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  • richardmitnick 11:32 am on August 14, 2017 Permalink | Reply
    Tags: , ATLAS sees first direct evidence of light-by-light scattering at high energy, , , , Particle Accelerators,   

    From ATLAS at CERN: “ATLAS sees first direct evidence of light-by-light scattering at high energy” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    14th August 2017
    Katarina Anthony

    1
    A light-by-light scattering candidate event measured in the ATLAS detector. (Image: ATLAS Collaboration/CERN).

    Physicists from the ATLAS experiment at CERN have found the first direct evidence of high energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics , confirms one of the oldest predictions of quantum electrodynamics (QED).

    “This is a milestone result: the first direct evidence of light interacting with itself at high energy,” says Dan Tovey (University of Sheffield), ATLAS Physics Coordinator. “This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism.”

    Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider’s second run began in 2015. As the accelerator collided lead ions at unprecedented collision rates, obtaining evidence for light-by-light scattering became a real possibility. “This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2,” says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

    Heavy-ion collisions provide a uniquely clean environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as ‘ultra-peripheral collisions’.

    Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

    “Finding evidence of this rare signature required the development of a sensitive new ‘trigger’ for the ATLAS detector,” says Steinberg. “The resulting signature — two photons in an otherwise empty detector — is almost the diametric opposite of the tremendously complicated events typically expected from lead nuclei collisions. The new trigger’s success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it.”

    ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of the result and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.

    See the full article here .

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  • richardmitnick 12:05 pm on July 20, 2017 Permalink | Reply
    Tags: , , , , Particle Accelerators,   

    From CERN: “HIE-ISOLDE: Nuclear physics gets further energy boost” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    17 July 2017
    Harriet Kim Jarlett

    CERN ISOLDE


    This is the Miniball germanium array, which is using the first HIE-ISOLDE beams for the experiments described below (Image: Julien Ordan /CERN)

    For the first time in 2017, the HIE- ISOLDE linear accelerator began sending beams to an experiment, marking the start of ISOLDE’s high-energy physics programme for this year.

    The HIE-ISOLDE (High-Intensity and Energy upgrade of ISOLDE) project incorporates a new linear accelerator (Linac) into CERN’s ISOLDE facility (which stands for the Isotope mass Separator On-Line). ISOLDE is a unique nuclear research facility, which produces radioactive nuclei (ones with too many, or too few, neutrons) that physicists use to research a range of topics, from studying the properties of atomic nuclei to biomedical research and to astrophysics.

    Although ISOLDE has been running since April, when the accelerator chain at CERN woke up from its technical stop over winter, HIE-ISOLDE had to wait until now as new components, specifically a new cryomodule, needed to be installed, calibrated, aligned and tested.

    Each cryomodule contains five superconducting cavities used to accelerate the beam to higher energies. With a third module installed, HIE-ISOLDE is able to accelerate the nuclei up to an average energy of 7.5 MeV per nucleon, compared with the 5.5 MeV per nucleon reached in 2016.

    This higher energy also allows physicists to study the properties of heavier isotopes – ones with a mass up to 200, with a study of 206 planned later this year, compared to last year when the heaviest beam was 142. From 2018, the HIE-ISOLDE Linac will contain four of these cryomodules and be able to reach up to 10 MeV per nucleon.

    “Each isotope we study is unique, so each experiment either studies a different isotope or a different property of that isotope. The HIE-ISOLDE linac gives us the ability to tailor make a beam for each experiment’s energy and mass needs,” explains Liam Gaffney, who runs the Miniball station where many of HIE-ISOLDE’s experiments are connected.

    The HIE-ISOLDE beams will be available until the end of November, with thirteen experiments hoping to use the facility during that time – more than double the number that took data last year. The first experiment, which begins today, will study the electromagnetic interactions between colliding nuclei of the radioactive isotope Selenium 72 and a platinum target. With this reaction they can measure whether or not the nuclei is more like a squashed disc or stretched out, like a rugby ball; or some quantum mechanical mixture of both shapes.

