATL-PHO-OREACH-2008-001-7"> <a href="//cds.cern.ch/images/ATL-PHO-OREACH-2008-001-7" title="View on CDS"> <img alt="Simulated production of a Higgs event in ATLAS" src="//cds.cern.ch/images/ATL-PHO-OREACH-2008-001-7/file?size=large"/> </a> <figcaption> <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <blockquote class="column-left image-align"> <p style="font-size: 1.3em !important;">“For me, it’s an incredible thing that it happened in my lifetime!”</p> </blockquote> <p class="column-left image-align"><strong>Peter Higgs</strong> was at a loss for words. The <a href="/science/experiments/cms">CMS</a> and <a href="/science/experiments/atlas">ATLAS</a> collaborations had just announced the discovery of a new, <a href="/science/physics/higgs-boson">Higgs-boson</a>-like particle at the Large Hadron Collider.</p> <figure class="cds-image align-right" id="CERN-HI-1207136-101"><a href="//cds.cern.ch/images/CERN-HI-1207136-101" title="View on CDS"><img alt="higgsjuly4,seminar,Milestones,Higgs Boson Discovery,360" src="//cds.cern.ch/images/CERN-HI-1207136-101/file?size=large" /></a> <figcaption>4 July 2012: François Englert (left) listens as Peter Higgs speaks, after ATLAS and CMS announce their discovery (Image: Maximilien Brice/CERN)</figcaption></figure> <p class="column-left image-align">It had been 48 years since the publication of his paper that first predicted the existence of the particle that bears his name, not long after Robert Brout and François Englert proposed a new mechanism that would give mass to elementary bosons. More than 30 years had elapsed <a href="/news/series/lhc-physics-ten/lhc-physics-ten-entering-uncharted-waters">since the LHC was first conceived</a> and around 20 years since the ATLAS and CMS collaborations were formed. After those long years filled with anticipation, it only took the Swedish Academy of Sciences a little over one year to award <strong>Englert and Higgs the 2013 Nobel Prize in Physics</strong>.</p> <p class="column-left image-align">For Peter Higgs, the discovery of the Higgs boson was the end of a remarkable journey. For particle physics, it was the <strong>beginning of a new one</strong>.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation effect_background_parallax is_half_height"> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2682635" data-filename="VHccmumu_plain" id="CMS-PHO-EVENTS-2019-006-12"> <a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2019-006-12" title="View on CDS"> <img alt="Displays of candidate VHcc events" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2019-006-12/file?size=large"/> </a> <figcaption> <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h1 id="higgs-like-higgs-ish-higgs-y">Higgs-like? Higgs-ish? Higgs-y?</h1> <p class="column-left image-align">“When you find something new, you have to understand exactly what it is that you have found,” remarks <strong>Giacinto Piacquadio</strong>, one of the conveners of the ATLAS collaboration’s Higgs group</p> <p class="column-left image-align">This understanding is built up gradually over time. Back in July 2012, physicists were cautious about calling the new particle <em>a</em> Higgs boson, let alone <em>the</em> Higgs boson predicted by the <strong><a href="/science/physics/standard-model">Standard Model of particle physics</a></strong>. And with good reason: while the simplest theoretical formulations required there to be only one kind of Higgs boson, some extensions of the Standard Model proposed that there could be as many as five kinds of bosons that are involved in the mass-giving mechanism. So for the first few months after the discovery, it was referred to as <strong>Higgs-<em>like</em></strong>, shorthand for “a particle that seems to behave like the Higgs boson predicted by the Standard Model but we need more data to be sure”.</p> <p class="column-left image-align">The identification of two quantum-mechanical properties of the particle – quantum spin and parity – gave credence to the Standard-Model interpretation. <strong>Spin</strong> is the intrinsic spatial orientation of quantum particles, and <strong>parity</strong> refers to whether the properties of the particle remain the same when some of its spatial coordinates are flipped, like comparing the particle with a hypothetical mirror image. In the Standard Model, the Higgs boson has no spin (“0”) and “even” parity. At the time of the discovery, the fact that the Higgs boson transformed into photons meant that – unlike all other elementary bosons we know – its spin could not be 1: photons have a quantum spin of 1 themselves, so a particle transforming into two photons would have a spin of 0 (with the two spins of the photon cancelling out) or 2 (if the two spins add up).</p> <figure class="align-right"><a href="//cds.cern.ch/record/1559925/files/figure_4a.png" title="View on CDS"><img alt="ATLAS Higgs spin/parity plot" src="//cds.cern.ch/record/1559925/files/figure_4a.png" /></a> <figcaption>Differences between the positive- and negative-parity theoretical scenarios (solid and dashed lines respectively) for a particle with spin 0. The data do not show evidence for the negative-parity scenario (Image: ATLAS/CERN)</figcaption></figure> <p class="column-left image-align">In science, you can never know something with 100% certainty, but you can rule out things that are not likely. Because spin-2 particles or parity-odd particles with spin 0 would leave subtly different signatures in the ATLAS and CMS detectors than the spin-0-parity-even particle they were looking for, the scientists were eventually able to rule out these more exotic possibilities by examining many more collision events and finding no evidence to support them. “We had to analyse two-and-a-half-times more data to drop the ‘-like’,” Piacquadio adds. By March 2013, scientists were confident calling the particle <strong><em>a</em> Higgs boson</strong>.