CERN: Physics updates en ATLAS reports first observation of WWW production <span>ATLAS reports first observation of WWW production</span> <span><span lang="" about="/user/147" typeof="schema:Person" property="schema:name" datatype="">cagrigor</span></span> <span>Mon, 07/26/2021 - 18:04</span> <div class="field field--name-field-p-news-display-list-cds field--type-cerncdsmedia field--label-hidden field--item"><figure class="cds-image" data-record-id="2776761" data-filename="WWW_event_3l_Run349169" id="ATLAS-PHOTO-2021-066-1"> <a href="//" title="View on CDS"> <img alt="ATLAS Event Display: Triple W-boson Production" src="//"/> </a> <figcaption> Display of a candidate WWW→ 3 leptons + neutrinos event. <span> (Image: CERN)</span> </figcaption> </figure></div> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>Today, at the <a href="">EPS-HEP Conference 2021</a>, the ATLAS collaboration <a href="">announced</a> the first <a href="">observation</a> of a rare process: the simultaneous production of three W bosons.</p> <p>As a carrier of the electroweak force, the W boson plays a crucial role in testing the Standard Model of particle physics. Though discovered nearly four decades ago, the W boson continues to provide physicists with new avenues for exploration.</p> <p>ATLAS researchers analysed the full LHC Run-2 dataset, recorded by the detector between 2015 and 2018, to observe the WWW process with a statistical significance of 8.2 standard deviations – well above the 5 standard-deviation threshold needed to declare observation. This result follows <a href="">an earlier observation by the CMS collaboration</a> of inclusive three weak boson production.</p> <p>Achieving this level of precision was no mean feat. Physicists analysed around 20 billion collision events recorded and pre-filtered by the ATLAS experiment in their search for just a few hundred events expected from the WWW process.</p> <p>As one of the heaviest known elementary particles, the W boson is able to decay in several different ways. The ATLAS physicists focused their search on the four WWW decay modes that have the best discovery potential due to their reduced number of background events. In three of these modes, two W bosons decay into charged leptons<em> </em>(electrons or muons), carrying the same positive or negative charge, and neutrinos, while the third W boson decays into a pair of light quarks. In the fourth decay mode, all three W bosons decay into a charged lepton and a neutrino.</p> <p>To pick out the WWW signal from the large number of background events, researchers used a machine-learning technique called Boosted Decision Trees (BDTs). BDTs can be trained to identify specific signals in the ATLAS detector, spotting small – but key – differences between the predicted event properties. The improved separation between signal and background provided by the BDTs – along with the massive dataset provided by Run 2 of the LHC – enhanced the precision of the overall measurement and enabled the first observation of WWW production.</p> <p>This exciting measurement also allows physicists to look for hints of new interactions that might exist beyond the current energy reach of the LHC. In particular, physicists can use the WWW production process to study the <em>quartic gauge boson coupling</em> – where two W bosons scatter off each other – a key property of the Standard Model.<em> </em>New particles could alter the quartic gauge boson coupling through quantum effects, modifying the WWW production cross section. The continued study of WWW and other electroweak processes offers an enticing road ahead.</p> <p><strong>Links</strong></p> <ul><li><a href="">Observation of WWW production in proton–proton collisions at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2021-039)</li> <li>EPS2021 presentation by Jessica Metcalfe: <a href="">Measurements of multi-boson production at ATLAS</a></li> <li>CMS Collaboration: <a href="">Observation of the production of three massive gauge bosons at 13 TeV</a> (arXiv: 2006.11191)</li> </ul></div> Mon, 26 Jul 2021 16:04:31 +0000 cagrigor 157686 at MoEDAL bags a first <span>MoEDAL bags a first</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Thu, 07/01/2021 - 16:29</span> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>The Monopole and Exotics Detector at the Large Hadron Collider (<a href="">MoEDAL</a>) does what it says on the tin. It searches for magnetic monopoles – hypothetical particles with either a “north” or a “south” magnetic charge instead of both – and other exotic theoretical particles. These searches have so far come up empty-handed, but they have <a href="">delivered crucial information</a> to help guide future searches. Now, in a first for an experiment at a particle collider, MoEDAL has <a href="">searched</a> for magnetic monopoles produced through a process called the Schwinger mechanism.