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Posts from home.cern
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LHCb experiment discovers a new pentaquarkThe LHCb collaboration has announced the discovery of a new pentaquark particle. The particle, named Pc(4312)+, decays to a proton and a J/ψ particle (composed of a charm quark and an anticharm quark). This latest observation has a statistical significance of 7.3 sigma, passing the threshold of 5 sigma traditionally required to claim a discovery of a new particle. In the conventional quark model, composite particles can be either mesons formed of quark–antiquark pairs or baryons formed of three quarks. Particles not classified within this scheme are known as exotic hadrons. When Murray Gell-Mann and George Zweig proposed the quark model in their 1964 papers, they mentioned the possibility of exotic hadrons such as pentaquarks, but it took 50 years to demonstrate their existence experimentally. In July 2015, the LHCb collaboration reported the Pc(4450)+ and Pc(4380)+ pentaquark structures. The new particle is a lighter companion to these pentaquark structures and its existence sheds new light into the nature of the entire family. Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. The five quarks might be assembled into a meson (one quark and one antiquark) and a baryon (three quarks), weakly bound together (Image: Daniel Dominguez/CERN) The analysis presented today at the Rencontres de Moriond quantum chromodynamics (QCD) conference used nine times more data from the Large Hadron Collider than the 2015 analysis. The data set was first analysed in the same way as before and the parameters of the previously reported Pc(4450)+ and Pc(4380)+ structures were consistent with the original results. As well as revealing the new Pc(4312)+ particle, the analysis also uncovered a more complex structure of Pc(4450)+ consisting of two narrow overlapping peaks, Pc(4440)+ and Pc(4457)+, with the two-peak structure having a statistical significance of 5.4 sigma. More experimental and theoretical study is still needed to fully understand the internal structure of the observed states. Read more on the LHCb website.
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The plot thickens for a hypothetical “X17” particleFresh evidence of an unknown particle that could carry a fifth force of nature gives the NA64 collaboration at CERN a new incentive to continue its searches. In 2015, a team of scientists spotted an unexpected glitch, or “anomaly”, in a nuclear transition that could be explained by the production of an unknown particle. About a year later, theorists suggested that the new particle could be evidence of a new fundamental force of nature, in addition to electromagnetism, gravity and the strong and weak forces. The findings caught the attention of physicists worldwide and prompted, among other studies, a direct search for the particle by the NA64 collaboration at CERN. A new paper from the same team, led by Attila Krasznahorkay at the Atomki institute in Hungary, now reports another anomaly, in a similar nuclear transition, that could also be explained by the same hypothetical particle. The first anomaly spotted by Krasznahorkay’s team was seen in a transition of beryllium-8 nuclei. This transition emits a high-energy virtual photon that transforms into an electron and its antimatter counterpart, a positron. Examining the number of electron–positron pairs at different angles of separation, the researchers found an unexpected surplus of pairs at a separation angle of about 140º. In contrast, theory predicts that the number of pairs decreases with increasing separation angle, with no excess at a particular angle. Krasznahorkay and colleagues reasoned that the excess could be interpreted by the production of a new particle with a mass of about 17 million electronvolts (MeV), the “X17” particle, which would transform into an electron–positron pair. The latest anomaly reported by Krasznahorkay’s team, in a paper that has yet to be peer-reviewed, is also in the form of an excess of electron–positron pairs, but this time the excess is from a transition of helium-4 nuclei. “In this case, the excess occurs at an angle of 115º but it can also be interpreted by the production of a particle with a mass of about 17 MeV,” explained Krasznahorkay. “The result lends support to our previous result and the possible existence of a new elementary particle,” he adds. Sergei Gninenko, spokesperson for the NA64 collaboration at CERN, which has not found signs of X17 in its direct search, says: “The Atomki anomalies could be due to an experimental effect, a nuclear physics effect or something completely new such as a new particle. To test the hypothesis that they are caused by a new particle, both a detailed theoretical analysis of the compatibility between the beryllium-8 and the helium-4 results, as well as independent experimental confirmation, is crucial.” The NA64 collaboration searches for X17 by firing a beam of tens of billions of electrons from the Super Proton Synchrotron accelerator onto a fixed target. If X17 did exist, the interactions between the electrons and nuclei in the target would sometimes produce this particle, which would then transform into an electron–positron pair. The collaboration has so far found no indication that such events took place, but its datasets allowed them to exclude part of the possible values for the strength of the interaction between X17 and an electron. The team is now upgrading their detector for the next round of searches, which, Gninenko says, are expected to be more challenging but at the same time more exciting. Among other experiments that could also hunt for X17 in direct searches is LHCb. Jesse Thaler, a theoretical physicist from the Massachusetts Institute of Technology, says: “By 2023, the LHCb experiment should be able to make a definitive measurement to confirm or refute the interpretation of the Atomki anomalies as arising from a new fundamental force. In the meantime, experiments such as NA64 can continue to chip away at the possible values for the hypothetical particle’s properties, and every new analysis brings with it the possibility (however remote) of discovery.”