    See the full article here.

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  • richardmitnick 12:01 pm on July 9, 2017 Permalink | Reply
    Tags: , , , , Particle Accelerators, , Probing physics beyond the Standard Model with heavy vector bosons   

    From ATLAS: “Probing physics beyond the Standard Model with heavy vector bosons” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    8th July 2017
    ATLAS Collaboration

    1
    Figure 1: The reconstructed mass of the selected candidate events decaying to WW or ZZ bosons, with the qqqq final state. The black markers represent the data. The blue and green curves represent the hypothesized signal for two different masses. The red curve represents the Standard Model processes. (Image: ATLAS Collaboration/CERN)

    Although the discovery of the Higgs boson by the ATLAS and CMS Collaborations in 2012 completed the Standard Model, many mysteries remain unexplained. For instance, why is the mass of the Higgs boson so much lighter than one would expect and why is gravity so weak?

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Numerous models beyond the Standard Model attempt to explain these mysteries. Some explain the apparent weakness of gravity by introducing additional dimensions of space in which gravity propagates. One model goes beyond that, and considers the real world as a higher-dimensional universe described by warped geometry, which leads to strongly interacting massive graviton states. Other models propose, for example, additional types of Higgs bosons.

    All these models predict the existence of new heavy particles that can decay into pairs of massive weak bosons (WW, WZ or ZZ). The search for such particles has benefited greatly from the increase in the proton–proton collision energy during Run 2 of the Large Hadron Collider (LHC).

    _______________________________________________________________________
    The search for new heavy particles has benefited greatly from the LHC’s increase in proton–proton collision energy.
    _______________________________________________________________________

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    Figure 2: The limit on the cross-section times branching ratio of hypothetical particle described by one of the models for the different final states. (Image: ATLAS Collaboration/CERN)

    The W and Z bosons are carrier particles that mediate the weak force. They decay into other Standard Model particles, like charged leptons (l), neutrinos (ν) and quarks (q). These particles are reconstructed differently in the detector. Quarks, for instance, are reconstructed as localized sprays of hadrons, denoted jets. The two bosons could yield several combinations of these particles in the final states. The ATLAS Collaboration has released results on searches involving all relevant decays of the boson pair: ννqq, llqq, lνqq and qqqq (where the lepton is an electron or muon).

    What do these searches have in common? In each of these, at least one of the bosons decays into a pair of quarks. When the sought-after particle is very massive, the two bosons from its decay are ejected with such large momenta that their respective decay products are collimated and the pair of quarks merge into a single large jet. This phenomenon provides a powerful means to distinguish the new physics signal from strong-interaction Standard Model processes. As some exemplary results of the searches, Figure 1 shows the distributions of the reconstructed mass of the candidate particle. Figure 2 shows the limit on the cross-section times branching ratio of a hypothetical particle described by one of the models.

    So far, no evidence of a new particle has been observed. The search continues with increased sensitivity as ATLAS collects more data.

    Links:
    See the full article for further references with links.

    See the full article here .

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  • richardmitnick 10:10 am on July 9, 2017 Permalink | Reply
    Tags: , , CERN LHC LHCb, CERN Physicists Find a Particle With a Double Dose of Charm, , , , Particle Accelerators,   

    From NYT: “CERN Physicists Find a Particle With a Double Dose of Charm” 

    New York Times

    The New York Times

    JULY 6, 2017
    KENNETH CHANG

    3
    The new particle, awkwardly known as Xi-cc++ (pronounced ka-sigh-see-see-plus-plus)

    1
    The Vertex Locator detector is part of an experiment at CERN’s Large Hadron Collider that discovered a particle that contains two charm quarks. Credit CERN

    Physicists have discovered a particle that is doubly charming.

    Researchers reported on Thursday that in debris flying out from the collisions of protons at the CERN particle physics laboratory outside Geneva, they had spotted a particle that has long been predicted but not detected until now.

    The new particle, awkwardly known as Xi-cc++ (pronounced ka-sigh-see-see-plus-plus), could provide new insight into how tiny, whimsically named particles known as quarks, the building blocks of protons and neutrons, interact with each other.