</p> <p>Nobel Prize winner Julian Schwinger showed that pairs of particles with electrical charge can be spontaneously created in a strong electric field. Similarly, pairs of magnetic monopoles could be spontaneously created in a strong magnetic field. Compared to other means of producing magnetic monopoles, this process, known as the Schwinger mechanism, has advantages, including that the monopoles should be created at a greater rate, thus increasing the chances of spotting them.</p> <p>The MoEDAL team usually looks for magnetic monopoles by exposing the experiment’s “magnetic monopole trappers”, which consist of 800 kg of aluminium blocks, to proton–proton collisions produced at the Large Hadron Collider (LHC). To search for Schwinger magnetic monopoles, however, the team exposed the blocks to lead–lead collisions produced by the LHC in November 2018, just before the collider was shut down for maintenance.</p> <p>Lead–lead collisions at the LHC generate extremely strong magnetic fields, and the November 2018 run generated a maximum magnetic field that was more than ten thousand times stronger than the strongest magnetic fields in the cosmos, which are found on the surfaces of fast-spinning neutron stars called magnetars, and ten million times stronger than the field strength required to create Schwinger monopoles. Therefore, these collisions could have produced such monopoles.</p> <p>After exposing the blocks to the lead–lead collisions, the MoEDAL researchers used a device called a SQUID magnetometer to scan the blocks for any trapped magnetic charges belonging to Schwinger monopoles. The researchers found no signs of such monopoles in the blocks, but the lead–lead collision data allowed them to rule out the existence of Schwinger monopoles that have masses up to 75 GeV/c<sup>2</sup>, where c is the speed of light, for magnetic charges ranging from 1 to 3 base units of magnetic charge.</p> <p>“A unique feature of the Schwinger monopoles is that they are not point-like, they have a finite size,” explains MoEDAL spokesperson James Pinfold. “Our mass bound is the first lower mass limit for finite-size monopoles from a collider search, and it’s tighter than previous similar mass bounds, such as that obtained from neutron-star data.”</p> <p>The MoEDAL team will continue its searches during the next run of the LHC, which will start in 2022 and deliver more proton–proton and lead–lead collision data for analysis.</p> </div> Thu, 01 Jul 2021 14:29:02 +0000 abelchio 157525 at Speeding up machine learning for particle physics <span>Speeding up machine learning for particle physics</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Mon, 06/21/2021 - 14:52</span> <div class="field field--name-field-p-news-display-caption field--type-string-long field--label-hidden field--item">An ultra-compressed deep neural network on a field-programmable gate array. (Image: Sioni P. Summers)</div> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p class="BodyA" style="border:medium none">Machine learning is everywhere. For example, it’s how Spotify gives you suggestions of what to listen to next or how Siri answers your questions. And it’s used in particle physics too, from theoretical calculations to data analysis. Now a team including researchers from CERN and Google has come up with a new method to speed up deep neural networks – a form of machine-learning algorithms – for selecting proton–proton collisions at the <a href="">Large Hadron Collider</a> (LHC) for further analysis. The technique, described in a <a href="">paper</a> just published in <em>Nature Machine Intelligence</em>, could also be used beyond particle physics.</p> <p>The particle detectors around the LHC ring use an electronic hardware “trigger” system to select potentially interesting particle collisions for further analysis. With the current rate of proton–proton collisions at the LHC, up to 1 billion collisions per second, the software currently in use on the detectors’ trigger systems chooses whether or not to select a collision in the required time, which is a mere microsecond. But with the collision rate set to increase by a factor of 5 to 7 with the future upgraded LHC, the <a href="">HL-LHC</a>, researchers are exploring alternative software, including machine-learning algorithms, that could make this choice faster.