</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h1 class="column-right image-align" id="the-goldilocks-zone">The Goldilocks zone</h1> <p class="column-right image-align">The Higgs boson was the last missing piece in the Standard Model. Crucially, its mass would determine how it could be observed. At <strong>125 gigaelectronvolts (GeV)</strong>, it turned out to be <em>just right</em> for studying the particle at the Large Hadron Collider.</p> <p class="column-right image-align">We can never directly see a Higgs boson. Like most kinds of particle in nature, it is unstable and – immediately after being produced – transforms into lighter particles through a process known as particle decay. The ATLAS and CMS detectors can therefore see only the remnants of transformations, signatures that a Higgs boson might have been produced in the LHC’s collisions. Further, the downstream remnants of a Higgs transformation hold clues for how the particle was produced in the first place.</p> <p class="column-right image-align">The Higgs boson’s mass was not predicted precisely by the Standard Model, but theorists knew that the processes that produced it and the kinds of particles it transformed into would depend on how heavy the boson actually was. They had prepared elaborate plots calculating the various probabilities for a Higgs boson of a given mass to transform into particular pairs of particles. According to these so-called “<strong>branching fractions</strong>”, a light Higgs boson of around 125 GeV would have the largest variety of transformation candidates that ATLAS and CMS could detect: pairs of W bosons, Z bosons, photons, bottom quarks, tau leptons and many others. The greater the variety of observable particles the Higgs can transform into, the greater the ability of scientists to study the interplay between these particles and the Higgs boson.</p> <figure class="align-left"><a href="//cds.cern.ch/record/1546765/files/dYRHXS2_BR_Fig1.png" title="View on CDS"><img alt="Higgs branching fraction" src="//cds.cern.ch/record/1546765/files/dYRHXS2_BR_Fig1.png" /></a> <figcaption>The rates at which a Higgs boson could undergo certain transformations (vertical axis) depending on its mass (horizontal axis) (Image: CERN)</figcaption></figure> <p class="column-right image-align">Although the Higgs field was conceived to explain the masses of the W and Z bosons, scientists realised that it could help account for the masses of the fermions, namely the particles of matter. If, due to its mass, they could only observe the interplay between the Higgs boson on one hand and the W and Z bosons on the other, the puzzle of the fermion masses would remain unsolved. Discovering the particle at a convenient mass was an unexpected kindness from nature. If it were slightly more massive, above 180 GeV or so, the options to study it at the time of its discovery would have been more limited.</p> <p class="column-right image-align">The variety of available transformation products means that data from the individual channels can be combined together through sophisticated techniques to build up a greater understanding of the particle. “Doing so is not trivial,” says <strong>Giovanni Petrucciani</strong>, co-convener of the Higgs analysis group in CMS. “You have to treat the uncertainties similarly across all the individual analyses and interpret the results carefully, once you have applied complicated statistical machinery.” <strong>Combining data</strong> from the transformation of the Higgs boson to pairs of Z bosons and pairs of photons allowed ATLAS and CMS to discover the Higgs boson in 2012.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation effect_background_parallax is_half_height"> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2666332" data-filename="_DSC5418" id="OPEN-PHO-MISC-2019-001-1"> <a href="//cds.cern.ch/images/OPEN-PHO-MISC-2019-001-1" title="View on CDS"> <img alt="Photograph corresponding to CERN courier article: Inspired by software (2019MarApr)" src="//cds.cern.ch/images/OPEN-PHO-MISC-2019-001-1/file?size=large"/> </a> <figcaption> Photograph featured in the CERN courier article for issue 2019MarApr. Contains an image of ATLAS Higgs event, accompanied with a piece of event selection code of an CMS analysis reimplemented by theorists in open code CheckMATE. <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h1 id="generation-gaps">Generation gaps</h1> <p class="column-left image-align">The LHC started operations at a collision energy of 7 teraelectronvolts (TeV) before ramping up to 8 TeV over the course of its first run (2010–2013). The data collected over this period not only led to the discovery of the Higgs boson but showed the relationship (“coupling”) <strong>between the Higgs boson and elementary bosons</strong>: it was observed transforming into pairs of Ws, Zs and photons. And, while transformations to gluons are impossible to observe, the scientists could probe this coupling through the Higgs production itself: the most abundant way for a Higgs to be created in proton–proton interactions is for two gluons – one from each proton – to fuse together, accounting for nearly 90% of Higgs bosons produced at the LHC.