    Protons and neutrons, which account for the bulk of ordinary matter, are made of two types of quarks: up and down. A proton consists of two up quarks and one down quark, while a neutron contains one up quark and two down quarks. These triplets of quarks are known as baryons.

    There are also heavier quarks with even quirkier names — strange, charm, top, bottom — and baryons containing permutations of heavier quarks also exist.

    An experiment at CERN, within the behemoth Large Hadron Collider, counted more 300 Xi-cc++ baryons, each consisting of two heavy charm quarks and one up quark.

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    The discovery fits with the Standard Model, the prevailing understanding of how the smallest bits of the universe behave, and does not seem to point to new physics.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “The existence of these particles has been predicted by the Standard Model,” said Patrick Spradlin, a physicist at the University of Glasgow who led the research. “Their properties have also been predicted.”

    Dr. Spradlin presented the findings on Thursday at a European Physical Society conference in Venice, and a paper describing them has been submitted to the journal Physical Review Letters.

    Up and down quarks have almost the same mass, so in protons and neutrons, the three quarks swirl around each other in an almost uniform pattern. In the new particle, the up quark circulates around the two heavy charm quarks at the center. “You get something far more like an atom,” Dr. Spradlin said.

    Quark interactions are complex and difficult to calculate, and the structure of the new particles will enable physicists to check the assumptions and approximations they use in their calculations. “It’s a new regime in quark-quark dynamics,” said Jonathan L. Rosner, a retired theoretical physicist at the University of Chicago.

    The mass of the Xi-cc++ is about 3.8 times that of a proton. The particle is not stable. Dr. Spradlin said the scientists had not yet figured out its lifetime precisely, but it falls apart after somewhere between 50 millionths of a billionth of a second and 1,000 millionths of a billionth of a second.

    For Dr. Rosner, the CERN results appear to match predictions that he and Marek Karliner of Tel Aviv University made.

    What is less clear is how the new particle fits in with findings from 2002, when physicists working at Fermilab outside Chicago made the first claim of a doubly charmed baryon, one consisting of two charm quarks plus a down quark (instead of the up quark seen in the CERN experiment).

    FNAL Tevatron

    FNAL/Tevatron map


    FNAL/Tevatron DZero detector


    FNAL/Tevatron CDF detector

    The two baryons should be very close in mass, but the Fermilab one was markedly lighter than what the CERN researchers found for Xi-cc++, and it appeared to decay instantaneously, in less than 30 millionths of a billionth of a second.

    Theorists like Dr. Rosner had difficulty explaining the behavior of the Fermilab particle within the Standard Model. “I didn’t have an honest alternative to allow me to believe that result,” he said.

    Peter S. Cooper, a deputy spokesman for the Fermilab experiment, congratulated the CERN researchers on their discovery. “That paper smells sweet,” he said. “From an experimental point of view, there’s nothing wrong. They definitely have something.”

    But he said the Fermilab findings still stood, too. He acknowledged that the two results do not readily make sense together.

    “I consider this a problem for my theoretical brethren to work out,” Dr. Cooper said. He added that it was a textbook example of the scientific method: “Our theoretical colleagues make a prediction. We go out and make a measurement and see if it’s right. If it isn’t, they go back and think harder.”

    It is possible one of the experiments is wrong. Researchers at other laboratories, including at CERN, have sought to detect the Fermilab baryon without success. Dr. Spradlin said he and his colleagues are searching the same data that revealed the Xi-cc++ for the baryon with two charm quarks and one down quark. That could confirm the Fermilab findings or reveal a mass closer to theorists’ expectations.

    “We should be able to see it with the data we have,” Dr. Spradlin said. “I think we are very close to resolving this controversy.”

    I presented an earlier post from LHCb, but it contained no reference to the paper in Physical Review Letters.

    See the full article here .