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FASER’s new detector expected to catch first collider neutrinoNo neutrino produced at a particle collider has ever been detected, even though colliders create them in huge numbers. This could now change with the approval of a new detector for the FASER experiment at CERN. The small and inexpensive detector, called FASERν, will be placed at the front of the FASER experiment’s main detector, and could launch a new era in neutrino physics at particle colliders. Ever since they were first observed at a nuclear reactor in 1956, neutrinos have been detected from many sources, such as the sun, cosmic-ray interactions in the atmosphere, and the Earth, yet never at a particle collider. That’s unfortunate, because most collider neutrinos are produced at very high energies, at which neutrino interactions have not been well studied. Neutrinos produced at colliders could therefore shed new light on neutrinos, which remain the most enigmatic of the fundamental particles that make up matter. The main reasons why collider neutrinos haven’t been detected are that, firstly, neutrinos interact very weakly with other matter and, secondly, collider detectors miss them. The highest-energy collider neutrinos, which are more likely to interact with the detector material, are mostly produced along the beamline – the line travelled by particle beams in a collider. However, typical collider detectors have holes along the beamline to let the beams through, so they can’t detect these neutrinos. Enter FASER, which was approved earlier this year to search for light and weakly interacting particles such as dark photons – hypothetical particles that could mediate an unknown force that would link visible matter with dark matter. FASER, supported by the Heising-Simons and Simons Foundations, will be located along the beamline of the Large Hadron Collider (LHC), about 480 metres downstream of the ATLAS experiment, so it will be ideally positioned to detect neutrinos. However, the detection can’t be done with the experiment’s main detector. “Since neutrinos interact very weakly with matter, you need a target with a lot of material in it to successfully detect them. The main FASER detector doesn’t have such a target, and is therefore unable to detect neutrinos, despite the huge number that will traverse the detector from the LHC collisions,” explains Jamie Boyd, co-spokesperson for the FASER experiment. “This is where FASERν, an idea previously considered by CERN theorist Alvaro de Rújula, comes in. It is made up of emulsion films and tungsten plates, and acts both as the target and the detector to see the neutrino interactions.” FASERν is only 25 cm wide, 25 cm tall and 1.35 m long, but weighs 1.2 tonnes. Current neutrino detectors are generally much bigger, for example Super-Kamiokande, an underground neutrino detector in Japan, weighs 50 000 tonnes, and the IceCube detector in the South Pole has a volume of a cubic kilometre. After studying FASER’s ability to detect neutrinos and doing preliminary studies using pilot detectors in 2018, the FASER collaboration estimated that FASERν could detect more than 20 000 neutrinos. These neutrinos would have a mean energy of between 600 GeV and 1 TeV, depending on the type of neutrino produced. Indeed there are three types of neutrinos – electron neutrino, muon neutrino and tau neutrino – and the collaboration expects to detect 1300 electron neutrinos, 20 000 muon neutrinos and 20 tau neutrinos. “These neutrinos will have the highest energies yet of man-made neutrinos, and their detection and study at the LHC will be a milestone in particle physics, allowing researchers to make highly complementary measurements in neutrino physics,” says Boyd. “What’s more, FASERν may also pave the way for neutrino programmes at future colliders, and the results of these programmes could feed into discussions of proposals for much larger neutrino detectors.” The FASERν detector will be installed before the next LHC run, which will start in 2021, and it will collect data throughout this run.
