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  • richardmitnick 8:11 am on March 25, 2019 Permalink | Reply
    Tags: "Highlights from Moriond: ATLAS explores the full Run 2 dataset", , , , , Particle Accelerators, ,   

    From CERN ATLAS: “Highlights from Moriond: ATLAS explores the full Run 2 dataset” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    From CERN ATLAS

    23rd March 2019
    Pierre Savard

    1
    Figure 1: The highest-mass dijet event measured by ATLAS (mass = 8.12 TeV). (Image: ATLAS Collaboration/CERN)

    This week, particle physicists from around the world gathered in La Thuile, Italy, for the annual Rencontres de Moriond conference on Electroweak Interactions and Unified Theories. It was one of the first major conferences to be held following the recent completion of the Large Hadron Collider’s (LHC) second operation period (Run 2). The ATLAS Collaboration unveiled a wide range of new results, including new analyses using the full Run 2 dataset, as well as some high-profile studies of Higgs, electroweak and heavy-ion physics.

    2
    Figure 2: The invariant mass spectrum of two electrons compared with the Standard Model prediction, and with putative signals from a Z’ boson. (Image: ATLAS Collaboration/CERN)

    First search results using the full Run 2 dataset

    Over the course of Run 2 of the LHC – from 2015 to 2018 – the ATLAS experiment collected 139 inverse femtobarn of proton-proton collision data for analysis. Though this data-taking period concluded just a few months ago, ATLAS physicists have already reported on a variety of new searches using the full Run 2 dataset. So far, all of these new searches are in agreement with the Standard Model expectation.

    The first of these analyses, released in a paper submitted to Physics Letters B, is a search for heavy neutral gauge bosons (denoted Z’) decaying into lepton pairs. The sensitivity of this analysis has increased significantly over the lifetime of the LHC, as seen in Figure 3. The new result sets exclusion limits on specific theoretical models up to a mass of 5.1 TeV.

    A similar search was also conducted by looking for new particles – or “resonances” – decaying to two jets of particles. These “dijet” searches reveal events with the highest energies observed at the LHC; an example of such an event can be seen in Figure 1. No evidence of significant resonant structures was observed in the mass spectrum probed with the Run 2 dataset.

    ATLAS physicists also presented a search for new particles decaying to two weak bosons (W of Z), where the weak bosons then decay to two jets each. As the resonance would be very heavy, the weak bosons produced would be highly energetic and would generate overlapping jets as they decay. Thus, identifying the weak bosons is particularly challenging, and required the development of new reconstruction and analysis techniques. These have substantially improved the sensitivity of the analysis, as illustrated in Figure 4, setting significantly improved constraints on the allowed parameter space for such heavy resonances decaying to W or Z bosons.

    New searches for supersymmetry were also presented. One analysis focused on electroweak production of supersymmetric particles called “charginos” and “sleptons” decaying into two electrons or muons, along with missing transverse momentum. A second analysis looked for long-lived supersymmetric partners of the top quark that decay further away from the collision point. Many other searches are ongoing at ATLAS that will probe vast regions of yet-unexplored supersymmetric parameter space.

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    Figure 3: Ratio of the observed limit to the Z’ cross section for the combination of the channels with two electrons and two muons. (Image: ATLAS Collaboration/CERN)

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    Figure 4: Comparison between the current and previous expected limits on the cross section times branching ratio for WW+WZ production. An extrapolation of the expected limits from the previous results to the current dataset size, assuming no change to the previous analysis strategy or its uncertainties, is also shown. (Image: ATLAS Collaboration/CERN)

    First measurement of the Higgs boson using the full Run 2 dataset

    ATLAS also released a new measurement of the rare production cross section of the Higgs boson in association with two top quarks (ttH), followed by the similarly rare decay of the Higgs boson to two photons. The ttH production process was first observed in 2018, though it required the combination of many Higgs decay channels. Using the full Run 2 dataset, the observation of ttH in a single Higgs decay channel – into a pair of photons – is now possible. This allowed for a measurement of the production rate with an uncertainty of 25%, with a central value that is compatible with the Standard Model prediction.

    An updated combination of Higgs analyses was also presented, setting new constraints on the Higgs coupling to other particles, as well as interesting indirect constraints on the elusive self-coupling of the Higgs boson with itself. This update includes analyses that use 80 fb-1 (the data taken from 2015 to 2017) and represents the most comprehensive and precise set of Higgs properties measurements presented by the collaboration to date. Other results reporting first evidence for the rare electroweak processes involving the production of three weak bosons were also shown at the conference.

    Observation of light-by-light scattering

    The scattering of light by light involves two incoming photons scattering off of each other and producing two outgoing photons. This is a purely quantum mechanical effect that is not predicted by the classical theory of electromagnetism. The scattering of light requires a very intense source of photons, which can be achieved by using the enormous electric fields generated by fully ionised lead ions. As the ions cross each other, the intense electric fields supply a beam of photons that can collide, effectively turning the Large Hadron Collider into a “Large Photon Collider”.

    Evidence for this process at the LHC was first reported by ATLAS in 2017 in Nature Physics, and was also seen by CMS. Using the much larger dataset collected in 2018, ATLAS was able to clearly observe this process with a significance of over 8 standard deviations and measure the cross section with an uncertainty of 19%. This was one of the first results presented at Moriond.

