CERN: Feature https://home.cern/ en Higgs10: inventing the future of Higgs research https://home.cern/news/series/higgs10/higgs10-inventing-future-higgs-research <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="1996997" data-filename="TUNNEL3D V2aCOMPO smooth panoramic montage" id="OPEN-PHO-ACCEL-2015-001-1"> <a href="//cds.cern.ch/images/OPEN-PHO-ACCEL-2015-001-1" title="View on CDS"> <img alt="3D dipole integration panoramic poster" src="//cds.cern.ch/images/OPEN-PHO-ACCEL-2015-001-1/file?size=large"/> </a> <figcaption> 3D dipole integration showing several parts with an interconnection open. Integration 3D du dipole et interconnection ouverte. <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>Higgs10: inventing the future of Higgs research</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/matthew-mccullough"><span class="cern-tag">Matthew McCullough</span></a></p> <p class="text-align-center">10 August, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs10-inventing-future-higgs-research">français</a></i></p> <hr /><p class="text-align-center">In the final part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, history teaches us that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.</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="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p>In 1975, three CERN theorists, John Ellis, Mary K. Gaillard and Dimitri Nanopoulos, undertook the first comprehensive study of the collider phenomenology of the Higgs boson. Almost 40 years later, it was discovered at the LHC. Now, ten years on, might we have such long-term foresight in anticipating the varied paths that future Higgs research may follow?</p> <blockquote>On 4 July 2022, enjoying the many beautiful presentations at the Higgs@10 symposium, a phrase kept ringing in my ears: “Compatible with Standard Model (SM) predictions”. Alarm bells were ringing. Really? Are we sure? Whether or not the Higgs is SM-like is a question that will shape the experimental future of Higgs research.</blockquote> <p>We may quantify an answer through the language of effective field theory, which is a mathematical manifestation of the notion that the most effective way to describe an object depends on the length scale you’re viewing it from. To astronauts, Earth is very effectively described as a smooth sphere. For summer students hiking to Le Reculet, it is not. So, too, of the quantum world. Far from a neutral atom, it effectively appears as a point-like particle with some leftover multipolar interactions with photons. At shorter distances, getting in amongst the electrons, this description fails entirely.</p> <p>Ditto the Higgs. Whatever’s going on in there, at energies near enough to m_H, it is effectively described as a point particle with a handful of additional “operators”, which are essentially new particle interactions that aren’t contained in the SM (don’t feature on <em>that</em> mug or T-shirt) but do involve SM particles. By eye, the astronaut may be able to make out some features on Earth and surmise that there may be mountains, but they couldn’t actually estimate the students’ elevation gain. Similarly, the non-SM Higgs operators can capture the long-distance leftover effects of the microscopic innards of the Higgs, but not reveal their full glory in detail. If all of these extra operators vanish, the Higgs is SM-like. Let’s consider two hand-picked examples and investigate just how SM-like the Higgs is...</p> <h4>How “fuzzy” is it?</h4> <p>Is it point-like down to the smallest distance scales or is it, like the pion, made up of other as-yet-unidentified new particles? In the latter case, much as for the pions and their constituent quarks and gluons, directly observing the new stuff would require going to higher energies. Alternatively, it could be point-like but probing it closely may reveal the telltale clues of a cloud of new particles that it interacts with. For your interest, the operator that can capture these properties is written (∂μ|H|2)2. If it vanishes, the Higgs is entirely point-like. If not, it’s fuzzier than expected. How fuzzy is it? Present LHC Higgs coupling measurements suggest it is effectively point-like down to a length scale merely a factor three below the electroweak scale. It could still be very fuzzy indeed! As fuzzy as a pion. If so, hardly an SM-like Higgs! We must do better and, through much more precise coupling measurements at the 0.2% level, a future Higgs factory like the FCC-ee could determine if the Higgs is point-like as far down as the 6% level.</p> <h4>Does the Higgs find itself attractive?</h4> <p>Yes, according to the SM. New particles means new forces and so it follows that if the Higgs boson interacts with new heavy particles they will generate a new force between the Higgs and itself. The operator effectively capturing this is |H|6 and it literally shapes the way in which the Higgs field gave mass to particles during the very nascence of our universe! So, how SM-like is the Higgs self-attraction? With present experimental constraints, we know the Higgs self-attraction could be 530% stronger than the SM value (not merely self-attraction, more like outright vanity) or even −140% less (self-repulsive, more like). Hardly SM-like in either case! To have any idea of whether the self-attraction is SM-like, we must do a lot better. A future facility, such as the FCC-hh, CLIC or a muon collider, could probe the self-attraction at the much more precise 5% level.</p> <h4>Patience is a virtue; complacency is not</h4> <p>It is far too early to call time at the bar for the Higgs boson. Who knows, we may even be served with something completely unexpected, like a new window into the dark sector of the universe. Truly exploring all facets of the nature of the Higgs boson, understanding whether or not it is SM-like, will take time (measured in decades) and a lot of hard work. But it can and should be done. This is the experimental future of Higgs research that we look forward to.</p> <p>All that said, it’s no secret that many theorists expected the Higgs to be much less SM-like than it appears to be already. Heads duly scratched, a theoretical coup d’état is now silently under way. There were good reasons to expect something different: chiefly the hierarchy problem. This problem is not simply aesthetic. The SM breaks down at high energies, ultimately making pathological predictions, thus it can only be a long-distance effective field theory description of something else more fundamental. If, as was the case for pions, the Higgs mass is determined by the more fundamental parameters, then for the Higgs there is no mechanism to keep it lighter than the mass scale of the new particles in that theory. Yet colliders tell us there is a gap between the mass of the Higgs and that of those new particles. In the past, this motivated the discovery and development of new mechanisms to explain a light Higgs, such as the venerated low-scale supersymmetry, thus far a no-show at the LHC physics party, with its attendant non-SM-like Higgs.