A new schedule for the LHC and its successorThe CERN Management has presented a new calendar for future accelerator runs to the Council, which met on 12 December. Under the new schedule, the LHC will restart in May 2021, two months after the initially planned date, and Run 3 will be extended by one year, until the end of 2024. All of the equipment needed for the High-Luminosity LHC, the LHC’s successor, and its experiments will be installed during Long Shutdown 3, between 2025 and mid-2027. The High-Luminosity LHC is scheduled to come into operation at the end of 2027. For the last year, extensive upgrades of CERN’s accelerator complex and experiments in preparation for the next LHC run and the High-Luminosity LHC have been under way. Major work is being carried out on all the machines and infrastructures: the particle accelerator chain is being entirely renovated as part of the LHC Injectors Upgrade (LIU) project, new equipment is being installed in the LHC, where upgrades are also ongoing, and the experiments are replacing numerous components, even entire subdetectors, in order to prepare for high luminosity (read also about upgrades at ALICE, ATLAS, CMS and LHCb). The High-Luminosity LHC will generate many more collisions than the LHC, accumulating ten times more data than its predecessor throughout its operation. This groundbreaking machine will thus be able to detect extremely rare phenomena and improve the precision of measurements of the infinitesimally small. In order to fully exploit the increased quantity of data, the experiments have embarked upon ambitious detector upgrade programmes. The extra time will enable them to ready themselves for Run 3 and, then, for the High-Luminosity LHC.
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LS2 Report: Linac4 knocking at the door of the PS BoosterBusy activity has returned to the CERN Control Centre (CCC), where the Operation group coordinates the current Linac4 test run, supported by the Accelerators and Beam Physics (ABP) group and all the involved equipment groups. As we write, the nominal 160 MeV beam has already reached the Linac4 dump. After the ongoing second long shutdown of CERN’s accelerator complex (LS2), Linac4 will replace the retired Linac2 to provide protons to the LHC and all the CERN experiments that are served by CERN’s proton-accelerator chain. “For this purpose, Linac4 has by now been connected to the LHC injector chain through its new transfer line to the PS tunnel and the existing transfer lines to the PS Booster (PSB),” says Bettina Mikulec, who coordinates the test run. In order to prepare Linac4 for the challenge of delivering reliably high-quality beam to the PSB from its first day of post-LS2 operation in 2020, a special beam run started on 30 September to measure its beam parameters in the completely renovated 15-m-long emittance-measurement line (LBE), only 50 m away from the PSB injection point. “In this line, we can of course measure the emittance of the beam – the transverse dimensions of the beam and its energy spread – but also its longitudinal parameters in the transfer line,” adds Bettina Mikulec. “The LBE line is also very close to the PSB entrance, so everything we measure there should not be far off from the final characteristics at the PSB injection point.” The new LBE line. At the end of this line, we can see the LBE dump (green shielding blocks) located just at the wall of the PSB (Image: CERN) Until 6 December, negative hydrogen ions* will cover the 86 m of the Linac4 to be sent along the new and renovated transfer lines (160 m in total) into the new LBE line, before terminating their flight in a dump located just at the wall of the PSB. “During this run, hardware and software changes deployed since the last Linac4 reliability run, which took place in 2018, will be commissioned, with the aim of solving some remaining issues and enhance the beam quality,” says Alessandra Lombardi, Linac4 project leader in the ABP group. In addition, the required operational beam configurations for the 2020 start-up will be prepared as much as possible in advance to allow a rapid Linac4 start in April 2020, when various beams will be set up for the PSB, so that the PSB can restart with beam under optimum conditions in September 2020. * Hydrogen atoms with an additional electron, giving it a net negative charge. Both electrons are stripped during injection from Linac4 into the PS Booster to leave only protons. ________ Follow the progress of the LBE run on the online Vistar.