    New measurement of CP violation

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    Figure 5: The measured values of φs and ∆Γs, compared to measurements by other LHC experiments. (Image: ATLAS Collaboration/CERN)

    The observed asymmetry between matter and antimatter in the Universe (a symmetry breaking known as “CP violation”) is one the unresolved puzzles in particle physics. As the Standard Model is only able to explain part of this asymmetry, there is great motivation to search for additional sources in the form of new or larger CP violation phases. The LHC produces copious samples of B mesons that are used to measure CP violating processes. In a new analysis using 80 fb-1 of data, ATLAS investigated the decay of B-sub-s (Bs) mesons, which are composed of a bottom quark and a strange quark. Specifically, J/ψ φ decays were investigated to measure the CP-violating phase φs, the average decay width (Γs), and the width difference (∆Γs) between the physical Bs meson states.

    In the Standard Model, φs is predicted to be small. However, physics beyond the Standard Model could increase the size of the observed CP violation by enhancing the mixing phase φs with respect to the Standard Model value. The measured values of φs and ∆Γs are shown in Figure 5, and compared to measurements by other LHC experiments and to the Standard Model prediction.

    A week of rich and exciting results

    This week, ATLAS and other LHC experiments presented important new results, deepening our understanding of particle physics. The presentation of the first results with the full Run 2 dataset represent the first steps in the realisation of what will be a rich and exciting Run 2 physics programme. Though Moriond EW is now drawing to a close, the Moriond QCD conference will start on its heels on Sunday 24 March – expect more exciting new results.

    See the full article here .


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  • richardmitnick 2:12 pm on March 22, 2019 Permalink | Reply
    Tags: , , , , Muoscope-a new small-scale portable muon telescope, Particle Accelerators, ,   

    From CERN CMS: “A ‘muoscope’ with CMS technology” 

    Cern New Bloc

    Cern New Particle Event

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    From CERN CMS

    22 March, 2019
    Cristina Agrigoroae

    1
    The resistive plate chambers (RPC) at CMS are fast gaseous detectors that provide a muon trigger system (Image: CERN)

    Particle physicists are experts at seeing invisible things and their detecting techniques have already found many applications in medical imaging or the analysis of art works. Researchers from the CMS experiment at the Large Hadron Collider are developing a new application based on one of the experiment’s particle detectors: a new, small-scale, portable muon telescope, which will allow imaging of visually inaccessible spaces.

    CERN CMS Muoscope- a new, small-scale, portable muon telescope developed by the CMS Collaborators from Ghent University and the University of Louvain in Belgium

    Earth’s atmosphere is constantly bombarded by particles arriving from outer space. By interacting with atmospheric matter, they decay into a cascade of new particles, generating a flux of muons, heavier cousins of electrons. These cosmic-ray muons continue their journey towards the Earth’s surface, travelling through almost all material objects.

    This “superpower” of muons makes them the perfect partners for seeing through thick walls or other visually challenging subjects. Volcanic eruptions, enigmatic ancient pyramids, underground caves and tunnels: these can all be scanned and explored from the inside using muography, an imaging method using naturally occurring background radiation in the form of cosmic-ray muons.

    Large-area muon telescopes have been developed in recent years for many different applications, some of which use technology developed for the LHC detectors. The muon telescope conceived by CMS researchers from two Belgian universities, Ghent University and the Catholic University of Louvain, is compact and light and therefore easy to transport. It is nonetheless able to perform muography at high resolution. It will be the first spin-off for muography using the CMS Resistive Plate Chambers (RPC) technology. A first prototype of the telescope, also baptised a “muoscope”, has been built with four RPC planes with an active area of 16×16 cm. The same prototype was used in the “UCL to Mars” project; it was tested for its robustness in a simulation of Mars-like conditions in the Utah Desert, where it operated for one month and later came back fully functional.

    Other CMS technologies have been used in muon tomography for security and environmental protection, as well as for homeland security.

    Learn more about the muon telescope here.

    See the full article here.


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  • richardmitnick 11:45 am on March 21, 2019 Permalink | Reply
    Tags: , , Particle Accelerators, , , , , "LHCb discovers matter-antimatter asymmetry in charm quarks", “They might look nearly identical from the outside but they behave differently” says Ivan Polyakov.“This is the puzzle of antimatter.”   

    From Symmetry: “LHCb discovers matter-antimatter asymmetry in charm quarks” 

    Symmetry Mag
    From Symmetry

    03/21/19
    Sarah Charley

    A new observation by the LHCb experiment finds that charm quarks behave differently than their antiparticle counterparts.

    1
    Artwork by Sandbox Studio, Chicago with Ana Kova

    CERN/LHCb detector

    Scientists on the LHCb experiment at the Large Hadron Collider at CERN have discovered a new way in which matter and antimatter behave differently.

    With 99.9999 percent statistical certainty, LHCb scientists have observed a difference between the decays of matter and antimatter particles containing charm quarks. This discovery opens up a new realm to study the differences between matter and antimatter and could help explain why we live in a matter-dominated universe.

    “This is a major breakthrough in experimental physics,” says Sheldon Stone, a professor at Syracuse University and collaborator on the LHCb experiment. “There’s been many attempts to make this measurement, but until now, no one had ever seen it. It’s a huge milestone in antimatter research.”