</p> <p>Rudely awoken by the deluge of exclusion plots, coffee reluctantly smelled, theorists have, in recent years, put forward what could well transpire to be revolutionary theoretical developments. The hierarchy problem hasn’t gone away and neither has the data, so the other foundational assumptions covertly injected into the old theories, often linked to symmetry or aesthetic principles such as simplicity or minimality, have been interrogated and found wanting. In response, intrepid new classes of theories have been developed that can address the hierarchy problem whilst being consistent with all those bothersome exclusion plots. They range from relatively modest conceptual tweaks of existing structures, to the abandonment of aesthetic principles, and then all the way out the other side to attempts to link the Higgs mass to the origins of the universe, cosmology, the nature of the Big Bang and, at an extreme, speculations about possible links between the Higgs mass and the existence of life itself. You name it, we’re boldly going.</p> <h4>It’s no <em>fait accompli</em></h4> <p>None of these ideas are as intoxicating as supersymmetry or as stupefying as extra dimensions, each leaving those who study them with more of a “watch this space” feeling than the “eureka” that Archimedes enjoyed. Variously, they’re not radical enough, too radical or simply not to taste. No Goldilocks moment just yet. However, in my view these issues are cause for hope. In similar moments in the past, we have been essentially on the right track, having to wait a little longer than expected for the confirming experimental data (top quark). At other times, the right ideas have been too radical for most to stomach in one sitting (quantum mechanics). Yet for others the correct approaches languished in relative obscurity far too long, simply for not being <em>à la mode</em> (quantum field theory). Look up the citation records of the original Brout-Englert, Higgs, Guralnik-Hagen-Kibble papers or Weinberg’s “A Model of Leptons”, all foundational to the physics of the Higgs boson, and you’ll see they are important cases in point that we would do well to remember. Nature made no promises that understanding the origins of the Higgs should have been easy, nor should it be in the future, but history teaches that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.</p> <h4>Where will all this go in coming years?</h4> <p>Will we be tenacious enough to build the accelerator, the detectors and the village it will take to measure the Higgs self-attraction or discover the fuzziness of the Higgs? Will some plucky theorists unlock the door to the fundamental theory beyond the SM? Will future phenomenologists lay the first foundational stones on the path to discovering it?</p> <p>As Dennis Gabor, the inventor of holography, put it: “The future cannot be predicted, but futures can be invented.”<br /> We’re working on it.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> History teaches that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth. </div> </div> </div> Fri, 19 Aug 2022 07:32:25 +0000 katebrad 185012 at https://home.cern Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years https://home.cern/news/series/higgs10/higgs10-ten-things-weve-learned-about-higgs-boson-past-ten-years-0 <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="2816153" data-filename="Screenshot%202022-07-19%20at%2015.31.24" id="CERN-HOMEWEB-PHO-2022-149-1"> <a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-1" title="View on CDS"> <img alt="Higgs10 Bulletin article - 7" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-1/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="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>Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years </span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/monica-dunford"><span class="cern-tag">Monica Dunford</span></a>&amp; <a href="/authors/andre-david"><span class="cern-tag">Andre David</span></a></p> <p class="text-align-center">19 July, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs10-ten-things-weve-learned-about-higgs-boson-past-ten-years-0">français</a></i></p> <hr /><p class="text-align-center">In the seventh part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, we give an overview of ten years of Higgs boson research.</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="text-component text-component-page clearfix"> <div class="text-component-text cern_full_html"> <p> </p> <p>Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today. But what have we physicists learned about the particle in the past ten years? </p> <p><strong>A scalar particle exists in nature</strong></p> <p>During the early hours of 4 July 2012, the foyer outside the main CERN lecture hall looked more like the lead-up to a rock concert than the main building of the world’s leading particle physics lab. Dozens of groggy-eyed students slowly rolled up their sleeping bags, stretching out after a long night on the hard floor. A line hundreds long snaked through the foyer, around the restaurant and out the door. The excitement in the line was pulsating – even though the odds of making it into the auditorium were small, just to be there was a thrill. We had found it. A scalar particle existed in nature and 4 July 2012 was its debut.</p> <p><strong>It’s heavy and short-lived</strong></p> <p>The first measurements of the new scalar particle, H(125), relied on two experimental channels: 4-lepton decays and 2-photon decays. Although these are not the most abundant decay channels, they are the best in determining the scalar particle’s mass. The measured mass of about 125 GeV is maximally interesting: it is much heavier than was expected for popular models of supersymmetry, it puts the universe in a precarious position between being stable and metastable, and it has a rich phenomenology. In contrast to its heavy mass, the particle’s lifetime is short; it is gone in 10<sup>-22</sup> of a second.</p> <p><strong>It has no electric charge and no spin</strong></p> <p>The discovery of the H(125) via its decay to two photons immediately established that the new particle had no electric charge and strongly disfavoured it to have spin of 1. The exact spin of the new particle can be probed by examining the angular distributions of the final-state products in decays to two protons, two W bosons and two Z bosons. The spin 0 hypothesis has held up against a myriad of other possible assignments.</p> <figure class="cds-image align-left" id="CERN-HOMEWEB-PHO-2022-149-2"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-2" title="View on CDS"><img alt="home.cern,Life at CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-2/file?size=large" /></a> <figcaption>Measurements of the interaction strength between the H(125) and some of the Standard Model particles. The red line represents the Standard Model expectation. Recent progress has increased the reach to second generations fermions, like the muon, and first results concerning charm quarks.