Successful tests of a cooler way to transport electricityLike a metal python, the huge pipe snaking through a CERN high-tech hall is actually a new electrical transmission line. This superconducting line is the first of its kind and allows vast quantities of electrical current to be transported within a pipe of a relatively small diameter. Similar pipes could well be used in towns in the future. This 60-metre-long line has been developed for CERN’s future accelerator, the High-Luminosity LHC, which is due to come into operation in 2026. Tests began last year and the line has transported 40 000 amps. This is 20 times more than what is possible at room temperature with ordinary copper cables of a similar cross-section. The line is composed of superconducting cables made from magnesium diboride (MgB2) and offers no resistance, enabling it to transport much higher current densities than ordinary cables, without any loss. The snag is that, in order to function in a superconducting state, the cables must be cooled to a temperature of 25 K (-248°C). It is therefore placed inside a cryostat, a thermally insulated pipe in which a coolant, namely helium gas, circulates. The real achievements are the development of a new, flexible superconducting system and the use of a new superconductor (MgB2). “The line is more compact and lighter than its copper equivalent, and it is cryogenically more efficient than a classical low temperature superconducting link that must be cooled to 4.5 K”, says Amalia Ballarino, the project leader. Having proven that such a system is feasible, at the end of March the team tested the connection to the room temperature end of the system. In the High-Luminosity LHC, these lines will connect power converters to the magnets. These converters are located at a certain distance from the accelerator. The new superconducting transmission lines, which measure up to 140 m in length, will feed several circuits and transport electrical current of up to 100 000 amps. “The magnesium diboride cable and the current leads that supply the magnets are connected by means of high-temperature ReBCO (rare-earth barium copper oxide) superconductors, also a challenging innovation for this type of application,” explains Amalia Ballarino. These superconductors are called “high-temperature” because they can operate at temperatures of up to around 90 kelvins (-183 °C), as opposed to just a few kelvins in the case of classical low-temperature superconductors. They can transport very high current densities, but are very tricky to work with, hence the impressiveness of the team’s achievement. Tests of the line with its new connection represent an important milestone in the project, as it proves that the whole system works correctly. “We have new materials, a new cooling system and unprecedented technologies for supplying the magnets in an innovative way,” says Amalia Ballarino. The project has also caught the attention of the outside world. Companies are using the work done at CERN to study the possibility of using similar transmission lines (at high voltage), instead of conventional systems, to transport electricity and power over long distances.
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Briefing book for 2020 update of European Strategy for Particle PhysicsThe particle physics community in Europe is in the midst of updating the European Strategy for Particle Physics. The latest input is a newly published 250-page physics briefing book, the result of an intense year-long effort to capture the status and prospects for experiment, theory, accelerators and computing for high-energy physics. The CERN Council first initiated the European Strategy process in 2005. It was updated in 2013 and the latest update was launched in 2018. In a truly collaborative initiative, the particle physics community submitted 160 contributions, and discussed the potential merits and challenges in an open symposium in Granada, Spain, in May. This briefing book now distills inputs to provide an objective scientific summary, which will form the basis of final discussions early next year. An important element of the European strategy update, given the long time scales involved, is to consider which major collider should follow the LHC. The Granada symposium revealed there is clear support for an electron-positron collider to study the Higgs boson in greater detail, but four possible options at different stages of maturity exist: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or Future Circular Collider (FCC-ee) at CERN and a Circular Electron Positron Collider (CEPC) in China. Also considered are design studies in Europe for colliders that push the energy frontier, including a 3 TeV upgrade of CLIC and a 100 TeV circular hadron collider (FCC-hh). The vast bulk of the briefing book details the current physics landscape and prospects for progress, including physics beyond the Standard Model and dark-sector exploration. It stresses the vital roles of detector and accelerator development as well as computing and instrumentation, with an emphasis on energy efficiency. The diversity of the global theoretical and experimental programme is a strong feature to tackle ongoing puzzles in particle physics. With this latest input to the process, the next steps involve drafting recommendations in Bad Honnef, Germany, in January, with their submission for the approval of the CERN Council foreseen in Budapest, Hungary, in May 2020. Read more in the full CERN Courier article.