    Every structure in the universe—from the tiniest speck of dust to the mightiest star—is built from matter. But there is an equally qualified material for the job: antimatter. Antimatter is nearly identical to matter, except that its charge and magnetic properties are reversed. Precision studies of antihydrogen atoms, for example, have shown that their characteristics are identical to hydrogen atoms to beyond the billionth decimal place.

    Matter and antimatter cannot coexist in the same physical space because if they come into contact, they annihilate each other. This equal-but-opposite nature of matter and antimatter poses a conundrum for cosmologists, who theorize that the same amount of matter and antimatter should have exploded into existence during the birth of our universe. But if that’s true, all of that matter and antimatter should have annihilated one another, leaving nothing but energy behind.

    Particle physicists are looking for any tiny differences between matter and antimatter which could help explain why matter won out over antimatter in the early universe.

    Lucky for them, antimatter is not a totally extinct species. “We don’t usually see antimatter in our world,” says Ivan Polyakov, a postdoc at Syracuse University and internal LHCb reviewer for this new analysis. “But it can be produced when ordinary matter particles are smashed together at high energies, such as they do inside the Large Hadron Collider.”

    The main way scientists study the tiny and rare particles produced during the LHC’s collisions is by mapping how they decay and transform into more-stable byproducts.

    “This gives us a sort of family lineage for our ­particle of interest,” says Cesar da Silva, a scientist from Los Alamos National Lab and also a LHCb collaborator. “Once stable particles are measured by the detector, we can trace their ancestors to find the primordial generation of particles in the collision.

    “Because of quantum mechanics, we cannot predict what each single unstable particle will decay into, but we can figure out the probabilities for each possible outcome.”

    The new LHCb study looked at the decays of particles consisting of two bound quarks—the internal structural components of particles like protons and neutrons. One version of this particle (called D0 by scientists) contained a charm quark and the antimatter version of the up quark, called an anti-up quark. The other version contained the reverse, an up quark and an anti-charm quark.

    Scientists on the LHCb experiment identified tens of millions of both D0 and anti-D0 particles and counted how many times each transformed into one set of byproducts (a pair of particles called pions) versus another possible set (a pair of particles called kaons).

    With everything else being equal, the ratio of these two possible outcomes should have been identical for both D0 and anti-D0 particles. But scientists found that the two ratios differed by about a tenth of a percent—evidence that these charmed matter and antimatter particles are not totally interchangeable.

    “They might look nearly identical from the outside, but they behave differently,” Polyakov says. “This is the puzzle of antimatter.”

    The idea that matter and antimatter particles behave slightly differently is not new and has been observed previously in studies of particles containing strange quarks and bottom quarks. What makes this study unique is that it is the first time this asymmetry has been observed in particles containing charm quarks.

    Previous experiments—including BaBar, Belle and CDF—endeavored to make this same measurement but fell short of collecting enough data to to tease out such a subtle effect.

    SLAC BaBar

    Belle II KEK High Energy Accelerator Research Organization Tsukuba, Japan

    FNAL/Tevatron CDF detector

    The huge amount of data generated since the start of LHC Run 2 combined with the introduction of more advanced methods to tag the particles of interest enabled scientists to collect enough matter and antimatter D0 particles to finally see these decay differences beyond a shadow of a doubt.

    The next step is to see how this measurement fits with the theoretical models, which are still a little fuzzy on this prediction.

    “Theorists will need to figure out if the Standard Model can explain this,” Stone says.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    “We’re pushing our field and this result will certainly be in the history books.”

    See the full article here .


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  • richardmitnick 10:40 am on March 21, 2019 Permalink | Reply
    Tags: "All together now: adding more pieces to the Higgs boson puzzle", , , , Particle Accelerators, ,   

    From CERN ATLAS: “All together now: adding more pieces to the Higgs boson puzzle” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    18th March 2019

    1
    Figure 1: Cross sections time branching fraction for the main Higgs production modes at the LHC (ggF, VBF, VH and ttH+tH) in each relevant decay mode (γγ, WW, ZZ, ττ, bb). All values are normalized to Standard Model predictions. In addition, the combined results for each production cross-section are also shown, assuming the Standard Model values for the branching ratios into each decay mode. (Image: ATLAS Collaboration/CERN)

    The Higgs boson was discovered in 2012 by the ATLAS and CMS experiments, but its rich interaction properties (its coupling to other particles) remain a puzzle whose pieces the experiments on the Large Hadron Collider (LHC) are bringing together.

    Fortunately, the LHC provides many windows into measuring Higgs boson couplings. There are four main ways to produce the Higgs boson: through the fusion of two gluon particles (gluon-fusion, or ggF); through the fusion of weak vector bosons (VBF); or in association with a W or Z boson (VH), or one or more top quarks (ttH+tH). There are also five main channels in which Higgs bosons can decay: into pairs of photons, W or Z bosons, tau leptons or b quarks. Each of these processes brings unique insights into the Higgs boson properties – and a separate piece in the puzzle of its true nature.

    Thanks to the unprecedented amount of Higgs bosons produced at the LHC, all of the above production and decay modes have now been observed. In a new result presented by the ATLAS Collaboration, utilising data collected up to 2017, the measurements for each of these processes have reached the five standard deviation significance threshold, past which their existence is considered established.