<span> (Image: ATLAS)</span></figcaption></figure><p><strong>It interacts with other bosons</strong></p> <p>How the new boson interacts with other particles can be probed in both how it decays and how it is produced. With its discovery via decays to two photons and two Z bosons, it was readily concluded that the H(125) particle couples to bosons (in the case of photons, indirectly). This was further reaffirmed with measurements of decays to two W bosons. Furthermore, the production of the H(125) through couplings to bosons is measured when two vector bosons (force carriers such as W and Z bosons) fuse to produce the scalar or when the scalar radiates from a heavy boson (so-called V+H production).</p> <p><strong>It interacts with fermions</strong></p> <p>The Standard Model (SM) predicts that the strength of the coupling between the H(125) and other particles is proportional to their masses. Studying fermions tests these couplings over three fermion generations spanning three orders of magnitude of masses. For the heaviest fermions, all couplings have been measured – to top quarks (via measurements of ttH production), to beauty quarks and to tau leptons. Now, the experimental challenge lies in reaching the second generation, whose coupling with the Higgs boson is weaker. First evidence of decays to muons are emerging and both the ATLAS and CMS experiments are homing in on decays to charm quarks. </p> <p><strong>It could be a portal for dark matter</strong></p> <p>If dark matter consists of elementary particle(s), the SM simply does not predict any of them. If the H(125) and dark matter particles interact in nature, one possible signature is that of “invisible” Higgs boson decays. Such searches limit these decays to be lower than 15% and, consequently, set limits on interactions between this Higgs boson and possible dark matter particles and on the models that predict them. The SM predicts only a diminutive branching fraction of 0.1% – to four neutrinos. </p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-149-3"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-3" title="View on CDS"><img alt="home.cern,Life at CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-149-3/file?size=large" /></a> <figcaption>Limits on Higgs boson pair production, a process that is sensitive to the Higgs boson self-interaction and the shape of the Higgs potential. Results are presented as a function of time along with projections for the full HL-LHC dataset that should provide enough sensitivity to challenge the SM prediction (red horizontal line).<span> (Image: CMS)</span></figcaption></figure><p><strong>It may touch the structure of the universe</strong></p> <p>The inclusion of the Brout-Englert-Higgs mechanism in the SM leads to precise predictions of how the universe evolved during one of its earliest stages, the electroweak epoch. A scalar field can influence several aspects of cosmology and even play a role in the observed matter–antimatter asymmetry in the universe. Depending on the shape of the vacuum potential, the universe could be metastable and decay, and one way to probe this shape is to measure the different ways in which the H(125) interacts with itself. One of the signatures that can be used to access this self-interaction is the production of Higgs boson pairs. While existing analyses of LHC data have already started to exclude some non-SM alternatives, more data and future accelerators – like Higgs factories – will allow us to explore this critical area.</p> <p><strong>It seems to be a lone child</strong></p> <p>The SM is minimalistic as far as scalars are concerned: it predicts one single elementary scalar particle, with distinct types of interactions. In straightforward extensions to the minimal SM, more than one Higgs boson is predicted, resulting in different sets of interactions. Therefore, a vigorous programme of searches for other Higgs bosons – lighter and heavier, neutral and charged (and doubly charged) – has been undertaken. With other possibilities being strongly reduced, H(125) is presently the only scalar we know of in nature.</p> <p><strong>It’s a new player in the team pushing past the SM</strong></p> <p>This Higgs boson is the newest player joining the team of particles that we use to understand the nature of the universe. Matter–antimatter asymmetry, dark matter, unification of all forces; these are some of the questions where a coherent and precise exploration of the properties of particles like the Z and W bosons, the beauty and top quarks and now the H(125), probe energy regimes far beyond those directly accessible at colliders. One possibility is to extend the SM with generic interactions that represent the effect of particles and interactions beyond the direct reach of present colliders. Making use of all the information from H(125) and its team members in a consistent fashion may point us in the direction of the next standard model.</p> <p><strong>It’s just the beginning</strong></p> <p>While we have established several properties and interactions of the H(125), much remains to be learned about this Higgs boson. Far from just being the last prediction from the SM, the discovery of the H(125) and its singular scalar quality provides an important instrument to further our understanding of nature at its deepest. Is there really only one Higgs boson in nature? Do its properties differ from the SM predictions? Can it show us what is beyond the electroweak scale? Might it interact with dark matter particles? Will we be able to use it to measure the shape of the vacuum potential of the universe?</p> <p>Ten years ago, before the discovery of this formidable tool, these questions were beyond our reach. The H(125) has opened new doors, inviting us to walk through.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today. </div> </div> </div> Wed, 27 Jul 2022 09:18:30 +0000 thortala 184435 at https://home.cern Higgs10: When spring 2012 turned to summer https://home.cern/news/series/higgs10/higgs10-when-spring-2012-turned-summer <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="1459503" data-filename="" id="CERN-HI-1207136-02"> <a href="//cds.cern.ch/images/CERN-HI-1207136-02" title="View on CDS"> <img alt="Higgs announcement seminar on 4 July 2012" src="//cds.cern.ch/images/CERN-HI-1207136-02/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="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>Higgs10: When spring 2012 turned to summer</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/fabiola-gianotti"><span class="cern-tag">Fabiola Gianotti</span></a>&amp; <a href="/authors/joe-incandela"><span class="cern-tag">Joe Incandela</span></a></p> <p class="text-align-center">04 July, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs10-when-spring-2012-turned-summer">français</a></i></p> <hr /><p class="text-align-center">In the sixth part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, the ATLAS and CMS experiments announce the discovery on 4 July 2012</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>At the Higgs boson search update seminar on 13 December 2011, things were already looking promising. The data had allowed us to constrain the Higgs boson mass to the range from around 115 to 130 GeV, and both ATLAS and CMS had tantalising hints of a new particle around a mass of 125 GeV. Those hints were not yet sufficiently strong to claim a discovery, the local significance was between 2.6 and 3.6 sigma, but they were enough to ensure that the eyes of the world would be on CERN as the data taking resumed in spring 2012 at a larger centre-of-mass energy: 8 TeV, compared with 7 TeV in 2011.