Developers revive first Web browser at week-long hackathonAn old NeXT Computer from the early ’90s, borrowed from a group of computer enthusiasts in Lausanne, sits in a corner, its screen showing a black-and-white command prompt on the old NeXTstep operating system. Programmers and developers from around the world gather around an oblong table with their computers, having animated conversations about “anti-aliased fonts” and “browser binaries”. Next door, a gigantic room houses the CERN Data Centre’s servers, where all of the Laboratory’s computing is done, as well as where the data from the Large Hadron Collider’s experiments are stored. The same room also hosted CERN’s first Internet connection in 1989 and today hosts the CERN Internet eXchange Point (CIXP). The developers are here to recreate the first Web browser, which was built at CERN in 1990 by Sir Tim Berners-Lee to browse pages on the Web, also his invention. You are presumably reading this article in a browser, which shows content in HyperText Markup Language or HTML, on a mobile or desktop operating system. Sir Tim’s original browser, initially called WorldWideWeb itself and later rebranded “Nexus”, could only run on the NeXT Computer on which he wrote his code. But this team of developers are now aiming to run the WorldWideWeb browser on today’s operating systems by building on the capabilities of the Web itself! They are doing this by emulating the original browser within a modern browser using the popular JavaScript programming language, allowing you to indulge in the early-Web experience without needing to get your hands on an archaic NeXT Computer yourself. The team was first assembled in 2013 to recreate the “line-mode browser”, originally written by Nicola Pellow in 1991. Now on their second stint here, with less than a month to go before the 30th anniversary of the Web, they are faced with new challenges. “We have retrieved the code of the WorldWideWeb browser,” developer Remy Sharp explained on Monday, the first day of the five-day-long sprint. “But we haven’t been able to get it onto the NeXT machine so far.” Indeed, it is not trivial to interface with hardware that is many decades old. The team needed to run the software on the machine it was designed for in order to replicate the exact look and feel on a modern system. This includes, for example, ensuring that the “blocky” fonts of the NeXTstep operating system render similarly in the emulated WorldWideWeb browser, rather than the smoother treatment they receive on screens nowadays. Eventually, fellow programmer Kimberly Blessing managed to load the WorldWideWeb browser onto the borrowed computer. The participants of the sprint shared lunch with Web pioneer Robert Cailliau on Tuesday, where they discussed the mechanics of the early Web browsers, including the fact that the WorldWideWeb browser offered the possibility not just of reading a Web page but also of editing it in real time. With a few hours left in their sprint, which is supported by the US Mission in Geneva through the CERN & Society Foundation, the developers are busy ensuring that their work can be publicly released. Their project notes and the resurrected WorldWideWeb browser can be found at cern.ch/worldwideweb. For more information about the project to preserve some of the digital assets associated with the birth of the Web, please visit cern.ch/first-website.
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CERN pays tribute to Murray Gell-MannMurray Gell-Mann, one of the foremost figures in the development of the Standard Model of particle physics, and recipient of the 1969 Nobel prize in physics, passed away on 24 May at the age of 89. Gell-Mann was responsible for naming quarks, the elementary particles found within hadrons such as protons and neutrons: he borrowed the term from James Joyce’s Finnegans Wake. In 1961, Gell-Mann had introduced a scheme called the Eightfold Way for classifying hadrons, based on the mathematical symmetry known as SU(3), for which he won the Nobel prize. Gell-Mann built upon this work in a new model that could successfully describe – among other phenomena – the magnetic properties of protons and neutrons. But Gell-Mann’s model required there to be three new elementary particles, which he called quarks, whose existence he proposed in 1964. Independently, and in the same year, Georg Zweig also described these elementary particles, calling them “aces”. The existence of quarks was experimentally demonstrated in the late 1960s by experiments at the Stanford Linear Accelerator Center (SLAC). Subsequently, results from the Gargamelle bubble chamber at CERN contributed evidence showing that these particles have charges of ⅓ or ⅔ that of an electron or proton, as predicted by Gell-Mann and Zweig. Numerous experiments at CERN are exploring the theory that describes quarks and their interactions, called quantum chromodynamics. At the Large Hadron Collider (LHC), physicists are still discovering novel combinations of Gell-Mann and Zweig’s particles, further testing the Standard Model. Gell-Mann spent some time at CERN in the ’60s, and returned in the late-’70s, when he lectured on the grand unification of the different forces in nature. In his later life, Gell-Mann turned his curiosity and attention to linguistics, among other fields, and led the Evolution of Human Languages programme at the Santa Fe Institute, which he co-founded. He was also the Robert Andrews Millikan Professor Emeritus at Caltech. An interview with Murray Gell-Mann on the occasion of his visit to CERN in 2012 (Video: CERN) Also read: Gell-Mann’s obituary published by Caltech Fifty years of quarks (published in 2014)