    The Higgs boson yields for most of the combinations of production and decay have been measured (see Figure 1) and have been found to agree with Standard Model predictions. The measurement of the cross sections for each production mode in proton–proton collisions at 13 TeV, assuming the decays occur as predicted by the Standard Model, are the most precise ones obtained to date.

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    Figure 2: Combined measurements of the cross section for the different production phase-space regions (STXS) considered in the analysis normalized by the Standard Model expectations. These regions are defined by different ranges of Higgs transverse momentum, number of associated jets, interval of vector boson transverse momentum in the VH production mechanism with the vector boson decaying leptonically. In this combination, the Higgs decay branching fractions are fixed to Standard Model values. (Image: ATLAS Collaboration/CERN)

    As physicists have placed these new pieces, they’ve also begun to explore the Higgs boson puzzle in a new way. In the latest analyses, instead of counting Higgs bosons inclusively in the major production and decay modes, ATLAS physicists have measured Higgs boson topologies separately for smaller regions of phase-space: different ranges of Higgs boson transverse momentum, numbers of associated jets, and numbers and kinematic properties of associated weak bosons and top quarks. Using these smaller puzzle pieces, called “simplified template cross sections” (STXS), allows physicists to better separate the measurement process from the interpretation in terms of theoretical properties. Ultimately, it provides a finer-grained picture of Higgs boson couplings at the LHC and more stringent tests of the Standard Model.

    Among the STXS regions considered in the analysis, some have already been measured with good precision at the LHC (see Figure 2), but no deviation from the Standard Model has been observed so far. These measurements allow physicists to further enhance the sensitivity on the coupling properties of the Higgs boson to the other elementary particles. Further, they have set constraints on new physics theories – such as the “two-Higgs doublet model”, which introduces additional Higgs bosons, and the hMSSM supersymmetric model – which are more stringent than those reported previously by ATLAS.

    These measurements will continue to improve as more data from Run 2 and beyond are included, providing a yet-finer picture of the properties of the Higgs boson.

    See the full article here .


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  • richardmitnick 10:24 am on March 21, 2019 Permalink | Reply
    Tags: "ATLAS measures Higgs boson coupling to top quark in diphoton channel with full Run 2 dataset", , , , Particle Accelerators, ,   

    From CERN ATLAS: “ATLAS measures Higgs boson coupling to top quark in diphoton channel with full Run 2 dataset” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    18th March 2019

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    Figure 1: Visualisation of an event from the tt̄H(γγ) analysis. The event contains two photon candidates (green towers), while the b-jets are shown as yellow (blue) cones. (Image: ATLAS Collaboration/CERN)

    In 2018, the ATLAS and CMS Collaborations announced the observation of the production of the Higgs boson in association with a top-quark pair, known as “ttH” production. This result was the first observation of the Higgs boson coupling to quarks. It was followed shortly by the observation of Higgs boson decays to bottom quarks.

    As only about 1% of the Higgs bosons are produced in association with a top-quark pair at the Large Hadron Collider (LHC), achieving this observation was especially challenging. It was accomplished by searching across many different Higgs boson decay channels, including decays to two W or Z bosons (WW* or ZZ*), a pair of tau leptons, a pair of b-quarks, and a pair of photons (“diphoton”). Their combination established ttH production with a significance of 6.3 standard deviations. The diphoton channel alone, using 80 fb-1 of data recorded by ATLAS between 2015 and 2017, provided an observed significance of 4.1 standard deviations (for 3.7 standard deviations expected when assuming ttH production to occur as predicted by the Standard Model).

    At the Rencontres de Moriond (La Thuile, Italy), the ATLAS Collaboration presented an updated measurement of ttH production in the diphoton channel. The result examines the full Run 2 dataset – 139 fb-1 collected between 2015 and 2018 – to observe ttH production in a single channel with a significance of 4.9 standard deviations (for 4.2 expected).

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    Figure 2: The ttH signal in the diphoton invariant mass spectrum. Events from the different analysis categories are weighted according to the category sensitivity to the ttH signal. The ttH signal manifests itself as a localised resonant bump in the red curve, representing the fit to the data of the signal and background shapes. The other Higgs production modes provide a small contribution to the resonant peak, as shown by by the green dashed line. (Image: ATLAS Collaboration/CERN)

    The analysis techniques utilised in the new result followed closely those employed in the previously published analysis – with a few exceptions. To cope with the intense 2018 data-taking conditions, ATLAS physicists revised their data calibration and selection mechanisms. In particular, the result utilises a revised procedure for differentiating photons arising, for example, from a Higgs boson decay from those induced by hadron jets, as well as an adapted photon energy calibration. Additionally, ATLAS implemented a new calibration for hadron jets, especially for those issued from bottom quarks, whose presence in the event is used to identify the decay of top quarks.

    The ttH cross section times the Higgs-to-diphoton branching fraction (the probability that a Higgs boson will decay into a photon pair) was measured to be 1.58 ± 0.39 fb. Its ratio to the Standard Model prediction is 1.38 ± 0.41, in agreement with unity.

    ATLAS is now working on extending the analysis of the diphoton channel – which is sensitive both to ttH and the other Higgs production modes – to the full Run 2 dataset. This complete diphoton measurement will allow for an even more sensitive test of the Higgs mechanism, and will further refine the ttH measurement.