</p> <figure class="cds-image align-right" id="CMS-PHO-EVENTS-2013-003-1"><a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2013-003-1" title="View on CDS"><img alt="Real Events,Higgs,collision,event display,CMS event display,Higgs event display,Higgs boson event display,Higgsgammagamma,gammagamma" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2013-003-1/file?size=large" /></a> <figcaption>A candidate for a Higgs boson transforming into two photons (Image: CMS/CERN)</figcaption></figure> <p class="column-left image-align">The next challenge was to observe the <strong>coupling to fermions</strong>, to cement the role of the Higgs field as the origin of mass of all elementary massive particles. These couplings had been probed indirectly: the Standard Model tells us that the gluon-fusion production mechanism and the Higgs transformation to photon pairs require the creation and annihilation of “virtual” top–antitop pairs. However, a direct observation of Higgs couplings to fermions was lacking.</p> <p class="column-left image-align">Curiously, both kinds of fermions – quarks, which make up compound particles like protons, and leptons, like the familiar electron – come in <strong>three generations of particles, each heavier than the previous</strong>. And unlike bosons, whose coupling strengths to the Higgs are proportional to their masses, the Higgs-coupling strengths of fermions is proportional to the square of their masses.</p> <p class="column-left image-align">The third generation of fermions – the heaviest – are therefore the most likely particles to manifest in processes involving the Higgs boson. “The connection between the Higgs and the top quark in particular is very exciting to look into,” remarks <strong>María Cepeda</strong>, Petrucciani’s fellow convener on CMS. Despite their relative abundance in such processes, these particles are challenging to identify. Since quarks cannot exist freely, two bottom quarks (a quark and an antiquark) emerging from a Higgs transformation rapidly combine with other quarks pulled out of the vacuum and form jets of particles. The experimentalists have to then tag jets of particles that carry the signature of a bottom quark, in order to isolate the signal. The top quark on the other hand is heavier than the Higgs and so a Higgs can never be observed transforming into two top quarks. Scientists have to therefore measure its coupling with the Higgs by looking for collision events in which a Higgs boson is produced in association with two top quarks. The second run of the LHC (2015–2018) was at an energy of 13 TeV and the large data volume collected allowed ATLAS and CMS to observe the interplay between the Higgs boson and the bottom quark, the top quark and the tau lepton.</p> <figure class="cds-image align-right" id="ATLAS-PHOTO-2018-022-7"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2018-022-7" title="View on CDS"><img alt="Higgs Candidates,Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2018-022-7/file?size=large" /></a> <figcaption>A candidate for a Higgs boson transforming into a b-quark and a b-antiquark (Image: ATLAS/CERN)</figcaption></figure> <p class="column-left image-align">Couplings to the second generation of fermions are much weaker and neither ATLAS nor CMS have so far observed Higgs transformations into charm quarks, strange quarks or muons. The next run of the LHC (2021 onwards) is expected to provide enough data to begin to shed light on some of these interactions. “The LHC’s instantaneous luminosity – <strong>the rate at which it collides protons</strong> – has increased dramatically over its first two runs,” notes Piacquadio with excitement. “This means that the number of Higgs bosons produced by the LHC continues to rise, as do the odds that we observe them undergoing rarer transformations.”</p> <p class="column-left image-align">But for the second generation of fermions, the LHC’s data volume over its whole operational life may not be enough to breach the 5σ statistical threshold to claim a Higgs transformation to all these particles. Although the High-Luminosity LHC, which will be the collider’s incarnation from 2026, is expected to allow ATLAS and CMS to see the Higgs transforming into pairs of muons, transformations to second-generation quarks will probably remain out of reach.</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h1 id="more-data-more-precision">More data, more precision</h1> <p class="column-left image-align">The Higgs boson holds the key to our understanding of nature beyond what is shown by the Standard Model.</p> <p class="column-left image-align">ATLAS and CMS are, for example, looking for so-called “invisible decays” of the Higgs boson, in which it transforms into particles that the detectors cannot observe. These invisible particles might be manifestations of <strong>dark matter</strong>. And measurements of couplings that deviate from the theoretical predictions could provide an alternative explanation for the masses of the different generations of fermions, explaining why they exist in distinct generations to start with and possibly hinting at the existence of other Higgs bosons.