</p> <p>The year’s big high-energy physics conference, ICHEP, was to be held in Melbourne, Australia, starting on 6 July 2012. We both had our tickets booked, and an update on the Higgs boson search was a key part of the agenda. Plans were made for a two-way link to relay the Higgs boson sessions live from Melbourne to CERN’s Main Auditorium. Meanwhile, both experiments got on with data taking and analysis.</p> <p>It was around mid-June that things started to get really interesting. By then, ATLAS had been seeing an excess of events in the two-photon channel, at the same mass as the excess reported at the end of 2011 based on an independent data sample, but nothing in the rarer four-lepton channels. It was clear to us both that we needed to see a signal in the gamma-gamma and lepton channels before going to the Director-General, Rolf Heuer. In the middle of June, CMS unblinded its analysis to find a four-sigma signal in the two-photon channel, and three-sigma in the leptons. Meanwhile, ATLAS’s Higgs boson sample received its first four-lepton candidates. We went to see Rolf.</p> <p>The following weeks were incredibly intense. It was imperative that the collaborations maintained complete confidentiality, and it was impressive how well that was respected, not only outside but also within CERN. ATLAS did not know what exactly CMS had, and vice versa. The two of us would keep each other informed almost daily, but we did not disclose the other experiment’s results to our respective collaborations. Together with Rolf Heuer, we were the only ones who had the full picture of what was going on. This was essential to maintain confidentiality, but also to avoid ATLAS and CMS influencing each other, and to ensure that emotions did not affect the ongoing work. The pressure was enormous, people were working around the clock making millions of checks and cross-checks, and they had to remain calm and focused. The rest of CERN, and indeed anyone following particle physics, must have felt the energy emanating from the community, because the sense of anticipation was tangible.</p> <p>The CERN Council met during the week of 18 June and decided that, whatever ATLAS and CMS had to say about the status of the Higgs boson searches, it should be said at CERN. We rapidly changed our travel plans, and CERN announced a Higgs boson update seminar for 4 July – the latest date compatible with both of us being in Melbourne in time for the plenary sessions of ICHEP. The primary direction of the two-way link with Melbourne was reversed: those arriving early for the conference would now follow the seminar at CERN remotely. The Council’s decision was taken as a sign that we had an announcement to make but, at that point, we were not telling anyone what we’d be announcing. Nevertheless, eminent theorists such as Carl Hagen and Gerry Guralnik decided to attend, and we invited all the other theorists who had been involved with developing the theory back in the 60s. As a result, François Englert and Peter Higgs also joined us on the day. Two years earlier, the four had shared the APS’s Sakurai Prize along with Robert Brout and Tom Kibble, both now sadly departed, for their work on spontaneous symmetry breaking in gauge theories.</p> <figure class="cds-image" id="CERN-HOMEWEB-PHO-2012-001-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2012-001-1" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2012-001-1/file?size=large" /></a> <figcaption>Fabiola Gianotti, Rolf Heuer and Joe Incandela in a packed CERN auditorium on the day of the announcement of the discovery of the Higgs boson<span> (Image: AFP/Denis Balibouse)</span></figcaption></figure><p>It went right down to the wire. The results were still being checked and double-checked until just days before the seminar, and we were putting the final touches to our presentations until minutes before the seminar began. Walking into the auditorium, past the people rolling up their sleeping bags because they’d camped out overnight to ensure their places, we felt tremendous pressure along with great pride for what our community had managed to achieve over the decades. Then, as soon as it began, it seemed that a huge weight was lifted. The room was a sea of faces, ranging from those of people whose working lives had been devoted to building the LHC, ATLAS and CMS, to those whose careers were just beginning. Everyone was with us and, because the results were so compelling, neither of us needed our banks of back-up slides in case we were called upon to justify the details.</p> <p>It was an amazing day, seen live around the world by half a million people, and reported by the media to over a billion. The media focused heavily upon us as the spokespersons but of course we were just the messengers. This success was the culmination of a multigenerational effort spanning decades. The capability of the particle physics community to deliver beyond expectation was truly inspiring. From the original theory through phenomenology to the design and construction of the accelerator, the detectors and the computing infrastructure, the tiny signal we were able to tease out from a large background was a credit to everyone who played a part. It was the triumph of a community that was able to achieve what many would have deemed impossible, bringing together expertise from every branch of the field.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-133-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-133-1" title="View on CDS"><img alt="home.cern,Diagrams and Charts" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-133-1/file?size=large" /></a> <figcaption>Plots shown by Joe Incandela for CMS and Fabiola Gianotti for ATLAS show a clear 5 sigma discovery signal.<span> (Image: CERN)</span></figcaption></figure><p>Peter Higgs was treated like a rock star on the day. His reaction gives the measure of our field: when pressed for comment by the media, he replied that this was a day for the experiments, and there would be plenty of time to talk to the theorists later. A little over a year later, François Englert and Peter Higgs shared the Nobel Prize in physics for “the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, which recently was confirmed through the discovery of the predicted fundamental particle by the ATLAS and CMS experiments at CERN's Large Hadron Collider.”</p> <p><em>Fabiola Gianotti and Joe Incandela, Spokespersons of the ATLAS and CMS experiments at the time of the Higgs boson’s discovery.