</p> <p>Enter the new study by CERN researchers and co-workers, which builds on <a href="">previous work</a> that introduced a software tool to deploy deep neural networks on a type of hardware, called field-programmable gate arrays (FPGAs), that can be programmed to perform different tasks, including selecting particle collisions of interest. The CERN researchers and their colleagues developed a technique that reduces the size of a deep neural network by a factor of 50 and achieves a network processing time of tens of nanoseconds – well below the time available to choose whether to save or discard a collision.</p> <p>“The technique boils down to compressing the deep neural network by reducing the numerical precision of the parameters that describe it,” says co-author of the study and CERN researcher Vladimir Loncar. “This is done during the training, or learning, of the network, allowing the network to adapt to the change. In this way, you can reduce the network size and processing time, without a loss in network performance.”</p> <p>In addition, the technique can find which numerical precision is best to use given certain hardware constraints, such as the amount of available hardware resources.</p> <p>If that wasn’t enough, the technique has the advantage that it is easy to use for non-experts, and it can be used on FPGAs in particle detectors and in other devices that require networks with fast processing times and small sizes.</p> <p>Looking forward, the researchers want to use their technique to design a new kind of trigger system for spotting collisions that would normally be discarded by a conventional trigger system but that could hide new phenomena. “The ultimate goal is to be able to capture collisions that could point to new physics beyond the <a href="">Standard Model</a> of particle physics,” says another co-author of the study and CERN researcher Thea Aarrestad.</p> </div> Mon, 21 Jun 2021 12:52:45 +0000 abelchio 157429 at Leptoquarks, the Higgs boson and the muon’s magnetism <span>Leptoquarks, the Higgs boson and the muon’s magnetism</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Mon, 06/14/2021 - 14:44</span> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>Zoom into an online particle physics conference, and the chances are you’ll hear the term muon anomaly. This is a longstanding tension with the <a href="">Standard Model</a> of particle physics, seen in the magnetism of a heavier cousin of the electron called a muon, that has recently been <a href="">strengthened by measurements made at Fermilab in the US</a>.</p> <p>In a <a href="">paper</a> accepted for publication in <em>Physical Review Letters</em>, a trio of theorists including Andreas Crivellin of CERN shows that a class of new unknown particles that could account for the muon anomaly, known as leptoquarks, could also affect the transformation, or “decay”, of the <a href="">Higgs boson</a> into muons.</p> <p>Leptoquarks are hypothetical particles that connect quarks and leptons, the two types of particles that make up matter at the most fundamental level. They are a popular explanation for the muon anomaly and other <a href="">anomalies</a> seen in certain decays of particles called B mesons.</p> <p>In their new study, Crivellin and his colleagues explored how two kinds of leptoquarks that could explain the muon anomaly would affect the rare decay of the Higgs boson into muons, of which the ATLAS and CMS experiments recently <a href="">obtained</a> the first indications.</p> <p>They found that one of the two kinds of leptoquarks increases the rate at which this Higgs decay takes place, while the other one decreases it.</p> <p>“The current measurements of the Higgs decay to muons are not sufficient to see this increase or decrease, and the muon anomaly has yet to be confirmed,” says Crivellin. “But if future measurements, at the LHC or future colliders, display such a change, and the muon anomaly is confirmed, it will be possible to pick out which of the two kinds of leptoquarks would be more likely to explain the muon anomaly.”