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  • richardmitnick 8:51 am on July 7, 2017 Permalink | Reply
    Tags: , , CERN Data Centre passes the 200-petabyte milestone, , Particle Accelerators,   

    From CERN: “CERN Data Centre passes the 200-petabyte milestone” 

    Cern New Bloc

    Cern New Particle Event

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    CERN

    6 July 2017
    Mélissa Gaillard

    1
    CERN’s Data Centre (Image: Robert Hradil, Monika Majer/ProStudio22.ch)

    On 29 June 2017, the CERN DC passed the milestone of 200 petabytes of data permanently archived in its tape libraries. Where do these data come from? Particles collide in the Large Hadron Collider (LHC) detectors approximately 1 billion times per second, generating about one petabyte of collision data per second. However, such quantities of data are impossible for current computing systems to record and they are hence filtered by the experiments, keeping only the most “interesting” ones. The filtered LHC data are then aggregated in the CERN Data Centre (DC), where initial data reconstruction is performed, and where a copy is archived to long-term tape storage. Even after the drastic data reduction performed by the experiments, the CERN DC processes on average one petabyte of data per day. This is how the the milestone of 200 petabytes of data permanently archived in its tape libraries was reached on 29 June.

    2
    This map shows the routes for the three 100 Gbit/s fibre links between CERN and the Wigner RCP. The routes have been chosen carefully to ensure we maintain connectivity in the case of any incidents. (Image: Google)

    See the full article here.

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  • richardmitnick 7:00 am on July 7, 2017 Permalink | Reply
    Tags: , , , , Particle Accelerators,   

    From ATLAS: “Why should there be only one? Searching for additional Higgs Bosons beyond the Standard Model” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    6th July 2017
    ATLAS Collaboration

    1
    Figure 1: Feynman diagram for leading order production of a neutral MSSM Higgs boson in association with b-quarks. (Image: ATLAS Collaboration/CERN)

    CERN CMS Higgs Event

    Since the discovery of the elusive Higgs boson in 2012, researchers have been looking beyond the Standard Model to answer many outstanding questions. An attractive extension to the Standard Model is Supersymmetry (SUSY), which introduces a plethora of new particles, some of which may be candidates for Dark Matter.

    Standard model of Supersymmetry DESY

    One of the most popular SUSY models – the Minimal Supersymmetric Standard Model (MSSM) – predicts the existence of five Higgs bosons. In this model, the recently discovered Higgs boson (h) would be considered to be the lightest of the set. Two charged Higgs (H+, H–) and two neutral Higgs (A/H) would complete the set, and could exist within a wide range of masses above that of the discovered Higgs boson. The LHC experiments are poised to search for these additional bosons using techniques similar to those used in the initial Higgs searches.

    In July 2017, the ATLAS collaboration presented a new result on the search for neutral (A/H) Higgs bosons decaying to two tau leptons. Taus are particularly interesting to the search as there is a stronger coupling between A/H and down-type fermions (e, μ, τ, d, s, b) for certain values of the MSSM parameter-space. This will enhance the probability of decays to tau leptons, as well as the production of A/H in association with b-quarks (Figure 1), providing a larger cross-section. Like with the Standard Model Higgs boson, gluon-fusion production of A/H remains an important production process in the MSSM to varying degrees (depending on the chosen model parameters). Thus, by classifying events by their probability of containing b-flavoured jets, the ATLAS search has been optimised for both b-associated and gluon-fusion production of A/H, respectively.

    2
    Figure 2 (left): The observed and expected 95% CL upper limits on the production cross section times di-tau branching fraction for a scalar boson produced via b-associated production. Figure 3 (right): The observed and expected 95% CL limits on tanβ as a function of the mass of the A boson in the hMSSM scenario. The area above the black curve has been excluded. The exclusion arising from the Standard Model Higgs boson coupling measurements and the exclusion limit from the ATLAS 2015 H/A→ ττ search are shown. (Images: ATLAS Collaboration/CERN)

    See the full article here .