</em></p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> It was just a few short weeks in mid-2012, but they were so intense that it felt like years. As 4 July drew near, the ATLAS and CMS experiments could sense that they were homing in on something big. </div> </div> </div> Thu, 07 Jul 2022 07:41:02 +0000 thortala 183779 at https://home.cern Higgs10: Big Bang Day https://home.cern/news/series/higgs10/higgs10-big-bang-day <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="2782003" data-filename="20210-138%20LHC%2008" id="CERN-PHOTO-202109-138-3"> <a href="//cds.cern.ch/images/CERN-PHOTO-202109-138-3" title="View on CDS"> <img alt="LHC dipole magnets in the tunnel" src="//cds.cern.ch/images/CERN-PHOTO-202109-138-3/file?size=large"/> </a> <figcaption> A chain of LHC dipole magnets inside the tunnel at point 1 (ATLAS) towards the end of Long Shutdown 2 (LS2).When the Large Hadron Collider (LHC) begins Run 3 next year, operators aim to increase the energy of the proton beams to an unprecedented 6.8 TeV. <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--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>Higgs10: Big Bang Day</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/lyn-evans"><span class="cern-tag">Lyn Evans</span></a></p> <p class="text-align-center">20 June, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs10-big-bang-day">français</a></i></p> <hr /><p class="text-align-center">In the fifth part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, the LHC circulates its first beams on 10 September 2008.</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>It was at 9.30 a.m. on 10 September 2008 that the LHC’s first beam was injected, in the full glare of the global media spotlight. Just under one hour later, a beam had been successfully steered all the way around the ring, to scenes of great emotion at the Laboratory. A long wait was over, LHC page 1 became the focus of everyone’s attention around the Lab, and a new era of research seemed about to get under way, but the sense of euphoria was to be short-lived.</p> <p>In the days that followed, things went well, but then disaster struck: during a ramp to full energy, one of the 10 000 superconducting joints between the magnets failed, causing extensive damage that took more than a year to recover from.</p> <p>It was unheard of to start a machine like the LHC in the public eye, but I’m assured we had little choice. In the months and weeks before the start-up, particle physics had never seen so much media attention. A small number of individuals on social media had managed to stir up the myth that the LHC would create a world-eating black hole, and the newspapers were full of it. They were going to come to CERN whether we asked them or not, so we invited them in on the basis that it would be better to have them inside the Lab than outside, telling the world that CERN was starting up the “black hole machine” behind locked doors. Over 300 media outlets came, BBC Radio 4 did an unprecedented full day of outside <a href="https://www.bbc.co.uk/radio4/bigbang/">broadcast from CERN</a>, and an estimated billion people watched as I gave the countdown to that first beam. I thought I was just talking to physicists in the main auditorium!</p> <figure class="cds-image" id="CERN-HOMEWEB-PHO-2022-124-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-124-1" title="View on CDS"><img alt="home.cern,Life at CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-124-1/file?size=large" /></a> <figcaption>The scene in the CERN Control Centre on 10 September 2008, when beams went round the LHC for the first time.<span> (Image: CERN)</span></figcaption></figure><p>Those joyful events of 10 September firmly established CERN’s place in the public eye, while the failure of a magnet interconnection just over a week later ensured the Laboratory would stay there. There was, and there remains, fascination with the human endeavour that particle physics represents, and the media were kind to us on the whole. But for me, the most important part of the story was somewhat lost.</p> <p>The LHC is unique. Like any energy-frontier accelerator, it is its own prototype, and building it was a learning experience from the start. Despite the serious nature of the setback in September 2008, it was really just another step, albeit a big one, on a long learning curve. As with previous setbacks, the LHC team was hard at work the next day to ensure that we could recover as fast as possible. We soon understood the problem, and we had all the spares we needed. It took a year to put right, but we knew straight away what we had to do.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-123-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-123-1" title="View on CDS"><img alt="home.cern,Accelerators" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-123-1/file?size=medium" /></a> <figcaption>Image of a LHC beam screen recorded on 10 September 2008, showing two spots corresponding to the successful circulation of protons once around the machine.<span> (Image: CERN)</span></figcaption></figure><p>It’s a great tribute to the global particle physics community that setbacks are confronted with a confident, positive approach. In 2004, after we’d installed a full sector of the cryogenic distribution line (QRL), it failed and had to be removed from the tunnel. To me, this was a much bigger issue than the 2008 event, since it required the whole LHC installation schedule to be rearranged while the contractor made good the problem with considerable help from CERN. Our Director-General at the time, Robert Aymar, was an engineer, and he understood the magnitude of the problem perfectly. He was the unsung hero in liberating the resources needed to get it fixed. It’s also thanks to him that we have Linac4, a key part of the HL-LHC project, whose construction began during his mandate. Later, in 2007, one of the so-called inner triplets, which perform the final focus of the beams, failed a high-pressure test in the LHC tunnel. It was remarkable how quickly CERN staff came up with an innovative and elegant solution, and implemented it with the help of colleagues from Fermilab, KEK and the Lawrence Berkeley National Laboratory.</p> <p>Following repairs and consolidation, on 29 November 2009 there were beams circulating again in the LHC, and full commissioning could get under way. The experiments had had an extra year to prepare, and although I’m sure they’d have preferred beam in 2008, they were in perfect shape to start data taking. Every cloud has a silver lining. This time, start-up went very quickly. The injector chain worked beautifully, as always, with even higher performance than we’d anticipated: a great tribute to our predecessors who built those machines from the 1950s onwards. We’d also learned a lot from LEP, and instrumentation was very much improved. The LHC physics programme, at an initial energy of 3.5 TeV per beam, began in earnest in March 2010.