    See the full article here .


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  • richardmitnick 3:55 pm on March 20, 2019 Permalink | Reply
    Tags: , , , , Particle Accelerators, ,   

    From ALICE at CERN: “The subterranean ballet of ALICE” 

    From From ALICE at CERN

    19 March, 2019
    Corinne Pralavorio

    During the long shutdown of CERN’s accelerators, the ALICE experiment at the LHC is removing and refurbishing or replacing the majority of its detectors.

    CERN ALICE Time Projection Chamber (Image: Maximilien Brice/CERN)

    The experiment caverns of the Large Hadron Collider (LHC) are staging a dazzling performance during Long Shutdown 2 (LS2). The resplendent sub-detectors, released from their underground homes, are performing a fascinating ballet. At the end of February, ALICE removed the two trackers, the inner tracker system and the time projection chamber, from the detector. At the very start of the long shutdown, on 3 December 2018, the teams began disconnecting the dozens of sub-detectors. And finally, on 25 February, the two trackers were ready to be removed.

    The trackers are located around the collision points and are used to reconstruct the tracks of the particles produced in the collisions. The data they generate are essential for identifying the particles and understanding what happened during the collision. ALICE’s inner tracker is a 1.5-metre-long tube, 1 metre in diameter. It will be replaced with a new, much more precise detector closer to the collision point, formed of seven pixel layers and containing a total of 12.5 billion pixels. The current detector is still in the cavern and could spend its retirement as a museum piece in an exhibition above ground.

    CERN ALICE internal tracker system (Image: Maximilien Brice/ Julien Ordan CERN)

    The time projection chamber is an imposing cylinder, measuring 5.1 metres in length and 5.6 metres in diameter, weighing an enormous 15 tonnes. The huge sub-detector was nonetheless hoisted out in just four hours, to be transferred to a building where it will undergo a complete metamorphosis. The current detector is based on multiwire proportional chamber technology. To increase the detector’s acquisition speed by a factor of 100, the readout system will be equipped with much faster components called gas electron multipliers (GEMs), and the electronics will be completely replaced. The teams have started the renovation work, which should take around 11 months.

    At present, the removal process is continuing in the cavern. Most of the calorimeters have been removed for refurbishment. Around 50 people are hard at work at the experiment.

    4
    After the removal of the two trackers, ALICE’s heart is now empty. (Image: Julien Ordan/CERN)

    To find out more about the major work in progress at ALICE, see these articles on the website and in the CERN Courier.

    See the full article here .


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  • richardmitnick 9:54 am on March 20, 2019 Permalink | Reply
    Tags: "Report reveals full reach of LHC programme", , , , , Particle Accelerators, ,   

    From CERN: “Report reveals full reach of LHC programme” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    19 March, 2019
    Matthew Chalmers

    1
    The excavation of the two new shafts for the HL-LHC at points 1 and 5 of the accelerator has recently been completed. © Antonino Panté, Reproduced with permission.

    The High-Luminosity LHC (HL-LHC), scheduled to operate from 2026, will increase the instantaneous luminosity of the LHC by at least a factor of five beyond its initial design luminosity. The analysis of a fraction of the data already delivered by the LHC – a mere 6% of what is expected by the end of HL-LHC in the late-2030s – led to the discovery of the Higgs boson and a diverse set of measurements and searches that have been documented in some 2000 physics papers published by the LHC experiments. “Although the HL-LHC is an approved and funded project, its physics programme evolves with scientific developments and also with the physics programmes planned at future colliders,” says Aleandro Nisati of ATLAS, who is a member of the steering group for a new report quantifying the HL-LHC physics potential.

    The 1000+ page report, published in January, contains input from more than 1000 experts from the experimental and theory communities. It stems from an initial workshop at CERN held in late 2017 (CERN Courier January/February 2018 p44) and also addresses the physics opportunities at a proposed high-energy upgrade (HE-LHC). Working groups have carried out hundreds of projections for physics measurements within the extremely challenging HL-LHC collision environment, taking into account the expected evolution of the theoretical landscape in the years ahead. In addition to their experience with LHC data analysis, the report factors in the improvements expected from the newly upgraded detectors and the likelihood that new analysis techniques will be developed. “A key aspect of this report is the involvement of the whole LHC community, working closely together to ensure optimal scientific progress,” says theorist and steering-group member Michelangelo Mangano.

    Physics streams

    The physics programme has been distilled into five streams: Standard Model (SM), Higgs, beyond the SM, flavour and QCD matter at high density.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    The LHC results so far have confirmed the validity of the SM up to unprecedented energy scales and with great precision in the strong, electroweak and flavour sectors. Thanks to a 10-fold larger data set, the HL-LHC will probe the SM with even greater precision, give access to previously unseen rare processes, and will extend the experiments’ sensitivity to new physics in direct and indirect searches for processes with low-production cross sections and more elusive signatures. The precision of key measurements, such as the coupling of the Higgs boson to SM particles, is expected to reach the percent level, where effects of new physics could be seen. The experimental uncertainty on the top-quark mass will be reduced to a few hundred MeV, and vector-boson scattering – recently observed in LHC data – will be studied with an accuracy of a few percent using various diboson processes.