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  • richardmitnick 1:18 pm on July 6, 2017 Permalink | Reply
    Tags: , , , Chasing the invisible, , , Particle Accelerators,   

    From ATLAS: “Chasing the invisible” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    6th July 2017
    ATLAS Collaboration

    1
    Figure 1: The second highest ETmiss monojet event in the 2016 ATLAS data. A jet with pT of 1707 GeV is indicated by the green and yellow bars corresponding to the energy deposition in the electromagnetic and hadronic calorimeters respectively. The ETmiss of 1735 GeV is shown as the white dashed line in the opposite side of the detector. No additional jets with pT above 30 GeV are found. (Image: ATLAS Collaboration/CERN)

    Cosmological and astrophysical observations based on gravitational interactions indicate that the matter described by the Standard Model of particle physics constitutes only a small fraction of the entire known Universe. These observations infer the existence of Dark Matter, which, if of particle nature, would have to be beyond the Standard Model.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Although the existence of Dark Matter is well-established, its nature and properties still remain one of the greatest unsolved puzzles of fundamental physics. Excellent candidates for Dark Matter particles are weakly interacting massive particles (WIMPs). These “invisible” particles cannot be detected directly by collision experiments.

    At the LHC, most collisions of protons produce sprays of energetic particles that bundle together into so-called “jets”. Momentum conservation requires that if particles are reconstructed in one part of the detector there have to be recoiling particles in the opposite direction. However, if WIMPs are produced they will leave no trace in the detector, causing a momentum imbalance called “missing transverse momentum” (ETmiss). However, a pair of WIMPs can be produced together with a quark or gluon that is radiated from an incoming parton (a generic constituent of the proton) producing a jet which allows to tag this kind of events.

    The jets+ETmiss search looks at final states where a highly energetic jet is produced in association with large ETmiss. Many beyond the Standard Model theories can be probed by looking for an excess of events with large missing transverse momentum compared to the Standard Model expectation. Among those theories, Supersymmetry and models which foresee the existence of Large Extra Spatial Dimensions (LED), predict additional particles that are invisible to collider experiments. These theories could give an elegant explanation to several anomalies still unsolved in the Standard Model.

    2
    Figure 2: Missing transverse momentum distribution after the jets+ETmiss selection in data and in the Standard Model predictions. The different background processes are shown in different colors. The expected spectra of LED, Supersymmetric and WIMP scenarios are also illustrated with dashed lines. (Image: ATLAS Collaboration/CERN)

    The combination of data-driven techniques and high-precision theoretical calculations has allowed ATLAS to predict the main Standard Model background processes with great precision. The shape of the ETmiss spectrum is used to increase the discovery potential of the analysis and increase the discrimination power between signals and background.

    The figure shows the missing transverse momentum spectrum compared to the measurement with the Standard Model expectation. Since no significant excess is observed, the level of agreement between data and the prediction is translated into limits on unknown parameters of the Dark Matter, Supersymmetry and LED models.

    In the WIMP scenario, the latest analysis using data collected in 2015 and 2016 in a specific interaction model are able to exclude Dark Matter masses up to 440 GeV and interaction mediators up to 1.55 TeV. Under the considered model, these represent competitive results when compared with other experiments using different detection approaches.

    Over the next two years the LHC aims to increase the data available by a factor of three. This will be a unique opportunity for ATLAS to investigate the energy frontier and the jets+ETmiss channel will continue to hold the potential to profoundly revise our understanding of the universe.

    Links:

    Search for dark matter and other new phenomena in events with an energetic jet and large missing transverse momentum using the ATLAS detector (ATLAS-CONF-2017-060): link coming soon
    EPS 2017 presentation by Shin-Shan Yu: Dark matter searches at colliders
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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  • richardmitnick 12:10 pm on July 6, 2017 Permalink | Reply
    Tags: , ATLAS takes a closer look at the Higgs boson’s couplings to other bosons, , , Particle Accelerators,   

    From ATLAS: “ATLAS takes a closer look at the Higgs boson’s couplings to other bosons” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    6th July 2017
    ATLAS Collaboration