</p> <p>I’m an accelerator physicist, but I want to finish by talking about the experiments. It’s not only the LHC that took technology way beyond anything that had ever been done before. Like the accelerator team, the experimental collaborations had also learned much from their predecessors. The previous generation of hadron collider experiments had luminosities two orders of magnitude lower to deal with, they had around a million readout channels compared with the LHC experiments’ up to 100 million, and their data rates and volumes were also much smaller. It’s thanks to the efforts of a global, multidisciplinary collaboration that the LHC project delivered so well on its promise right from the moment data taking began, re-measuring everything we’d learned before about the Standard Model of particle physics in the first few months of operation, and then going on to new discoveries. But that’s a story for another day.</p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> On 10 September 2008, the LHC circulated its first beams. It may not have been all plain sailing from then on, but the adventure had begun. </div> </div> </div> Thu, 07 Jul 2022 07:40:55 +0000 katebrad 183778 at https://home.cern Higgs10: The dramatic last year of CERN’s flagship LEP collider https://home.cern/news/series/higgs10/higgs10-dramatic-last-year-cerns-flagship-lep-collider <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="1221380" data-filename="" id="CERN-AC-9903039-03"> <a href="//cds.cern.ch/images/CERN-AC-9903039-03" title="View on CDS"> <img alt="At work on LEP, the world’s most powerful electron–positron collider" src="//cds.cern.ch/images/CERN-AC-9903039-03/file?size=large"/> </a> <figcaption> The LHC will be built inside the same tunnel as an existing accelerator, the Large Electron Positron (LEP) collider which came on stream in 1989. LEP will be removed from the tunnel at the end of this year to make way for the LHC. Here technicians make delicate adjustments to one of LEP’s thousands of magnets. <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--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>Higgs10: The dramatic last year of CERN’s flagship LEP collider</span> <span class="header-block__name__underline"></span> </h3> <span class="header-block__subhead" ><p class="text-align-center">By: <a href="/authors/luciano-maiani"><span class="cern-tag">Luciano Maiani</span></a>&amp; <a href="/authors/roger-cashmore"><span class="cern-tag">Roger Cashmore</span></a></p> <p class="text-align-center">31 May, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/higgs10-dramatic-last-year-cerns-flagship-lep-collider">français</a></i></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>The year 2000 was set to be the last year of running for CERN’s <a href="/science/accelerators/large-electron-positron-collider">Large Electron–Positron (LEP)</a> collider, and it ended in dramatic fashion. <a href="/about/who-we-are/our-people/biographies/luciano-maiani">Luciano Maiani</a> was Director-General and Roger Cashmore Research Director as the new millennium dawned.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-108-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-108-1" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-108-1/file?size=medium" /></a> <figcaption>Luciano Maiani (left) and Lyn Evans look from the LHC transfer tunnel, TI2, into the LEP/LHC tunnel just after the tunneling machine broke through on 15 May 2001. The decision to close LEP in 2000 allowed LHC works to proceed at full pace.<span> (Image: CERN)</span></figcaption></figure><p><em><strong>Roger Cashmore :</strong></em></p> <p>The final year of LEP operation, 2000, had been agreed on at CERN by all of the relevant committees. By this time, the LEP experiments – <a href="/science/experiments/aleph">ALEPH</a>, <a href="/science/experiments/delphi">DELPHI</a>, <a href="/science/experiments/l3">L3</a> and <a href="/science/experiments/opal">OPAL</a> – had established the <a href="/science/physics/standard-model">Standard Model of particle physics</a> with great precision. LEP had achieved its mission, and the only thing missing from the Standard Model was the elusive <a href="/science/physics/higgs-boson">Higgs particle</a>. Nobody knew whether the Higgs was within LEP’s reach, but detailed analysis suggested that its mass might be not much more than 100 GeV and that it would be produced in electron–positron collisions in association with a <a href="/science/physics/z-boson">Z</a> particle. In other words, the LEP experiments might have a chance of crowning their achievements with a spectacular discovery to start the new millennium.</p> <p>There was nothing to lose and, as the 2000 run got underway, the machine was pushed to its limits. A cut-off date of 1 September had been set, and a closing celebration planned for the following month. Throughout the year, regular reports were made to the LEP Experiments Committee (LEPC), but there was no sign of a Higgs up to a mass of about 110 GeV. The decision was taken to push the beam energy beyond the limits through July and August: at this stage, if something broke, it really didn’t matter. And that was when the situation became exciting. A small excess of events was observed by the ALEPH experiment at a mass of about 114 GeV, but with no supporting evidence from the other experiments. Nevertheless, I telephoned Luciano to keep him informed that we might have an exciting time on our hands, and potentially a very difficult one! As a result of the ALEPH candidates, LEP’s final run was extended through to the end of October.</p> <p><em><strong>Luciano Maiani : </strong></em></p> <p>I remember Roger’s call like it was yesterday. Whatever happened next was going to require some difficult decisions. In October, we celebrated the conclusion of the LEP programme in the presence of eminent representatives of the <a href="/about/who-we-are/our-governance/member-states">Member States</a>, even though the machine was still running. ALEPH’s excess was still there so, after the speeches were done, we discreetly started to work out the cost of running LEP for another year, and the repercussions it would have on the construction of the <a href="/science/accelerators/large-hadron-collider">LHC</a>.</p> <p>The problem was that LHC excavations would soon reach the LEP tunnel, so an extra year of running would mean that work would have to stop, contracts be terminated and penalties paid to the companies involved, not to mention the extra running costs that had not been budgeted for. In total, we worked out that it would cost some 120 MCHF, and deal a major psychological blow to the LHC community. We had no way of anticipating how the LHC experiments’ funding agencies would react to the news of a year’s delay.