</p> <p class="column-left image-align">Yet, the Brout-Englert-Higgs mechanism remains among the least-understood phenomena in the Standard Model. Indeed, while scientists have dropped the “-like” suffix and have understood the Higgs boson remarkably since its discovery, they still do not know if what was observed is the Higgs boson predicted by the Standard Model. Couplings to the second-generation fermions remain elusive and the couplings that <em>have</em> been observed are known with an uncertainty of 10 to 20%, expected to reduce to the 2–4% range with the <strong>High-Luminosity LHC</strong>. Observation of as-yet-unseen phenomena and precision measurements of those that have been seen may require data volumes far greater than the LHC can provide over its lifetime.</p> <p class="column-left image-align">The global particle-physics community is therefore keen on building a “Higgs factory”, a dedicated accelerator with a focus on producing Higgs bosons in unimaginably large quantities, to allow the continued exploration of this strange particle. A high-energy Higgs factory would also enable scientists to produce two Higgs bosons at a time, to address the question of the so-called “Higgs self-interaction”, the process through which the Higgs boson itself gains mass.</p> <p class="column-left image-align">Since its discovery nearly eight years ago, ATLAS and CMS have published hundreds of papers on the Higgs boson and our understanding of the particle has grown incrementally but greatly. Today, we know with great precision what its mass is, what its most abundant transformation channels are and how it is produced in the first place. But a lot remains unknown, about both the Higgs boson and the quantum world in general.</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <hr /> <p><em>The Higgs may be the most important discovery of the LHC so far, but there is much still to learn from this remarkable machine. Our next story in this series will take a look at searches for dark matter at the Large Hadron Collider.</em></p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Our third story in the LHC Physics at Ten series takes us on a deeper dive into the Higgs boson </div> </div> </div> Mon, 08 Jun 2020 17:55:32 +0000 achintya 155006 at https://home.cern The Higgs boson: What makes it special? https://home.cern/news/series/lhc-physics-ten/higgs-boson-what-makes-it-special <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--18" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>The Higgs boson: What makes it special?</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/achintya-rao"><span class="cern-tag">Achintya Rao</span></a></p> <p class="text-align-center">4 MAY, 2020 · <em>Voir en <a href="/fr/news/series/lhc-physics-ten/higgs-boson-what-makes-it-special">français</a></em></p> <hr /> <p class="text-align-center">Our second story in the <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series visits the LHC’s most important discovery so far</p> </span> </div> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2642472" data-filename="Hbb_v2" id="CMS-PHO-EVENTS-2018-008-1"> <a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2018-008-1" title="View on CDS"> <img alt="Display of a event observed in the CMS detector in which a Higgs boson decays to bottom quarks" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2018-008-1/file?size=large"/> </a> <figcaption> <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <blockquote class="column-left image-breakout"> <p style="font-size: 1.3em !important;">As a layman I would now say… I think we have it.</p> </blockquote> <p class="column-left image-breakout">“<em>It</em>” was the <a href="/science/physics/higgs-boson">Higgs boson</a>, the almost-mythical entity that had put particle physics in the global spotlight, and the man proclaiming to be a mere layman was none other than <strong>CERN’s Director-General</strong>, Rolf Heuer. Heuer spoke in the Laboratory’s main auditorium on <strong>4 July 2012</strong>, moments after <a href="/news/press-release/cern/cern-experiments-observe-particle-consistent-long-sought-higgs-boson">the CMS and ATLAS collaborations at the Large Hadron Collider announced the discovery of a new elementary particle</a>, which we now know is a Higgs boson. Applause reverberated in Geneva from as far away as Melbourne, Australia, where delegates of the International Conference on High Energy Physics were connected via video-conference.</p> <figure class="cds-image breakout-right" id="CERN-HI-1207136-57"><a href="//cds.cern.ch/images/CERN-HI-1207136-57" title="View on CDS"><img alt="higgsjuly4,seminar,Milestones,Higgs Boson Discovery,360" src="//cds.cern.ch/images/CERN-HI-1207136-57/file?size=large" /></a> <figcaption>4 July 2012: A packed auditorium at CERN listens keenly to the announcement from CMS and ATLAS (Image: Maximilien Brice/CERN)</figcaption></figure> <p class="column-left image-breakout">So what exactly is so special about this particle?</p> <p class="column-left image-breakout">“<strong>Easy!</strong> It is the first and only elementary scalar particle we have observed,” grins <strong>Rebeca Gonzalez Suarez</strong>, who, as a doctoral student, was involved in the CMS search for the Higgs boson. Easy for a physicist, perhaps…</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation effect_background_parallax is_half_height"> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="2312440" data-filename="ATLASHiggsbb" id="ATLAS-PHOTO-2018-010-1"> <a href="//cds.cern.ch/images/ATLAS-PHOTO-2018-010-1" title="View on CDS"> <img alt="ATLAS Event Display: Higgs boson decaying to a b-quark pair" src="//cds.cern.ch/images/ATLAS-PHOTO-2018-010-1/file?size=large"/> </a> <figcaption> <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="elegance-and-symmetries">Elegance and symmetries</h2> <p class="column-left image-breakout">At the subatomic scale, the universe is a <strong>complex choreography of elementary particles</strong> interacting with one another through fundamental forces, which can be explained using a term that physicists of all persuasions turn to: <em>elegance</em>.