    1
    Figures 1 and 2: Measurement of the Higgs boson production cross sections in its main production modes and normalised to the Standard Model predictions, as obtained by the H→ZZ*→4ℓ and H→γγ decay channels respectively. (Image: ATLAS Collaboration/CERN)

    Since resuming operation for Run 2, the LHC has been producing about 20,000 Higgs bosons per day in its 13 TeV proton–proton collisions. At the end of 2015, the data collected by the ATLAS and CMS collaborations were already enough to re-observe the Higgs boson at the new collision energy. Now, having recorded more than 36,000 trillion collisions between 2015 and 2016, ATLAS can perform ever more precise measurements of the properties of the Higgs boson.

    Measuring how the Higgs boson is produced and it decays is one of the major goals of the LHC experiments. Greater precision in these measurements allows us to refine our understanding of the Higgs sector of the Standard Model, and also constrain new phenomena beyond the Standard Model that would modify the coupling of the Higgs with the other Standard Model particles. By studying the Higgs boson decays to photon pairs (H→γγ) and to four leptons via intermediate Z bosons (H→ZZ*→4ℓ, where the ‘*’ indicates that one Z boson is produced off its mass shell), the ATLAS experiment can measure the coupling properties of the Higgs boson with unprecedented precision.

    At the LHC, the Higgs boson is produced through different processes with very different rates: gluon fusion, vector-boson fusion, WH, ZH, and ttH. To probe these production modes, ATLAS has introduced a set of criteria to categorize the Higgs events with the H→γγ and H→ZZ*→4ℓ final states. The results of this study are displayed in Figures 1 and 2, where the measured cross section, normalized to the value predicted by the Standard Model, is shown.

    ___________________________________________________________________________________

    Combining these separate measurements allowed ATLAS to bring the experimental sensitivity closer to the precision of the Standard Model predictions.
    ___________________________________________________________________________________

    With the LHC producing an ever-increasing number of Higgs bosons, ATLAS has been able to start measuring the cross section of each production mode in different phase-spaces, setting an additional stress test for the Standard Model. These results are used to constrain possible modifications of the Higgs boson couplings from those predicted by the Standard Model. No significant deviation from the prediction has yet been observed.

    The H→γγ decay channel is also used to measure several differential cross sections for observables sensitive to Higgs boson production and decay, where good agreement was found between the data and Standard Model predictions (see Figure 4). Similar measurements have already been performed with H→ZZ∗→4ℓ decays.

    Combining these separate measurements allowed ATLAS to bring the experimental sensitivity closer to the precision of the Standard Model predictions. The total Higgs boson production cross section is measured to be 57.0 +6.0−5.9 +3.2−2.7 pb, where the first uncertainty is statistical and the second of systematic origin. The result is consistent with the Standard Model prediction of 55.6+2.4−3.4 pb. (Figure 3)

    ATLAS will continue to study the Higgs boson properties for the rest of Run 2, isolating its rare production modes and measuring its more elusive properties. Uncovering these secrets will either further cement the Standard Model, or give us insight of what lies beyond.

    3
    Figure 3: Total Higgs boson production cross sections measured at centre-of-mass energies of 7, 8, and 13 TeV by the H→γγ and H→ZZ*→4ℓ* channels and their combinations, compared to the Standard Model predictions. (Image: ATLAS Collaboration/CERN)

    4
    Figure 4: Transverse momentum of the Higgs boson as measured in the Hyy decay, and compared to the Standard model predictions. (Image: ATLAS Collaboration/CERN)

    Links

    Measurement of the Higgs boson coupling properties in the H→ZZ→4l decay channel at 13 TeV with the ATLAS detector (ATLAS-CONF-2017-043): link coming soon
    Measurements of Higgs boson properties in the diphoton decay channel with 36.1 fb−1 pp collision data at the center-of-mass energy of 13 TeV with the ATLAS detector (ATLAS-CONF-2017-045): link coming soon
    Combined measurements of Higgs boson production and decay in the H→ZZ*→4ℓ and H→γγ channels using 13 TeV pp collision data collected with the ATLAS experiment (ATLAS-CONF-2017-047): link coming soon
    EPS 2017 presentation by Ruchi Gupta: Measurement of the Higgs boson couplings and properties in the diphoton, ZZ and WW decay channels using the ATLAS detector : and Tamara Vazquez Schroeder Determination of the Higgs boson properties with the ATLAS detector
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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  • richardmitnick 10:58 am on July 6, 2017 Permalink | Reply
    Tags: , , , , Observation of an exceptionally charming particle, Particle Accelerators,   