    The 2012 discovery of the Higgs boson opens brand-new studies of its properties, the SM in general, and of possible physics beyond the SM. Outstanding opportunities have emerged for measurements of fundamental importance at the HL-LHC, such as the first direct constraints on the Higgs trilinear self-coupling and the natural width. The experience of LHC Run 2 has led to an improved understanding of the HL-LHC’s ability to probe Higgs pair production, a key measure of its self-interaction, with a projected combined ATLAS and CMS sensitivity of four standard deviations. In addition to significant improvements on the precision of Higgs-boson measurements, the HL-LHC will improve searches for heavier Higgs bosons motivated by theories beyond the SM and will be able to probe very rare exotic decay modes thanks to the huge dataset expected.

    The new report considers a large variety of new-physics models that can be probed at HL-LHC. In addition to searches for new heavy resonances and supersymmetry models, it includes results on dark matter and dark sectors, long-lived particles, leptoquarks, sterile neutrinos, axion-like particles, heavy scalars, vector-like quarks, and more. “Particular attention is placed on the potential opened by the LHC detector upgrades, the assessment of future systematic uncertainties, and new experimental techniques,” says steering-group member Andreas Meyer of CMS. “In addition to extending the present LHC mass and coupling reach by 20–50% for most new-physics scenarios, the HL-LHC will be able to potentially discover, or constrain, new physics that is not in reach of the current LHC dataset.”

    Pushing for precision

    The flavour-physics programme at the HL-LHC comprises many different probes – the weak decays of beauty, charm, strange and top quarks, as well as of the τ lepton and the Higgs boson – in which the experiments can search for signs of new physics. ATLAS and CMS will push the measurement precision of Higgs couplings and search for rare top decays, while the proposed second phase of the LHCb upgrade will greatly enhance the sensitivity with a range of beauty-, charm-, and strange-hadron probes. “It’s really exciting to see the full potential of the HL-LHC as a facility for precision flavour physics,” says steering-group member Mika Vesterinen of LHCb. “The projected experimental advances are also expected to be accompanied by improvements in theory, enhancing the current mass-reach on new physics by a factor as large as four.”

    Finally, the report identifies four major scientific goals for future high-density QCD studies at the LHC, including detailed characterisation of the quark–gluon plasma and its underlying parton dynamics, the development of a unified picture of particle production, and QCD dynamics from small to large systems. To address these goals, high-luminosity lead–lead and proton–lead collision programmes are considered as priorities, while high-luminosity runs with intermediate-mass nuclei such as argon could extend the heavy-ion programme at the LHC into the HL-LHC phase.

    High-energy considerations

    High Energy LHC (HE-LHC)

    One of the proposed options for a future collider at CERN is the HE-LHC, a new pp collider in the LHC ring with CM energy in the range of 27 TeV, which would occupy the same tunnel but be built from advanced high-field dipole magnets that could support roughly double the LHC’s energy. Such a machine would be expected to deliver an integrated proton–proton luminosity of 15,000 fb–1 at a centre-of-mass energy of 27 TeV, increasing the discovery mass-reach beyond anything possible at the HL-LHC. The HE-LHC would provide precision access to rare Higgs boson (H) production modes, with approximately a 2% uncertainty on the ttH coupling, as well as an unambiguous observation of the HH signal and a precision of about 20% on the trilinear coupling. An HE-LHC would enable a heavy new Z´ gauge boson discovered at the HL-LHC to be studied in detail, and in general double the discovery reach of the HL-LHC to beyond 10 TeV.

    The HL/HE-LHC reports were submitted to the European Strategy for Particle Physics Update in December 2018, and are also intended to bring perspective to the physics potential of future projects beyond the LHC. “We now have a better sense of our potential to characterise the Higgs boson, hunt for new particles and make Standard Model measurements that restrict the opportunities for new physics to hide,” says Mangano. “This report has made it clear that these planned 3000 fb–1 of data from HL-LHC, and much more in the case of a future HE-LHC, will play a central role in particle physics for decades to come.”

    See the full article here.


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  • richardmitnick 12:25 pm on March 17, 2019 Permalink | Reply
    Tags: "ATLAS observes light scattering off light", , , , Heavy Ion, , HION group, Particle Accelerators, ,   

    From CERN ATLAS: “ATLAS observes light scattering off light” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    17th March 2019
    ATLAS Collaboration

    New result studies photons interacting at high energies.

    1
    Figure 1: ATLAS event display showing the energy deposits of two photons in the electromagnetic calorimeter (green) on opposite sides and no other activity in the detector, which is the clean signature of light-by-light scattering. The Feynman diagram of this process is shown in the lower right corner. (Image: ATLAS Collaboration/CERN)

    Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing again a pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism, and is forbidden in classical physics (such as Maxwell’s theory of electrodynamics).

    Direct evidence for light-by-light scattering at high energy had proven elusive for decades, until the Large Hadron Collider (LHC) began its second data-taking period (Run 2). Collisions of lead ions in the LHC provide a uniquely clean environment to study light-by-light scattering. The bunches of lead ions that are accelerated to very high energy are surrounded by an enormous flux of photons. Indeed, the coherent action from the large number of 82 protons in a lead atom with all the electrons stripped off (as is the case for the lead ions in the LHC) give rise to an electromagnetic field of up to 1025 Volt per metre. When two lead ions pass close by each other at the centre of the ATLAS detector, but with a distance greater than twice the lead ion radius, those photons can still interact and scatter off one another without any further interaction between the lead ions, as the reach of the (much stronger) strong force is bound to the radius of a single proton. These interactions are known as ultra-peripheral collisions.