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From cosmic rays to cloudsCERN’s colossal complex of accelerators is in the midst of a two-year shutdown for upgrade work. But that doesn’t mean all experiments at the Laboratory have ceased to operate. The CLOUD experiment, for example, has just started a data run that will last until the end of November. The CLOUD experiment studies how ions produced by high-energy particles called cosmic rays affect aerosol particles, clouds and the climate. It uses a special cloud chamber and a beam of particles from the Proton Synchrotron to provide an artificial source of cosmic rays. For this run, however, the cosmic rays are instead natural high-energy particles from cosmic objects such as exploding stars. “Cosmic rays, whether natural or artificial, leave a trail of ions in the chamber,” explains CLOUD spokesperson Jasper Kirkby, “but the Proton Synchrotron provides cosmic rays that can be adjusted over the full range of ionisation rates occurring in the troposphere, which comprises the lowest ten kilometres of the atmosphere. That said, we can also make progress with the steady flux of natural cosmic rays that make it into our chamber, and this is what we’re doing now.” In its 10 years of operation, CLOUD has made several important discoveries on the vapours that form aerosol particles in the atmosphere and can seed clouds. Although most aerosol particle formation requires sulphuric acid, CLOUD has shown that aerosols can form purely from biogenic vapours emitted by trees, and that their formation rate is enhanced by cosmic rays by up to a factor 100. Most of CLOUD’s data runs are aerosol runs, in which aerosols form and grow inside the chamber under simulated conditions of sunlight and cosmic-ray ionisation. The run that has just started is of the “CLOUDy” type, which studies the ice- and liquid-cloud-seeding properties of various aerosol species grown in the chamber, and direct effects of cosmic-ray ionisation on clouds. The present run uses the most comprehensive array of instruments ever assembled for CLOUDy experiments, including several instruments dedicated to measuring the ice- and liquid-cloud-seeding properties of aerosols over the full range of tropospheric temperatures. In addition, the CERN CLOUD team has built a novel generator of electrically charged cloud seeds to investigate the effects of charged aerosols on cloud formation and dynamics. “Direct effects of cosmic-ray ionisation on the formation of fair-weather clouds are highly speculative and almost completely unexplored experimentally,” says Kirkby. “So this run could be the most boring we’ve ever done – or the most exciting! We won’t know until we try, but by the end of the CLOUD experiment, we want to be able to answer definitively whether cosmic rays affect clouds and the climate, and not leave any stone unturned.”
Highlights from the 2019 Moriond conference (electroweak physics)At the 66th Rencontres de Moriond conference, which is taking place in La Thuile, Italy, physicists working at CERN are presenting their most recent results. Since the start of the conference on 16 March, a wide range of topics from measurements of the Higgs boson and Standard Model processes to searches for rare and exotic phenomena have been presented. The Standard Model of particle physics is a successful theory that describes how elementary particles and forces govern the properties of the Universe, but it is incomplete as it cannot explain certain phenomena, such as gravity, dark matter and dark energy. For this reason, physicists welcome any measurement that shows discrepancies with the Standard Model, as these give hints of new particles and new forces – of new physics, in other words. At the conference, the ATLAS and CMS collaborations have presented new results based on up to 140 fb–1 of proton-proton collision data collected during Run 2 of the Large Hadron Collider (LHC) from 2015 to 2018. Many of these analyses benefited from novel machine-learning techniques used to extract data from background processes. Since the discovery of the Higgs boson in 2012, ATLAS and CMS physicists have made significant progress in understanding its properties, how it is formed and how it interacts with other known particles. Thanks to the large quantity of Higgs bosons produced in the collisions of Run 2, the collaborations were able to measure most of the Higgs boson’s main production and decay modes with a statistical significance far exceeding five standard deviations. In addition, many searches for new, additional Higgs bosons have been presented. From a combination of all Higgs boson measurements, ATLAS obtained new constraints on the Higgs self-coupling. CMS has presented updated results on the Higgs decay to two Z bosons and has also derived new information on the strength of the interaction between Higgs bosons and top quarks. This interaction is measured in two ways, using top quark pairs and using a rare process in which four top quarks are produced. The probability of four top quarks being produced at the LHC is about a factor of ten less likely than the production of Higgs bosons together with two top quarks, and about a factor of ten thousand less likely than the production of just a top quark pair. ATLAS event display showing the clean signature of light-by-light scattering (Image: ATLAS/CERN) The ATLAS collaboration has also reported first evidence for the simultaneous production of three W or Z bosons, which are the mediator particles of the weak force. Tri-boson production is a rare process predicted by the Standard Model, and is sensitive to possible contributions from yet unknown particles or forces. The very large new dataset has also been used by the ATLAS and CMS collaborations to expand