    From LHCb at CERN: ” Observation of an exceptionally charming particle” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    LHCb at CERN

    Today, at the EPS Conference on High Energy Physics, EPS-HEP 2017, in Venice, Italy, the LHCb collaboration presented the first observation of a doubly charmed particle. This particle, called the Ξcc++, is a baryon (particle composed of three quarks) containing two charm quarks and one up quark, resulting in an overall doubly positive charge. It is a doubly charm counterpart of the well-known lower mass Ξ0 baryon, which is composed of two strange quarks and an up quark.

    1

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    The Ξcc++ baryon is identified via its decay into a Λc+ baryon and three lighter mesons K-, π+ and π+. The image above shows an example of a Feynman diagram contributing to this decay. The Λc+ baryon decays in turn into a proton p, a K- and a π+ meson. The image shows the Λc+K-π+π+ invariant mass spectrum obtained with 1.7 fb-1 of data collected by LHCb in 2016 at the LHC centre-of-mass energy of 13 TeV. The mass is measured to be about 3621 MeV/c2 which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed-quark content. The signal candidates are consistent with particles that traveled a significant distance before decaying: even selecting only those Ξcc++ particles that survived more than approximately five times the expected decay time resolution, the signal remains highly significant. This state is therefore incompatible with a strongly decaying particle, but is consistent with a longer-lived decay involving weak interactions as would be expected for this particle.

    3
    The new particle, awkwardly known as Xi-cc++ (pronounced ka-sigh-see-see-plus-plus)

    The existence of doubly charmed baryons was already known to be a possibility in the 1970s, after the discovery of the charm quark. In the early 2000s the observation of a similar particle was reported by the SELEX collaboration. This observation was not confirmed by subsequent experiments and the measured properties of this particle are not compatible with those of the Ξcc++ baryon discovered by LHCb. The discovery of the Ξcc++ performed by LHCb has been made possible by the high production rate of heavy quarks at the LHC and thanks to the unique capabilities of the experiment, which can identify the decay products with excellent efficiency and purity. The image shows an artist view of this new particle. This animation shows how the signal accumulated in the Λc+K-π+π+ invariant-mass spectrum throughout 2016.

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    This discovery opens a new field of particle physics research. An entire family of doubly charmed baryons related to the Ξcc++ is predicted, and will be searched for with added enthusiasm. Furthermore, other hadrons containing different configurations of two heavy quarks, for example two beauty quarks or a beauty and charm quark, are waiting to be discovered. Measurements of the properties of all these particles will allow for precise tests of QCD, the theory of strong interactions, in a unique environment. LHCb is very well equipped to face this very exciting challenge.

    More information can be found in the LHCb EPS-HEP presentation, in the LHCb publication and soon in a forthcoming CERN seminar. Read also the CERN Press Release in English and French as well as the CERN Courier article in near future.

    click here to get direct access to all LHCb published papers

    See the full article here.

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    THE FOUR MAJOR PROJECT COLLABORATIONS

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  • richardmitnick 5:25 pm on July 4, 2017 Permalink | Reply
    Tags: , , Higgs prior to the announcements, Particle Accelerators, ,   

    From Symmetry: “Ten things you may not know about the Higgs boson” 03/01/12 

    Symmetry Mag

    Symmetry

    03/01/12
    Kathryn Jepsen

    This year [2012], results from the Large Hadron Collider in Europe and the Tevatron in the United States will either prove or refute the existence of the Standard Model Higgs particle, a keystone in theorists’ proposed explanation for the origin of mass. Symmetry looks at little-known facts about the elusive particle.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    1. Peter Higgs’ best-known paper on the new particle was initially rejected.