</p> <p class="column-left image-breakout">“In the 1960s, theoretical physicists were working on an <strong>elegant way</strong> of describing the fundamental laws of nature in terms of quantum field theory,” says <strong>Pier Monni</strong>, of CERN’s Theory department. In quantum field theory, both matter particles (<em>fermions</em> such as electrons, or the quarks inside protons) and the force carriers (<em>bosons</em> such as the photon, or the gluons that bind quarks) are manifestations of underlying, fundamental quantum fields. Today we call this elegant description the <a href="/science/physics/standard-model"><strong>Standard Model of particle physics</strong></a>.</p> <figure class="cds-image breakout-right" id="CERN-GE-0710029-07"><a href="//cds.cern.ch/images/CERN-GE-0710029-07" title="View on CDS"><img alt="ALEPH,John Ellis,Lagrangian,Higgs,Penguin,diagram" src="//cds.cern.ch/images/CERN-GE-0710029-07/file?size=large" /></a> <figcaption>The Standard Model of particle physics represented in a single equation<span> (Image: CERN)</span></figcaption></figure> <p class="column-left image-breakout">The Standard Model is based on the notion of symmetries in nature, that the physical properties they describe remain unchanged under some transformation, such as a rotation in space. Using this notion, physicists can provide a unified set of equations for both electromagnetism (electricity, magnetism, light) and the weak nuclear force (radioactivity). The force which is thus unified is dubbed the electroweak force.</p> <p class="column-left image-breakout">But these very symmetries presented a glaring problem: “The symmetries explained the electroweak force but in order to keep the symmetries valid, they forbid its force-carrying particles from having mass,” explains <strong>Fabio Cerutti</strong>, who co-led Higgs groups at ATLAS on two separate occasions. “The photon, which carries electromagnetism, we knew was massless; the <strong><a href="/science/physics/w-boson-sunshine-and-stardust">W</a> and <a href="/science/physics/z-boson">Z</a> bosons</strong>, carriers of the weak force, could not be.” Although the W and Z had not been directly observed at the time, physicists knew that if they <em>were</em> to have no mass, processes such as beta decay would have occurred at infinite rates – a physical impossibility – while other processes would have probabilities greater than one at high energies.</p> <p class="column-left image-breakout">In 1964, two papers – one by <strong>Robert Brout and François Englert</strong>, the other by <strong>Peter Higgs</strong> – purported to have a solution: a new mechanism that would break the electroweak symmetry. The Brout-Englert-Higgs mechanism introduced a new quantum field that today we call the Higgs field, whose quantum manifestation is the Higgs boson. Only particles that interact with the Higgs field acquire mass. “It is exactly this mechanism,” Cerutti adds, “that creates all the complexity of the Standard Model.”</p> <p class="column-left image-breakout">Originally conceived to explain the masses of the W and Z bosons only, scientists soon found they could extend the Brout-Englert-Higgs mechanism to account for the mass of all massive elementary particles. “To accommodate the mass of the W and Z bosons, we don’t need the same Higgs field to give mass to any other particles such as electrons or quarks,” remarks <strong>Kerstin Tackmann</strong>, a co-convener of the Higgs group on ATLAS. “But it is a convenient way to do so!”</p> <p class="column-left image-breakout">The mathematical puzzle had been solved decades ago but whether the maths described physical reality remained to be tested.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation effect_background_parallax is_half_height"> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="1606851" data-filename="Higgsfield" id="OPEN-PHO-ACCEL-2013-052-1"> <a href="//cds.cern.ch/images/OPEN-PHO-ACCEL-2013-052-1" title="View on CDS"> <img alt="Artistic view of the Higgs Field" src="//cds.cern.ch/images/OPEN-PHO-ACCEL-2013-052-1/file?size=large"/> </a> <figcaption> Artistic view of the Brout-Englert-Higgs Field <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="something-in-nothing">Something in nothing</h2> <p>The Higgs field is peculiar in two particular ways.</p> <p>Imagine an empty region of space, a perfect vacuum, without any matter present in it. Quantum field theory tells us that this hypothetical region is not <em>really</em> empty: particle–antiparticle pairs associated with different quantum fields pop into existence briefly before annihilating, transforming into energy. However, the “expectation value” of these fields in a vacuum is zero, implying that on average we can expect there to be no particles within the perfect vacuum. The Higgs field on the other hand has a really high vacuum expectation value. “This non-zero vacuum expectation value,” Tackmann elaborates, “means that <strong>the Higgs field is everywhere</strong>.” Its omnipresence is what allows the Higgs field to affect all known massive elementary particles in the entire universe.