</p> </div> Mon, 14 Jun 2021 12:44:58 +0000 abelchio 157201 at ALICE finds that charm hadronisation differs at the LHC <span>ALICE finds that charm hadronisation differs at the LHC</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Andrea Dainese</div> </div> <span><span lang="" about="/user/147" typeof="schema:Person" property="schema:name" datatype="">cagrigor</span></span> <span>Thu, 06/10/2021 - 14:13</span> <div class="field field--name-field-p-news-display-list-cds field--type-cerncdsmedia field--label-hidden field--item"><figure class="cds-image" data-record-id="2758788" data-filename="DSC00643" id="ALICE-PHO-ITS-2021-001-3"> <a href="//" title="View on CDS"> <img alt="ALICE ITS Outer Barrel Installation" src="//"/> </a> <figcaption> Installation of the Outer Barrel of the new silicon Inner Tracking System of ALICE inside the solenoidal magnet. <span> (Image: CERN)</span> </figcaption> </figure></div> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>Quarks are among the elementary particles of the Standard Model of Particle Physics. Besides up and down quarks, which are the basic building blocks of ordinary matter in the Universe, four other quark flavours exist and are also abundantly produced in collisions at particle accelerators like the CERN Large Hadron Collider. Quarks are not observed in isolation due to a fundamental aspect of the strong interaction, known as colour charge confinement. Confinement requires particles that carry the charge of the strong interaction, called colour, to form states that are colour-neutral. This in turn forces quarks to undergo a process of hadronisation, i.e. to form hadrons, which are composite particles mostly made of a quark and an antiquark (mesons) or of three quarks (baryons). The only exception is the heaviest quark, the top, which decays before it has time to hadronise.</p> <p>At particle accelerators, quarks with a large mass, such as the charm quark, are produced only in the initial interactions between the colliding particles. Depending on the type of beam used, these can be electron-positron, electron-proton or proton-proton collisions (as at the LHC). The subsequent hadronisation of charm quarks into mesons (D<sup>0</sup>, D<sup>+</sup>, D<sub>s</sub>) or baryons (Λ<sub>c</sub>, Ξ<sub>c</sub>, …) occurs on a long space-time scale and was considered to be universal - that is, independent of the species of the colliding particles - until the recent findings by the ALICE collaboration.</p> <p>The large data samples collected during Run 2 of the LHC allowed ALICE to count the vast majority of charm quarks produced in the proton-proton collisions by reconstructing the decays of all charm meson species and of the most abundant charm baryons (Λ<sub>c</sub> and Ξ<sub>c</sub>). The charm quarks were found to form baryons almost 40% of the time, which is four times more often than what was expected based on measurements previously made at colliders with electron beams (e<sup>+</sup>e<sup>-</sup> and ep in the figure below).</p> <p>These measurements show that the process of colour-charge confinement and hadron formation is still a poorly understood aspect of the strong interaction. Current theoretical explanations of baryon enhancement include the combination of multiple quarks produced in proton-proton collisions and new mechanisms in the neutralisation of the colour charge. Additional measurements during the next run of the LHC will allow these theories to be scrutinised and further our knowledge of the strong interaction.</p> <p><em>Read more in the <a class="bulletin" href="">article</a> by ALICE and on <a href="">the ALICE website</a>.</em></p> <figure role="group"><img alt="Graphic" data-entity-type="file" data-entity-uuid="78b896b0-db05-4249-9c6d-875ae30532a7" src="/sites/" /><figcaption>Fraction of charm quarks that hadronise to form each species of mesons (quark-antiquark) or baryons (three quarks). The ALICE measurements in proton-proton collisions show a larger fraction of baryons than those at colliders using electron beams. (Image: CERN)</figcaption></figure></div> Thu, 10 Jun 2021 12:13:20 +0000 cagrigor 157183 at LHCb measures tiny mass difference between particles <span>LHCb measures tiny mass difference between particles</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Tue, 06/08/2021 - 08:57</span> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>The <a href="">LHCb</a> collaboration has measured a difference in mass between two particles of 0.00000000000000000000000000000000000001 grams – or, in scientific notation, 10<sup>-38</sup> g. The result, reported in a <a href="">paper</a> just submitted for publication in the journal <em>Physical Review Letters</em> and presented today at a <a href="">CERN seminar</a>, marks a milestone in the study of how a particle known as a D<sup>0</sup> meson changes from matter into <a href="">antimatter</a> and back.</p> <p>The D<sup>0</sup> meson is one of only four particles in the <a href="">Standard Model</a> of particle physics that can turn, or “oscillate”, into their antimatter particles, which are identical to their matter counterparts in most ways. The other three are the K<sup>0</sup> meson and two types of B mesons.