</p> <p>As October progressed, the other LEP experiments did not see anything, and ALEPH did not find any more candidates. LEP’s illustrious career seemed to be coming to an uneventful end, but there was to be one final twist: towards the end of month, the L3 experiment announced an event that seemed to change everything. It was a two-jet event. Each jet contained a b quark, and there was missing energy corresponding to the mass of a Z particle. Significantly, the jets had the fateful energy of around 114 GeV.</p> <figure class="cds-image align-left" id="CERN-HOMEWEB-PHO-2022-108-2"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-108-2" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-108-2/file?size=large" /></a> <figcaption>Michel Spiro (left) and Roger Cashmore speaking at the LEP Fest, a celebration of the achievements of LEP on 10 October 2000.<span> (Image: CERN)</span></figcaption></figure><p>L3’s event could be interpreted as the production of the same particle that ALEPH seemed to see decaying into a b-anti-b quark pair, with the accompanying Z decaying into two invisible neutrinos. In short, it could be another trace of the existence of the Higgs boson.</p> <p>We discussed the L3 event thoroughly with LEPC Chair Michel Spiro and concluded that it was inconclusive. It could be a Higgs, but it could equally well be something much more mundane: there was no imbalance in transverse energy as there had been in the 1980s when <a href="/about/who-we-are/our-people/biographies/carlo-rubbia">Carlo Rubbia</a> had announced the discovery of the Z boson. Without that, the missing energy could have been lost down the beam pipes and so gone undetected and, importantly, there were well-known electromagnetic processes that would produce just such an outcome.</p> <p>The L3 event was not a smoking gun after all, and we were left at the end of the month with a very difficult decision to take. Whatever we decided, some part of the community would be disappointed. Events proceeded quickly. On 3 November, LEPC delivered its verdict: not conclusive. Similar verdicts were then delivered by the Research Board and the Scientific Policy Committee (SPC). The decision was left to us and, along with Roger and the whole Directorate, we made our decision. For us, LEP was over; the LHC was the best machine to tell us whether there was a Higgs at 114 GeV, or whether LEP had been chasing phantoms.</p> <p>By 4 November I had already written to George Kalmus, the Chair of the SPC. “The idea that we may find ourselves in September 2001 with 3.5–4 sigma, CERN’s financial position aggravated, LHC delayed and LHC people disbanded is not very encouraging. I am not going to go this way.” On 17 November, we recommended no additional year of LEP running to the Committee of Council. Faced with the alternative of betting 120 MCHF on the roulette wheel of a few anomalous events, the Council wisely accepted our advice.</p> <p>LEP’s final year had been an emotionally charged rollercoaster ride. The lights never went out at CERN as analyses were refined around the clock and, when our decision became known, it was greeted with relief, shock and disbelief in equal measure. At the end of 2000, the Council’s decision moved us firmly into the LHC era, ready to fully explore the Higgs and much more.</p> <p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> The year 2000 was set to be the last year of running for CERN’s Large Electron–Positron (LEP) collider, and it ended dramatically </div> </div> </div> Wed, 08 Jun 2022 14:25:18 +0000 katebrad 182747 at https://home.cern Higgs10: Three-quarters of the way there https://home.cern/news/series/higgs10/higgs10-three-quarters-way-there <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="2810697" data-filename="Screenshot%202022-05-30%20at%2008.49.39" id="CERN-HOMEWEB-PHO-2022-106-1"> <a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-106-1" title="View on CDS"> <img alt="Higgs10 Bulletin article - 3" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-106-1/file?size=large"/> </a> <figcaption> Higgs10 Bulletin article - 3 <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--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>Higgs10: Three-quarters of the way there</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">25 May, 2022 · <i>Voir en <a href="/fr/news/series/higgs10/three-quarters-way-there">français</a></i></p> <hr /><p class="text-align-center">In the third part of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, we see how the direct discovery of the W and Z bosons at the SppS in 1983 provided solid experimental support for the existence of the Higgs boson</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"> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-106-1"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-106-1" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-106-1/file?size=large" /></a><figcaption>Press conference on the announcement of the W and Z bosons. From left to right: Carlo Rubbia, spokesman of the UA1 experiment; Simon van der Meer, responsible for developing the stochastic cooling technique; Herwig Schopper, Director-General of CERN; Erwin Gabathuler, Research Director at CERN, and Pierre Darriulat, spokesman of the UA2 experiment<span> (Image: CERN)</span></figcaption></figure><p>4 July 2012 wasn’t the first time physicists had packed themselves into the CERN auditorium to witness the discovery of a new elementary particle. To rapturous applause on 20 January 1983, <a href="/about/who-we-are/our-people/biographies/carlo-rubbia">Carlo Rubbia</a>, spokesperson of the <a href="/science/experiments/ua1">UA1</a> experiment at the Spp-barS collider, presented six candidate events for the <a href="/science/physics/w-boson-sunshine-and-stardust">W boson</a>, the electrically charged carrier of the weak interaction responsible for radioactive decay. In similar scenes the following afternoon, Luigi Di Lella of the <a href="/science/experiments/ua2">UA2</a> experiment announced four W candidates. Along with the <a href="/science/physics/z-boson">Z boson</a> and massless photon, the W boson is one of three “gauge” bosons of a unified electroweak interaction that demands the existence of a fourth “scalar” particle called the Higgs boson.</p> <p><a href="/news/series/higgs10/higgs-boson-and-rise-standard-model-particle-physics-1970s">Indirect evidence for the Z boson had been obtained a decade earlier at Gargamelle</a>, driving the community to seek a direct discovery of the massive electroweak bosons. But their predicted masses – around 80 and 90 GeV for the W and Z, respectively – were beyond the reach of experiments at the time. In 1976, Rubbia, Peter McIntyre and David Cline suggested modifying the CERN <a href="/science/accelerators/super-proton-synchrotron">SPS</a> from a one-beam accelerator into a machine that would collide beams of protons and antiprotons, greatly increasing the available energy. <a href="https://cerncourier.com/cws/article/cern/46057">Simon van der Meer</a> had already invented <a href="/science/engineering/stochastic-cooling">a way of producing and storing dense beams</a> of protons or antiprotons, while his “stochastic cooling” method to reduce the energy spread and angular divergence of the beams had been honed at the <a href="/science/accelerators/intersecting-storage-rings">Intersecting Storage Rings</a> (the world’s first hadron collider). Many doubted the wisdom of the decision, however, especially as CERN was keen to push its visionary <a href="/science/accelerators/large-electron-positron-collider">Large Electron–Positron (LEP) collider</a>.