the searches for new particles beyond the Standard Model at the energy available at the LHC. One of the possible theories is supersymmetry, an extension of the Standard Model, which features a symmetry between matter and force and introduces many new particles, including possible candidates for dark matter. These hypothetical particles have not been detected in experiments so far, and the collaborations have set stronger lower limits on the possible range of masses that they could have. A collision event recorded by CMS, containing a missing-transverse-energy signature, which is one of the characteristics sought in the search for SUSY (Image: CMS/CERN) The CMS collaboration has placed new limits on the parameters of new physics theories that describe hypothetical slowly moving heavy particles. These are detected by measuring how fast particles travel through the detector: while the regular particles propagate at speeds close to that of light, straight from the proton collisions, these heavy particles are expected to move measurably slower before decaying into a shower of other particles, creating a “delayed jet”. CMS has also presented first evidence for another rare process, the production of two W bosons in not one but two simultaneous interactions between the constituents of the colliding protons. In addition, ATLAS and CMS have presented new studies on the search for hypothetical Z′ (Z-prime) bosons. The existence of such neutral heavy particles is predicted by certain Grand Unified theories that could provide an elegant extension of the Standard Model. Although no significant signs of Z′ particles have been observed thus far, the results provide constraints on their production rate. The LHCb collaboration has presented several new measurements concerning particles containing beauty or charm quarks. Certain properties of these particles can be affected by the existence of new particles beyond the Standard Model. This allows LHCb to search for signs of new physics via a complementary, indirect route. One much anticipated result, shown for the first time at the conference, is a measurement using data taken from 2011 to 2016 of the ratio of two related rare decays of a B+ particle. These decays are predicted in the Standard Model to occur at the same rate to within 1%; the data collected are consistent with this prediction but favour a lower value. This follows a pattern of intriguing hints in other, similar decay processes; while none of these results are significant enough to constitute evidence of new physics on their own, they have captured the interest of physicists and will be investigated further with the full LHCb data set. LHCb also presented the first observation of matter–antimatter asymmetry known as CP violation in charm particle decays, as reported in a dedicated press release last week. Finally, using the results of lead-ion collisions taken in 2018, the ATLAS collaboration has been able to clearly observe a very rare phenomenon in which two photons – particles of light – interact, producing another pair of photons, with a significance of over 8 standard deviations. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism, and is forbidden by Maxwell's classical theory of electrodynamics. Additional information: ATLAS news CMS news LHCb news
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CMS gets first result using largest ever LHC data sampleJust under three months after the final proton–proton collisions from the Large Hadron Collider (LHC)’s second run (Run 2), the CMS collaboration has submitted its first paper based on the full LHC dataset collected in 2018 – the largest sample ever collected at the LHC – and data collected in 2016 and 2017. The findings reflect an immense achievement, as a complex chain of data reconstruction and calibration was necessary to be able to use the data for analysis suitable for a scientific result. “It is truly a sign of effective scientific collaboration and the high quality of the detector, software and the CMS collaboration as a whole. I am proud and extremely impressed that the understanding of the so recently collected data is sufficiently advanced to produce this very competitive and exciting result,” said CMS spokesperson Roberto Carlin. Quantum chromodynamics (QCD) is one of the pillars of the Standard Model of elementary particles and describes how quarks and gluons are confined within composite particles called hadrons, of which protons and neutrons are examples. However, the QCD processes behind this confinement are not yet well understood, despite much progress in the last two decades. One way to understand these processes is to study the little known Bc particle family, which consists of hadrons composed of a beauty quark and a charm antiquark (or vice-versa). The high collision energies and rates provided by the Large Hadron Collider opened the path for the exploration of the Bc family. The first studies were published in 2014 by the ATLAS collaboration, using data collected during LHC’s first run. At the time, ATLAS reported the observation of a Bc particle called Bc(2S). On the other hand, the LHCb collaboration reported in 2017 that their data showed no evidence of Bc(2S) at all. Analysing the large LHC Run 2 data sample, collected in 2016, 2017 and 2018, CMS has now observed Bc(2S) as well as another Bc particle known as Bc*(2S). The collaboration has also been able to measure the mass of Bc(2S) with a good precision. These measurements provide a rich source of information on the QCD processes that bind heavy quarks into hadrons. For more information about the results visit the CMS webpage. The results were submitted to Physical Review Letters and presented at CERN this week.