</p> <p>When the universe had just come into being and was extremely hot, its energy density was higher than the energy associated with the vacuum expectation value of the Higgs field. As a result, the symmetries of the Standard Model could hold, allowing particles such as the W and Z to be massless. As the universe started to cool down, the energy density dropped, until – fractions of a second after the Big Bang – it fell below that of the Higgs field. This resulted in the symmetries being broken and certain particles gained mass.</p> <p>The other property of the Higgs field is what makes it impossible to observe directly. Quantum fields, both observed and hypothesised, come in different varieties. <strong>Vector fields</strong> are like the wind: they have both magnitude and direction. Consequently, vector bosons have an intrinsic angular momentum that physicists call quantum spin. <strong>Scalar fields</strong> have only magnitude and no direction, like temperature, and scalar bosons have no quantum spin. Before 2012 we had only ever observed vector fields at the quantum level, such as the electromagnetic field.</p> <p>“You can observe a field by observing a particle interacting with it, like electrons bending in a magnetic field,” Monni explains. “Or you can observe it by producing the quantum particle associated with the field, such as a photon.” But the Higgs field, with its constant non-zero value, cannot be switched on or off like the electromagnetic field. Scientists had only one option to prove it exists: create – and observe – the Higgs boson.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation effect_background_parallax is_half_height"> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="1459462" data-filename="eemm_run195099_evt137440354_ispy_rz-annotated-2" id="CMS-PHO-EVENTS-2012-004-6"> <a href="//cds.cern.ch/images/CMS-PHO-EVENTS-2012-004-6" title="View on CDS"> <img alt="CMS Higgs Search in 2011 and 2012 data: candidate ZZ event (8 TeV) with two electrons and two muons" src="//cds.cern.ch/images/CMS-PHO-EVENTS-2012-004-6/file?size=large"/> </a> <figcaption> r-z view (vertical plane containing the beam) with labels <span> (Image: CERN)</span> </figcaption> </figure> </div> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <h2 id="bump-hunting-at-the-large-hadron-collider">Bump-hunting at the Large Hadron Collider</h2> <p class="column-left image-breakout">Particle collisions at sufficiently high energies are necessary to produce a Higgs boson, but for a long time physicists were hunting in the dark: they did not know what this energy range was.</p> <p class="column-left image-breakout">They had searched for signs of the Higgs boson in particle-collision debris at the Large Electron–Positron collider (LEP), which was the Large Hadron Collider’s direct predecessor, and at Fermilab’s Tevatron in the US. The Large Hadron Collider had the capacity to explore the entire predicted energy range where the Higgs boson could appear, and the two general-purpose particle detectors at the LHC – ATLAS and CMS – were meant to provide a definitive answer on its existence. For some, like Monni, the LHC’s calling was irresistible, leading him to switch careers from aerospace engineering to theoretical physics.</p> <p class="column-left image-breakout">Gonzalez Suarez’s colleagues and friends were in the CMS and ATLAS control rooms when <a href="/news/series/lhc-physics-ten/lhc-physics-ten-entering-uncharted-waters">the LHC embarked on its high-energy journey on <strong>30 March 2010</strong></a>. She herself was in her office at CERN’s main site in Geneva. “I was writing my doctoral thesis on one screen and looking at the live stream of the collisions on a second. I wanted to know if the code I had written to identify particles produced in the collisions worked!”</p> <p class="column-left image-align">When two protons collide within the LHC, it is their constituent quarks and gluons that interact with one another. These high-energy interactions can, through well-predicted quantum effects, produce a Higgs boson, which would immediately transform – or “decay” – into lighter particles that ATLAS and CMS could observe. The scientists therefore needed to build up enough evidence to suggest that particles that <em>could</em> have appeared from a Higgs production and transformation were indeed the result of such a process.</p> <figure class="align-right"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2012-001-2" title="View on CDS"><img src="https://cds.cern.ch/record/2230893/files/HiggsGammaGamma.gif" /></a> <figcaption>ATLAS (and CMS) observed the Higgs boson in transformations to two photons by collecting and analysing lots of data over time. (Image: ATLAS/CERN)</figcaption></figure> <p class="column-left image-align">“When the LHC programme started, popular belief was that we would only see a Higgs boson after several years of data collection,” recounts <strong>Vivek Sharma</strong>, who co-led the CMS search when the LHC began operations. Sharma and his colleagues presented a plan to CMS in September 2010 of how to tackle the problem with half that data. It required not only a thorough understanding of one’s own detector hardware, its reach and its limitations, but also a team with a variety of technical expertise. “By the time ATLAS and CMS gave a joint talk to CERN’s Scientific Policy Committee in March 2011,” Sharma continues, “<strong>there was a force building up</strong> that the Higgs boson could be hunted with even smaller datasets.”