</p> <figure class="cds-image align-left" id="CERN-HOMEWEB-PHO-2022-102-3"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-102-3" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-102-3/file?size=large" /></a> <figcaption>First direct production of the W boson in the UA1 detector in late 1982<span> (Image: CERN)</span></figcaption></figure><p>As former UA2 spokesperson Pierre Darriulat wrote in <a href="https://cerncourier.com/a/the-w-and-z-particles-a-personal-recollection/ "><em>CERN Courier</em></a> in 2004: “The pressure to discover the W and Z was so strong that the long design, development and construction time of the LEP project left most of us, even the most patient, dissatisfied. A quick (but hopefully not dirty) look at the new bosons would have been highly welcome. But when proton–proton colliders such as the Superconducting Intersecting Storage Rings were proposed in this spirit, they were ‘killed in the egg’ by the management at CERN, with the argument that they would delay – or, even worse, endanger – the LEP project. The same argument did not apply to the proton–antiproton collider, as it did not require the construction of a new collider ring and could be proposed as an experiment … Another argument also made it possible for the proton–antiproton project to break the LEP taboo: if CERN did not buy Carlo’s idea, it was most likely that he would sell it to Fermilab.”</p> <p>Two detectors, UA1 and UA2, built around the Spp-barS beam pipe to search for signatures of the W and Z particles, started taking collision data in 1981. When they confirmed the existence of the W boson – which was announced at a press conference at CERN on 25 January 1983, followed by the discovery of the Z boson a few months later and the <a href="https://www.nobelprize.org/prizes/physics/1984/summary/">Nobel Prize in Physics for Rubbia and Van der Meer</a> the following year – the case for the existence of the Higgs boson grew stronger.</p> <figure class="cds-image align-right" id="CERN-HOMEWEB-PHO-2022-102-2"><a href="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-102-2" title="View on CDS"><img alt="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-102-2/file?size=large" /></a> <figcaption><span>First direct production of the Z boson in the UA1 detector in April 1983 (Image: CERN)</span></figcaption></figure><p>That’s because all three bosons hail from the same “Mexican hat”-shaped Brout-Englert-Higgs (BEH) field that broke the electroweak symmetry a fraction of a nanosecond after the Big Bang and left the universe with a non-zero vacuum expectation value. As the universe transitioned from a symmetrical state at the top of the hat to a more stable configuration in the rim, three of the BEH field’s four mathematical components were absorbed to generate masses for the W and Z bosons (while keeping the photon massless); the fourth, corresponding to an otherworldly oscillation up and down the rim of the Mexican hat, is the Higgs boson.</p> <p>In 1983, assuming that the electroweak <a href="/science/physics/standard-model">Standard Model</a> and BEH mechanism were correct, three quarters of the BEH field had been discovered. LEP went on to measure the properties of the W and Z bosons in great detail, helping to constrain the possible hiding places for the “remaining quarter”. The Standard Model does not predict the mass of the Higgs boson. Finding it would require an even more powerful machine. Thanks to the foresight of CERN Director-General <a href="/about/who-we-are/our-people/biographies/john-bertram-adams">John Adams</a> in 1977, the LEP tunnel was designed to be large enough to accommodate the proton–proton collider that, 35 years later, would uncover the final quarter of the mysterious scalar field that pervades the universe and gives mass to elementary particles.</p> <figure class="cds-video" id="CERN-VIDEO-2013-003-001"><div><iframe allowfullscreen="true" frameborder="0" height="450" src="//cds.cern.ch/video/CERN-VIDEO-2013-003-001" width="100%"></iframe></div> <figcaption>Carlo Rubbia, spokesperson for the UA1 experiment, announces six candidate W boson events in a seminar on 20 January 1983 <span> (<a href="https://videos.cern.ch/record/1507644">Video: CERN</a>)</span></figcaption></figure><p> </p> </div> </div> </div> </div> </div> </div> </div> <div class="field--item"> The direct discovery of the W and Z bosons at the SppS in 1983 provided solid experimental support for the existence of the Higgs boson </div> </div> </div> Tue, 31 May 2022 09:04:34 +0000 katebrad 182630 at https://home.cern Higgs10: The Higgs boson and the rise of the Standard Model of Particle Physics in the 1970s https://home.cern/news/series/higgs10/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--14" 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>Higgs10: 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> <hr /><p class="text-align-center">In part two of the <a href="/news/series/higgs10"><span class="cern-tag">Higgs10</span></a> series, we see how breakthroughs in the 1970s helped the Brout–Englert–Higgs (BEH) field and its boson emerge as the most promising theoretical model to explain the origin of mass</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>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> <figure class="cds-image align-right" 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="home.cern,Personalities and History of CERN" src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-081-1/file?size=large" /></a> <figcaption>Paul Musset (centre), then representative of the Gargamelle collaboration, standing in the control room of the eponymous bubble chamber in 1974. Gargamelle provided the first direct evidence for the existence of neutral currents in 1973.<span> (Image: CERN)</span></figcaption></figure><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 (centre), 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 Higgs10: A boson is born https://home.cern/news/series/higgs10/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--16" 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>Higgs10: 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<br />  </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 electroweak 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> <blockquote>“We’ve scratched the surface,” said Peter Higgs in 2019. “But we have clearly much more to discover.”</blockquote> </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--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>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--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>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