    In a result published in Nature Physics in 2017, the ATLAS Collaboration found thirteen candidate events for light-by-light scattering in lead-lead collision data recorded in 2015, for 2.6 events expected from background processes. The corresponding significance of this result was 4.4 standard deviations – making it the first direct evidence of high-energy light-by-light scattering.

    2
    Figure 2: Invariant mass distribution of the measured final state photon pairs (markers), compared to the expected light-by-light scattering signal (red line) and expected background contributions (shaded areas). (Image: ATLAS Collaboration/CERN)

    Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS Collaboration reported the observation of light-by-light scattering with a significance of 8.2 standard deviations. The result utilises data from the most recent heavy-ion operation of the LHC, which took place in November 2018. About 3.6 times more events (1.73 nb−1) were collected compared to 2015. The increased dataset, in combination with improved analysis techniques, allowed the measurement of the scattering of light-by-light with greatly improved precision. A total of 59 candidate events were observed (see Figure 2), for 12 events expected from background processes. From these numbers, the cross section of this process, restricted to the kinematic region considered in the analysis, was calculated as 78 ± 15 nb.

    Curiously, the signature of this process – two photons in an otherwise empty detector (see the event display in Figure 1) – is almost the opposite of the tremendously rich and complex events typically observed in high-energy collisions of two lead nuclei. Observing it required the development of improved trigger algorithms for fast online event selection, as well as a specifically-adjusted photon-identification algorithm using a neural network, as the studied photons have about ten times less energy than the lowest energetic photons usually measured with the ATLAS detector. Being able to record these events demonstrates the power and flexibility of the ATLAS detector and its event reconstruction, which was designed for very different event topologies.

    This new measurement opens the door to further study the light-by-light scattering process, which is not only interesting in itself as a manifestation of an extremely rare QED phenomenon, but may be sensitive to contributions from particles beyond the Standard Model. It allows for a new generation of searches for hypothetical light and neutral particles.

    See the full article here .


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  • richardmitnick 11:57 am on March 17, 2019 Permalink | Reply
    Tags: "ATLAS finds evidence of three massive vector boson production", , , , , Particle Accelerators, ,   

    From CERN ATLAS: “ATLAS finds evidence of three massive vector boson production” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN

    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    17th March 2019
    ATLAS Collaboration

    Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS collaboration released evidence for the simultaneous production of three W or Z bosons in proton–proton collisions at the Large Hadron Collider (LHC). The W and Z bosons are the mediator particles of the weak force – one of the four known fundamental forces – which is responsible for the phenomenon of radioactivity as well as an essential ingredient to our Sun’s thermonuclear process.

    A new window for exploration

    The new ATLAS result is based on data collected by ATLAS during 2015–2017 at a collision energy of 13 TeV. It provides evidence of “tri-boson” events with a significance of 4 standard deviations. This indication is but the latest chapter in a decades-long history of measurements with weak bosons. The W and the Z bosons were discovered in 1983 at CERN’s proton-antiproton collider.

    CERN Proton-Antiproton Collider

    In 1996, at CERN’s Large Electron-Positron (LEP) collider, events with two W bosons were first observed, and shortly thereafter ZZ events were found.

    CERN LEP Collider


    CERN LEP Collider

    A decade after that, WW, WZ and ZZ events were observed at Fermilab’s Tevatron collider.

    FNAL/Tevatron map


    FNAL/Tevatron

    Large rates of diboson events are now produced at the LHC, allowing for precise measurements.

    Tri-boson production are rare processes predicted by the Standard Model of particle physics.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    Their production involves self-interaction among the weak bosons, so-called triple and quartic gauge boson couplings, which are sensitive to possible contributions from yet unknown particles or forces.

    1
    Figure 1: Data compared to expectation for the distribution of the invariant mass of two jets. The signal (VVV, in yellow) is scaled to the measured value. (Image: ATLAS Collaboration/CERN)

    2
    Figure 2: Data compared to expectation for the distribution of the BDT in one of the WVZ channels. The signal (VVV, in yellow) is scaled to the measured value. (Image: ATLAS Collaboration/CERN)

    A result years in the making

    3
    Figure 3: Combination of two measurements (normalised to their Standard Model predictions) in the WWW and two in the WVZ channels, in final states with a different number of leptons. (Image: ATLAS Collaboration/CERN)

    Since weak bosons are unstable, they are reconstructed in the detector via their decays to pairs of leptons (including invisible neutrinos) or quarks – the latter forming sprays of particles, called “jets”. ATLAS physicists combined searches for different decay modes and different types of tri-boson production, including events with three W bosons (“WWW”), and events with one W boson, one Z boson and a third boson of either variety. The latter are known as “WVZ” events, where the “V” is a shorthand for “W or Z”.

    One technique employed by ATLAS physicists to search for “WWW” events used the calculated invariant mass of two jets and compared this to the mass of the W boson (Figure 1). This allowed them to determine whether the jets were the outcome of a W boson decay. Such techniques have been used by physicists for decades (including in the 2012 discovery of the Higgs boson).