</p> <p>Mesons are part of the large class of particles made up of fundamental particles called quarks, and contain one quark and one antimatter quark. The D<sup>0</sup> meson consists of a charm quark and an up antiquark, while its antiparticle, the anti-D<sup>0</sup>, consists of a charm antiquark and an up quark.</p> <p>In the strange world of quantum physics, just as Schrödinger's notorious cat can be dead and alive at the same time, the D<sup>0</sup> particle can be itself and its antiparticle at once. This quantum “superposition” results in two particles, each with their own mass – a lighter and a heavier D meson (known technically as D<sub>1</sub> and D<sub>2</sub>). It is this superposition that allows the D<sup>0</sup> to oscillate into its antiparticle and back.</p> <p>The D<sup>0</sup> particles are produced in proton–proton collisions at the <a href="">Large Hadron Collider</a> (LHC), and they travel on average only a few millimetres before transforming, or “decaying”, into other particles. By comparing the D<sup>0</sup> particles that decay after travelling a short distance with those that travel a little further, the LHCb collaboration has measured the key quantity that controls the speed of the D<sup>0</sup> oscillation into anti-D<sup>0</sup> – the difference in mass between the heavier and lighter D particles.</p> <p>The result, 10<sup>-38</sup> g, crosses the “five sigma” level of statistical significance that is required to claim an observation in particle physics.</p> <p>“To put this incredibly small mass difference in context, it is still a small number even when compared with the mass of the D<sup>0</sup> particle – the same as the mass of a snowball compared to the mass of the entire Mont Blanc, the highest peak in Europe, standing at over 4800 metres,” says LHCb spokesperson Chris Parkes. “And it’s a big step in the study of the oscillatory behaviour of the D<sup>0</sup> particles.”</p> <p>With the tiny mass difference now observed, a new phase of particle exploration can begin. Researchers can make further measurements of the D<sup>0</sup> decays to obtain a more precise mass difference and look for the effect on the D<sup>0</sup> oscillation of unknown particles not predicted by the Standard Model.</p> <p>Such new particles could increase the average speed of the oscillation or the difference between the speed of the matter-to-antimatter oscillation and that of the antimatter-to-matter oscillation. If observed, such a difference could shed light on why the universe is made up entirely of matter, even though matter and antimatter should have been created in equal amounts during the Big Bang.</p> <figure><div><iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" frameborder="0" height="315" src="" width="560"></iframe></div> <figcaption>LHCb spokesperson Chris Parkes explains the new result. (Video: CERN)</figcaption></figure><p>_____</p> <p><em>Read more on the <a class="bulletin" href="">LHCb website</a>.</em></p> </div> Tue, 08 Jun 2021 06:57:33 +0000 abelchio 157160 at FASER catches first candidate collider neutrinos <span>FASER catches first candidate collider neutrinos</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Wed, 06/02/2021 - 15:24</span> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>It’s a first at the Large Hadron Collider (LHC), or indeed at any particle collider: the FASER collaboration has detected the first candidate particle interactions for neutrinos produced in LHC collisions. The result, described in a <a href="">paper posted online</a>, paves the way for studies of high-energy neutrinos at current and future colliders.</p> <p>Neutrinos are the most abundant fundamental particles that have mass in the universe, and they have been detected from many sources. Yet, no neutrino produced at a particle collider has ever been directly detected, even though colliders produce them in abundance. Studying such collider neutrinos could shed new light on the still enigmatic nature of these fundamental particles, not least because collider neutrinos are produced at high energies, at which their weak interactions with matter have been little studied.</p> <p>The FASER experiment’s <a href="">FASERν detector</a> and the newly approved <a href="">SND@LHC detector</a> have both been designed to catch and study collider neutrinos, and they are expected to be installed at the LHC over the course of 2021 and to begin taking data when the collider starts up again in 2022. However, the FASER collaboration was in for an early treat when it took four weeks’ worth of proton–proton collision data with a smaller pilot version of FASERν shortly before the LHC was shut down for maintenance and upgrades at the end of 2018.