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Upgrading ALICE: What’s in store for the next two years?With massive red doors weighing 350 tonnes each, it takes more than uttering “open sesame” to open the ALICE detector. Behind the doors lie the inner workings of a unique detector built to study the conditions of matter moments after the birth of the Universe, conditions which are recreated in the LHC. When the CERN accelerator complex was switched off in December 2018, scientists and technicians entered the ALICE cavern, 56 metres underground, to open the massive shielding around the magnet and to start work on the detector. This maintenance and upgrade work will last two years, the time CERN has allocated for a technical break called Long Shutdown 2 (LS2). For ALICE, LS2 activities started at a fast pace, with a full programme planned of upgrades or replacements of subdetectors as well as of trigger and data-acquisition systems. The 16-metre-tall doors of the ALICE experiment magnet, each weighing 350 tonnes, are now open to allow scientists and technicians to work on the detector upgrade. (Image: Julien Marius Ordan/CERN) ALICE is dedicated to the study of quark-gluon plasma (QGP), a state of matter that prevailed in the first instants of the Universe. By colliding particles, namely protons and lead nuclei, from the Large Hadron Collider (LHC), ALICE can harvest data at the high-energy frontier. Increased luminosity, first in 2021 and later in the High-Luminosity LHC (HL-LHC) project, will open up a range of possibilities and challenges for ALICE. An increase in luminosity – a measure of the number of collisions per unit of time – will allow ALICE to study rare phenomena and perform high-precision measurements, shedding light on the thermodynamics, evolution and flow of the QGP, as well as on quark and gluon interactions. Hunting for the right tracks, starting from the core During this upgrade, a smaller-diameter beam pipe will replace ALICE’s existing one. Inside the beam pipe, particles travel at almost the speed of light and smash together inside the core of the detector, generating many new particles. Scientists are interested in determining the position of the interaction point, and reducing the beam pipe’s diameter improves this measurement by a factor of three with respect to the present detector. ALICE will also become better at detecting particles with a shorter lifetime, i.e. those decaying closer to the interaction point. The need for a new beam pipe is linked to the replacement of the inner tracking system (ITS), which surrounds it. The new ITS will be equipped with innovative, compact pixel sensor chips. This tracking system measures the properties of the particles emerging from the collisions, so it must be fast-acting and fine-grained to handle the higher collision rates in the future. The new system will dramatically improve the capacity of the detector to pinpoint and reconstruct the particle trajectories. The sensor and readout chips built into the same piece of silicon for the new inner tracking system will also be employed in the muon forward tracker (MFT), which tracks muons close to the beam pipe. This promises excellent spatial resolution, making ALICE not only more sensitive to several measurements, but also able to access new ones currently beyond reach. A major upgrade of the ALICE time projection chamber (TPC), an 88-cubic-metre cylinder filled with gas and read-out detectors that follows particles’ trajectories in 3D, is also ongoing. Charged particles spraying out from the collision point ionise the gas along their path, liberating clouds of electrons that drift towards the endplates of the cylinder. These make up a signal that is amplified and then read. The current read-out, based on multi-wire proportional-chamber technology, will not be able to cope with increased interaction rates, so it will be replaced with multi-stage gas electron multiplier (GEM) chambers. This upgrade will increase the read-out rate of the detector by about two orders of magnitude. In addition, a new fast interaction trigger detector (FIT) will detect particles that scatter with a small angle relative to the beam direction and will replace three current trigger detectors. It will remove unwanted signals, including interactions of the beam with the residual gas in the beam pipe. A factor of 100 gain in statistics As a consequence of the increased luminosity and interaction rate, a significantly larger amount of data will have to be processed and selected. More powerful electronics, data processing and computing systems have therefore been designed to sustain high throughput and performance. The ALICE collaboration is currently installing a new data centre above ground to improve computing capacity. When the new LHC run starts in 2021, the significantly improved detector will offer a factor of 100 gain in statistics. Work has begun on the inner sub-detectors of the ALICE experiment ahead of the installation of new equipment. (Image: Maximilien Brice/Julien Marius Ordan/CERN) When ALICE’s magnet doors close again in summer 2020, they will hide an even more powerful instrument, ready to embark on more collisions and more data-taking. Take a 360° tour of ALICE (Video: CERN) Read more in “ALICE revitalised” in the latest CERN Courier, which also has LS2 highlights from ATLAS, CMS and LHCb. More photos from LHCb are available on CDS
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