    The WVZ analysis, on the other hand, employs machine learning techniques to identify tri-boson events. Several multivariate algorithms in form of boosted decision trees (BDTs) were trained to learn which events in data are from tri-boson production and which arise from other Standard Model processes (Figure 2). By considering various features of the event – such as the momenta of the leptons, the overall momentum imbalance and the number of jets – the BDTs are able to deduce (more efficiently than humans) the origin of the data. Ultimately, the BDTs identified some of the data as likely originating from WVZ production.

    Altogether, the resulting ATLAS measurement (Figure 3) is found to be in agreement with the Standard Model prediction, thus providing one more piece of the puzzle in our understanding of particle physics.

    See the full article here .


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  • richardmitnick 1:10 pm on March 15, 2019 Permalink | Reply
    Tags: , , , , , Particle Accelerators, ,   

    From Fermi National Accelerator Lab: “Fermilab, international partners break ground on new state-of-the-art particle accelerator” 

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

    March 15, 2019
    Andre Salles, Fermilab Office of Communication
    asalles@fnal.gov
    630-840-6733

    With a ceremony held today, the U.S. Department of Energy’s Fermi National Accelerator Laboratory officially broke ground on a major new particle accelerator project that will power cutting-edge physics experiments for many decades to come.

    The new 700-foot-long linear accelerator, part of the laboratory’s Proton Improvement Plan II (PIP-II), will be the first accelerator project built in the United States with significant contributions from international partners. When complete, the new machine will become the heart of the laboratory’s accelerator complex, vastly improving what is already the world’s most powerful particle beam for neutrino experiments and providing for the long-term future of Fermilab’s diverse research program.

    The new PIP-II accelerator will make use of the latest superconducting technology, a key research area for Fermilab. Its flexible design will enable it to work as a new first stage for Fermilab’s chain of accelerators, powering both the laboratory’s flagship project — the international Deep Underground Neutrino Experiment (DUNE), hosted by Fermilab — and its extensive suite of on-site particle physics experiments, including searches for new particles and new forces in our universe.

    1
    On Friday, March 15, Fermilab broke ground on the PIP-II accelerator project, joined by dignitaries from the United States and international partners on the project. From left: Senator Tammy Duckworth (IL), Senator Dick Durbin (IL), Rep. Sean Casten (IL-6), Rep. Robin Kelly (IL-2), Rep. Bill Foster (IL-11), Fermilab Director Nigel Lockyer, Rep. Lauren Underwood (IL-14), Illinois Governor JB Pritzker, DOE Under Secretary for Science Paul Dabbar, PIP-II Project Director Lia Merminga, DOE Associate Director for High Energy Physics Jim Siegrist, University of Chicago President Robert Zimmer, Consul General of India Neeta Bhushan, British Consul General John Saville, Consul General of Italy Giuseppe Finocchiaro, Consul General of France Guillaume Lacroix, DOE Fermi Site Office Manager Mike Weis, DOE PIP-II Federal Project Director Adam Bihary and Consul General of Poland Piotr Janicki. Photo: Reidar Hahn

    DUNE is under construction now and will be the most advanced experiment in the world studying ghostly, invisible particles called neutrinos. These particles may hold the key to cosmic mysteries that have baffled scientists for decades. The DUNE collaboration brings together more than 1,000 scientists from over 180 institutions in more than 30 countries, all with a single goal: to better understand these elusive particles and what they can tell us about the universe.

    The PIP-II accelerator will enable the beam that will send trillions of neutrino particles 800 miles (1,300 kilometers) through the earth to the four-story-high DUNE detector, to be built a mile beneath the surface at the Sanford Underground Research Facility [SURF] in Lead, South Dakota. With the improved particle beam enabled by PIP-II, scientists will use the DUNE detector to capture the most vivid 3-D images of neutrino interactions ever seen.

    3
    Shortly after breaking ground on the PIP-II accelerator project on Friday, March 15, Fermilab employees were joined by the governor of Illinois, six members of Congress and partners from around the world in this group photo. Photo: Reidar Hahn

    PIP-II is itself a groundbreaking scientific instrument, and its construction is pioneering a new paradigm for accelerator projects supported by DOE. The accelerator would not be possible without the contributions and world-leading expertise of partners in France, India, Italy and the UK. Scientists in each country are building components of the accelerator, to be assembled at Fermilab. This will be the first accelerator project in the United States completed using this approach.

    With PIP-II at the center of the laboratory’s accelerator complex, Fermilab will remain at the forefront of particle physics research and accelerator science for the foreseeable future.

    Today’s groundbreaking ceremony for the PIP-II accelerator was attended by dignitaries from around the globe. Speakers included Sen. Dick Durbin (IL), Sen. Tammy Duckworth (IL), Rep. Lauren Underwood (IL-14), Rep. Bill Foster (IL-11), Rep. Robin Kelly (IL-2), Rep. Sean Casten (IL-6), DOE Under Secretary for Science Paul Dabbar, University of Chicago President Robert Zimmer, and national and international partners in the project.

    4
    This architectural rendering shows the buildings that will house the new PIP-II accelerators. Architectural rendering: Gensler. Image: Diana Brandonisio.

    See the full article here .


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    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

    FNAL MINERvA front face Photo Reidar Hahn

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