</p> <p>After analysing the pilot detector data and estimating a background of particle events that could mimic the signal from neutrino interactions, the FASER team found several candidate events for collider neutrinos. The result has a statistical significance of 2.7 standard deviations, a little below the 3 standard deviations required to claim evidence of a particle or process in particle physics.</p> <p>“The goal of the pilot detector was to demonstrate the feasibility of neutrino measurements in the experimental environment of the LHC,” says FASER co-spokesperson Jamie Boyd. “So we are very excited that this small detector, which is only about 1% of the final detector, allowed us to see the first candidate events for neutrino interactions at a collider.”</p> <p>The team expects to observe about 20 000 collider neutrino interactions with the full-fledged FASERν detector in the next LHC run, from 2022 to 2024.</p> <figure role="group"><img alt="Shows particle tracks from neutrino interactions" data-entity-type="file" data-entity-uuid="60a57715-02b2-4acc-a3b1-2ec0b53418d4" src="/sites/" /><figcaption>Two candidate events for neutrinos produced in LHC collisions and interacting in the FASERν pilot detector. The neutrinos enter the detector from the left, and interact with the detector material to produce a number of charged particles. The different lines in each event show tracks from these charged particles, originating from the neutrino interaction point. (Image: FASER/CERN)</figcaption></figure><p>_____</p> <p><em>See also <a class="bulletin" href="">this story in the CERN Courier</a>.</em></p> </div> Wed, 02 Jun 2021 13:24:49 +0000 abelchio 157127 at RADES joins the hunt for dark matter <span>RADES joins the hunt for dark matter</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Ana Lopes</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Tue, 05/25/2021 - 11:18</span> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>Long-hypothesised particles called axions could solve two problems in one strike: they could explain the puzzling symmetry properties of the strong force and they could make up the mysterious <a href="">dark matter</a> that permeates the cosmos. One of the newest detectors of the <a href="">CAST</a> experiment at CERN, RADES, has now joined the worldwide hunt for axions, searching for axions from the Milky Way’s “halo” of dark matter and setting a limit on the strength of their interaction with photons. The results are described in a <a href="">paper</a> submitted for publication in the <em>Journal of High Energy Physics</em>.</p> <p>One way of searching for axions from the Milky Way’s dark-matter halo is to look for their conversion into photons in a “resonating cavity”. If such axions surround and enter a resonating cavity that is placed in a strong magnetic field and resonates at a frequency corresponding to their mass, the chances of detecting them through their conversion into photons are increased.</p> <p>Many experiments have used this search method and set limits on the interaction strength of axions with two photons in the case of small axion masses, mainly below 25 µeV (for comparison, the proton mass is about 1 GeV). Searching for larger axion masses using this approach requires a smaller cavity resonating at a higher frequency, but the smaller volume of a smaller cavity decreases the chances of spotting the particles.</p> <p>A workaround involves dividing the cavity into smaller cavities that resonate at a higher frequency and collectively don’t result in a loss of cavity volume. This is exactly the concept behind the RADES detector, which was installed inside one of CAST’s dipole magnet bores in 2018 and can search for axions from the Milky Way’s dark-matter halo that have a mass of around 34.67 µeV.</p> <p>Researchers are developing complementary approaches to searching for axions, and some have searched for larger-mass axions using new cavity designs and placed limits on their interaction strength with two photons. But the best limit so far for an axion mass of 34.67 µeV was placed by CAST’s previous <a href="">searches for axions from the Sun</a>.</p> <p>In its latest <a href="">paper</a>, the CAST team describes the results of the first RADES search for axions. Sifting through data taken for more than 100 hours within a period of 20 days in 2018, the team saw no signs of axions. However, the data places a limit on the interaction strength of axions with two photons in the case of axions with a mass of or close to 34.67 µeV – a limit that is more than 100 times more stringent than CAST’s previous best limit for this mass.