</p> <p class="column-left image-align">A routine end-of-year seminar by ATLAS and CMS in December 2011 overloaded CERN’s webcast servers, as thousands tuned in to hear the latest updates from the collaborations. Early signs of the Higgs boson were there: both detectors had seen bumps in their data that were starting to look distinct from any statistical fluctuations or noise. But the results lacked the necessary statistical certainty to claim discovery. The world had to wait nearly seven months before Joe Incandela of CMS and Fabiola Gianotti of ATLAS could do so in July 2012. The collaborations had performed better than expected to discover the Higgs boson with just two years of data from the LHC.</p> <p class="column-left image-breakout">In CERN’s auditorium, Peter Higgs wiped away tears of joy, and François Englert paid tribute to his late colleague and collaborator, Robert Brout, who did not live to see proof of the mechanism that bears his name.</p> <p class="column-left image-breakout">Gonzalez Suarez celebrated with mixed emotions. Her post-doctoral research took her away from Higgs research before the discovery, and eventually from CMS, to the ATLAS collaboration. “The discovery of the Higgs boson was a historic event, but we are still only <strong>at the beginning</strong> in our understanding of this new particle.”</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <hr /> <p><em>The road from data to discovery was challenging. But what have we learnt about the Higgs boson since then? Find out more in part two of the Higgs saga (coming soon).</em></p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Our second story in the LHC Physics at Ten series visits the LHC’s most important discovery so far </div> </div> </div> Mon, 04 May 2020 08:21:16 +0000 achintya 154643 at https://home.cern LHC Physics at Ten: Entering Uncharted Waters https://home.cern/news/series/lhc-physics-ten/lhc-physics-ten-entering-uncharted-waters <div class="layout layout__region featured-story-page-node-layout-content"> <div class="field--items"> <div class="field--item"> <div class="component-row component-row__display__fluid section-navigation component-row__has-header effect_none is_full_height"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="cern-component-header-blocks component-header"> <div id="header-blocks--20" class="owl-carousel owl-theme component-header__carousel header-carousel"> <div class="header-block"> <div class="header-block__title"> <h3 class="header-block__name" > <span>LHC Physics at Ten: Entering Uncharted Waters</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/achintya-rao"><span class="cern-tag">Achintya Rao</span></a></p> <p class="text-align-center">30 MARCH, 2020 · <i>Voir en <a href="/fr/news/series/lhc-physics-ten/lhc-physics-ten-entering-uncharted-waters">français</a></i></p> <hr /> <p class="text-align-center">We start our <a href="/news/series/lhc-physics-ten"><span class="cern-tag">LHC Physics at Ten</span></a> series with a trip down memory lane to the day when it all began</p> </span> </div> <div class="background__veil"></div> <div class="background-component background__cds_media" style="height: 100%;"> <figure class="cds-image" data-record-id="1255406" data-filename="" id="CERN-AC-1003061-112"> <a href="//cds.cern.ch/images/CERN-AC-1003061-112" title="View on CDS"> <img alt="CERN Control Centre on 30 March 2010" src="//cds.cern.ch/images/CERN-AC-1003061-112/file?size=large"/> </a> <figcaption> <span> (Image: CERN)</span> </figcaption> </figure> </div> </div> </div> <span class="component-header__scroll"></span> </div> <a class="endof-cern-header-blocks"></a> </div> </div> </div> </div> </div> <div class="field--item"> <div class="component-row component-row__display__fluidcenter section-navigation effect_none"> <div class="component-row__row"> <div class="component-row__column component-row__center section-has-no-column col-md-12 col-sm-12 col-xs-12"> <div class="box-effects-wrapper "> <div class="component-margin component-margin-small" ></div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p class="column-left image-breakout">On 30 March 2010, <strong>exactly ten years ago</strong>, a metaphorical champagne bottle was smashed across the bow of the <a href="/science/accelerators/large-hadron-collider">Large Hadron Collider</a> (and several non-metaphorical ones were popped) as CERN’s flagship accelerator embarked upon its record-breaking journey to explore strange new worlds at the high-energy frontier: <a href="/news/press-release/cern/lhc-research-programme-gets-underway">it collided protons at an energy of <strong>3.5 teraelectronvolts (TeV) per beam</strong> for the first time</a>. Since then, the largest scientific instrument ever built has enabled scientists to study a variety of physics phenomena, with its crowning achievement being the <a href="/news/press-release/cern/cern-experiments-observe-particle-consistent-long-sought-higgs-boson">discovery of the Higgs boson in 2012</a>.</p> <figure class="cds-image breakout-right" id="CERN-HOMEWEB-PHO-2010-001-1"><img alt="A screenshot of a control screen showing the LHC’s status at 13:30 on 30 March 2010. The text on top says 'Proton Physics: Stable Beams' and the image shows a graph for two proton beams at 3.5 teraelectronvolts each." src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2010-001-1/file?size=large" /><figcaption>LHC Page 1 shortly after first high-energy collisions in the accelerator (Image: CERN)</figcaption></figure> <p class="column-left image-breakout">The LHC wasn’t built just to find the Higgs boson – or prove that it didn’t exist! Over the last ten years, it has allowed scientists to test the <strong><a href="/science/physics/standard-model">Standard Model of particle physics</a></strong> with higher precision than ever before, demonstrating the theory’s robustness. In addition to the proton–proton collisions that are the LHC’s staple, scientists have used collisions of lead nuclei to recreate and examine the <strong>conditions that prevailed in the very early universe</strong>, when quarks and gluons existed freely. And the Higgs boson itself has brought entirely new perspectives to physics – an elementary particle with no intrinsic angular momentum, the first of its kind.