</p> <p>“This result is a significant first step in the direct search for axions using dipole magnets,” says RADES scientist Sergio Arguedas Cuendis. “And as far as axion searches go, it’s one of the most stringent limits ever set for axions with masses above 25 µeV.”</p> </div> Tue, 25 May 2021 09:18:54 +0000 abelchio 157084 at Why precision luminosity measurements matter <span>Why precision luminosity measurements matter</span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Piotr Traczyk</div> </div> <span><span lang="" about="/user/159" typeof="schema:Person" property="schema:name" datatype="">abelchio</span></span> <span>Wed, 05/12/2021 - 17:31</span> <div class="field field--name-field-p-news-display-list-cds field--type-cerncdsmedia field--label-hidden field--item"><figure class="cds-image" data-record-id="2758629" data-filename="202103-040_016" id="CERN-PHOTO-202103-040-24"> <a href="//" title="View on CDS"> <img alt="CMS central beampipe installation" src="//"/> </a> <figcaption> The beam pipe, the "heart" of the detector, is being installed inside the CMS detector. Here the LHC beams collide at the Interaction Point. It will be one of the last elements of the experiment to be installed before closing the CMS detector. <span> (Image: CERN)</span> </figcaption> </figure></div> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>The <a href="/science/experiments/atlas">ATLAS</a> and <a href="/science/experiments/cms">CMS</a> experiments at the <a href="/science/accelerators/large-hadron-collider">Large Hadron Collider</a> (LHC) have performed luminosity measurements with spectacular precision. A recent physics briefing from<a href=""> CMS</a> complements earlier<a href=""> ATLAS</a> results and shows that, by combining multiple methods, both experiments have reached a precision better than 2%. For physics analyses – such as searches for new particles, rare processes or measurements of the properties of known particles – it is important not only for accelerators to increase luminosity, but also for physicists to understand it with the best possible precision.</p> <p>Luminosity is one of the fundamental parameters to measure an accelerator’s performance. In the LHC, the circulating beams of protons are not continuous beams but are grouped into packets, or “bunches”, of about 100 billion protons. These bunches collide with oncoming bunches 40 million times per second at the interaction points within particle detectors. But when two such bunches pass through each other, only a few protons from each bunch end up interacting with the protons circulating in the opposite direction. Luminosity is a measure of the number of these interactions. The two main aspects of luminosity are instantaneous luminosity, describing the number of collisions happening in a unit of time (for example, every second), and integrated luminosity, measuring the total number of collisions produced over a period of time.</p> <p>Integrated luminosity is usually expressed in units of “inverse femtobarns” (fb<sup>-1</sup>). A femtobarn is a unit of cross-section, a measure of the probability for a process to occur in a particle interaction. This is best illustrated with an example: the total cross-section for Higgs boson production in proton–proton collisions at 13 TeV at the LHC is of the order of 6000 fb. This means that every time the LHC delivers 1 fb<sup>-1</sup> of integrated luminosity, about 6000 fb x 1 fb<sup>-1</sup> = 6000 Higgs bosons are produced.</p> <p>Knowing the integrated luminosity allows physicists to compare observations with theoretical predictions and simulations. For example, physicists can look for dark matter particles that escape collisions undetected by looking at the energies and momenta of all particles produced in a collision. If there is an imbalance, it could be caused by an undetected, potentially dark matter, particle carrying energy away. This is a powerful method of searching for a large class of new phenomena, but it has to take into account many effects, such as neutrinos produced in the collisions. Neutrinos also escape undetected and leave an energy imbalance, so in principle, they are indistinguishable from the new phenomena. To see if something unexpected has been produced, physicists have to look at the numbers.</p> <p>If 11 000 events show an energy imbalance, and the simulations predict 10 000 events containing neutrinos, this could be significant. But if physicists only know luminosity with a precision of 10%, they could have easily had 11 000 neutrino events, but there were just 10% more collisions than assumed. Clearly, a precise determination of luminosity is critical.