</p> <p class="column-left image-breakout">The path to proton–proton collisions at the teraelectronvolt scale – whose story goes as far back as 1977, when such a machine was first conceived – was fraught with challenges. No hadron collider <strong>of this size and energy</strong> had been built before, and technical and scientific expertise had to be cultivated to bring it to fruition. Global collaborations were formed to design and build the detectors at each of the four collision points around the ring.</p> <p class="column-left image-breakout"><a href="/news/press-release/cern/first-beam-lhc-accelerating-science">Proton beams flew through the machine for the first time</a> on <strong>10 September 2008</strong>, but an electrical fault only nine days later put the accelerator out of action for over a year. The first low-energy collisions were achieved on 23 November 2009. A week later, the LHC <a href="/news/press-release/cern/lhc-sets-new-world-record">took over the mantle from Fermilab’s Tevatron as the world’s highest-energy collider</a>, achieving <strong>1.18 TeV in each beam</strong>. The following March, it left the shallow waters and entered uncharted territory by colliding beams at an energy of 3.5 TeV per beam. Tears of joy and relief accompanied thunderous applause in the CERN Control Centre and the experiments’ control rooms. That Tuesday, when all four of the LHC’s big detectors – <a href="/science/experiments/alice">ALICE</a>, <a href="/science/experiments/atlas">ATLAS</a>, <a href="/science/experiments/cms">CMS</a> and <a href="/science/experiments/lhcb">LHCb</a> – saw high-energy collision debris for the first time, was the culmination of <strong>over 30 years of dreams, plans and dedication</strong>. The first papers showing early results were presented days later and, within a few months, the LHC had helped “rediscover” Standard-Model particles that had originally taken decades to find.</p> <figure class="breakout-right"><div style="position: relative; padding-top: 56.25%;"><iframe allowfullscreen="" frameborder="0" src="https://www.youtube-nocookie.com/embed/cnJvpbLp4p8?rel=0&amp;hl=en&amp;cc_lang_pref=en&amp;cc_load_policy=1" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></div> <figcaption>Relive the moments leading up to the first high-energy collisions at the LHC (Credit: CERN)</figcaption></figure> <p class="column-left image-breakout">In the ten years since, we have witnessed the awesome capabilities of not only the LHC but also the detectors that collect data from the collisions. While the accelerator has performed <strong>beyond expectations</strong>, so too have these experimental apparatuses, receiving far greater collisions every instant than they had been designed for and filtering out the interesting ones for analysis. The collaborations operating them have published <strong>hundreds of scientific papers</strong> using data that are unique in every sense.</p> <p class="column-left image-breakout">The LHC’s saga, though, has just begun. The machine is expected to operate until the end of the ’30s and nearly 95% of the LHC’s promised data volume is still to come. However, the analysis of the data collected thus far – in particular phenomena associated with the Higgs boson – has already begun to show where a future accelerator should point its bow.</p> <p class="column-left image-breakout">In the coming weeks, to mark the first ten years of one of <strong>humanity’s greatest scientific endeavours</strong>, we will publish a series of features on home.cern covering the physics results that have shaped our understanding of the universe – from probing the Standard Model and the early universe, to the new vistas that the Higgs boson has opened up, to the mysteries of dark matter and more. Celebrate <strong>ten years of LHC physics</strong> with us.</p> </div> </div> <div class="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <hr class="column-left image-breakout" /> <p class="column-left image-breakout"><i>Meanwhile, the celebrations have already begun: the latest issue of the </i><strong>CERN Courier</strong><i> has several stories that might interest you. <i><a href="https://cerncourier.com/a/bang-beam-bump-boson/">Bang, beam, bump, boson</a><i> describes life at the helm of the LHC; </i><a href="https://cerncourier.com/a/a-labour-of-love/">A labour of love</a><i> focuses on the lives of the experimentalists operating the gigantic detectors; and </i><a href="https://cerncourier.com/a/lhc-at-10-the-physics-legacy/">LHC at 10: the physics legacy</a><i> provides an in-depth look at the new knowledge we have gained from theory and experiment.</i></i></i></p> <p><i><i> </i></i></p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> The Large Hadron Collider began its physics programme on this day a decade ago </div> </div> </div> Mon, 30 Mar 2020 11:43:23 +0000 achintya 153509 at https://home.cern