</p> <p>There are also types of analyses that depend much less on absolute knowledge of the number of collisions. For example, in measurements of ratios of different particle decays, such as the <a href="/news/news/physics/intriguing-new-result-lhcb-experiment-cern">recent LHCb measurement</a>. Here, uncertainties in luminosity get cancelled out in the ratio calculations. Other searches for new particles look for <a href="">peaks in mass distribution</a> and so rely more on the shape of the observed distribution and less on the absolute number of events. But these also need to know the luminosity for any kind of interpretation of the results. </p> <p>Ultimately, the greater the precision of the luminosity measurement, the more physicists can understand their observations and delve into hidden corners beyond our current knowledge.</p> </div> Wed, 12 May 2021 15:31:01 +0000 abelchio 157015 at Connecting the smallest and largest scales <span>Connecting the smallest and largest scales </span> <div class="field field--name-field-p-news-display-byline field--type-entity-reference field--label-hidden field--items"> <div class="field--item">Matthew Chalmers</div> </div> <span><span lang="" about="/user/40" typeof="schema:Person" property="schema:name" datatype="">katebrad</span></span> <span>Thu, 05/06/2021 - 15:48</span> <div class="field field--name-field-p-news-display-listing-img field--type-image field--label-hidden field--item"> <img src="/sites/" width="842" height="595" alt="DESY Blazar artists impression" typeof="foaf:Image" class="img-responsive" /> </div> <div class="field field--name-field-p-news-display-caption field--type-string-long field--label-hidden field--item">Cosmic particle accelerators like blazars (artist&#039;s impression) are typical objects for multi-messenger astronomy. (Image credit: DESY, Science Communication Lab)</div> <div class="field field--name-field-p-news-display-body field--type-text-long field--label-hidden field--item"><p>Established in 2019 with its central hub at CERN, the European Consortium for Astroparticle Theory (<a href="">EuCAPT</a>) aims to bring together the European community of theoretical astroparticle physicists and cosmologists to tackle some of the greatest mysteries in science.</p> <p>There are strong hints that explanations for dark matter and dark energy, the origin of high-energy cosmic rays, the matter-antimatter asymmetry, and other enigmas about the universe at large lie in the domain of particle physics. Addressing them therefore demands a highly interdisciplinary approach by a strong and diverse community.</p> <p>"Astroparticle physics is undergoing a phase of profound transformation", says EuCAPT Director Gianfranco Bertone of the Centre for Gravitation and Astroparticle Physics at the University of Amsterdam. "We have recently obtained extraordinary results, such as the discovery of high-energy cosmic neutrinos with IceCube and the direct detection of gravitational waves with LIGO and Virgo, and we have witnessed the birth of multi-messenger astrophysics. Yet we have formidable challenges ahead of us."</p> <p>The <a href="">symposium</a> featured 29 invited presentations and 42 lightning talks given by young researchers, covering<strong> </strong>every aspect of astroparticle physics and cosmology, from early-universe inflationary dynamics to late-universe structure formation. The event also included a plenary session dedicated to the planning of a community-wide white paper, followed by thematic parallel discussions. An award ceremony congratulated Hannah Banks from the University of Cambridge, Francesca Capel from TU Munich and Charles Dalang from the University of Geneva for the best talks by young scientists.</p> <p>"The symposium has been a successful opportunity for community building and for looking into the future of astroparticle physics and cosmology," said Gian Giudice, the Head of CERN’s Theoretical Physics department. "The emphasis on the future was underlined by our choice of selecting almost all speakers from among young researchers."</p> <p>EuCAPT is led by an international steering committee comprising 12 theorists from institutes in France, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom, and from CERN. Its aim is to coordinate scientific and training activities, help researchers attract adequate resources for their projects, and promote a stimulating and open environment in which young scientists can thrive. CERN will act as the central hub of EuCAPT for the first five years.</p> </div> Thu, 06 May 2021 13:48:28 +0000 katebrad 156962 at