    But this was a blessing in disguise, since it led Higgs to add a paragraph introducing the now-famous Higgs particle. In 1964, Higgs wrote two papers, each just two pages long, on what is now known as the Higgs field. The journal Physics Letters accepted the first but sent the second back. Yoichiro Nambu, a highly regarded physicist who had reviewed the second paper, suggested Higgs add a section explaining his theory’s physical implications. Higgs added a paragraph predicting that an excitation of the field, like a wave in the ocean, would yield a new particle. He then submitted the revised paper to the competing journal Physical Review Letters, which published it.

    2. The science minister for the United Kingdom once held a national competition to find the best Higgs explanation.

    According to the Higgs model, elementary particles gain mass by interacting with an invisible, omnipresent field. The more a particle interacts with the Higgs field, the more mass it will have. Scientists had such difficulty explaining the Higgs field to the British government that in 1993, UK Science Minister William Waldegrave challenged them to send him their best one-page descriptions. Waldegrave handed out champagne to the winners, who included physicist David Miller of University College London. Miller compared the Higgs field to a crowd of political party workers spread evenly through a room. An anonymous person could move through the crowd unhindered. However, then Prime Minister Margaret Thatcher would attract a lot of attention: Party workers would clump around her, slowing her down, giving her metaphorical “mass.” Creative types have since swapped the characters in the metaphor for Albert Einstein mobbed by fellow scientists or pop stars swarmed by paparazzi.

    3. The Higgs mechanism explains only a small fraction of the mass in the universe.

    Most popular science articles give the Higgs model broad credit for lending mass to everything in the universe. However, the Higgs field gives mass only to elementary particles such as quarks and electrons. Most of the visible universe is made of composite particles such as protons and neutrons, which contain quarks. Just as a loaf of raisin bread weighs more than the sum of its raisins, protons and neutrons get their mass from more than just the quarks inside them. The strong nuclear force that holds those quarks together does most of the mass-giving work.

    4. Higgs was not the only physicist who contributed to the idea of how to give particles mass.

    At least a dozen theorists played some part in developing the theoretical framework that led to the Higgs particle. In 2010, the American Physical Society awarded the J.J. Sakurai Prize to six physicists who had published papers on the topic in 1964. But other theorists came up with similar ideas, and earlier publications helped pave the way. The size of this crowd may trouble a certain Swedish committee, as the annual Nobel Prize for physics can be awarded to three living scientists at most.

    5. The term “boson” comes from the name of Indian physicist and mathematician Satyendra Nath Bose.

    Particles come in two varieties: bosons and fermions. The Higgs particle falls into the category of bosons, named for a physicist best known for his collaborations in the 1920s with Albert Einstein. Some of the pair’s work resulted in the invention of Bose-Einstein statistics, a way to describe the behavior of a class of particles that now shares Bose’s name. Two bosons with identical properties can be in the same place at the same time, but two fermions cannot. This is why photons, which are bosons, can travel together in concentrated laser beams. But electrons, which are fermions, must stay away from each other, which explains why electrons must reside in separate orbits in atoms. Bose never received a doctorate, nor was he awarded a Nobel Prize for his work, though the Nobel committee recognized other scientists for research related to the concepts he developed.

    6. The nickname “God particle” originated from a book by Nobel laureate Leon Lederman.

    2

    Physicist Leon Lederman unwittingly gave the Higgs boson what may be its most-disliked descriptor with the title of his book, The God Particle. Lederman likes to joke that he actually wanted to call the Higgs boson the “goddamn particle” because it’s so darned difficult to find. The nickname makes for attention-grabbing headlines, but it also makes most particle physicists cringe.

    7. Standard particle theory will be incomplete even if the Higgs particle is discovered.

    The Standard Model of elementary particles today (2017) (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    The Higgs boson is the last undiscovered particle predicted by the Standard Model, a beautiful mathematical framework physicists use to describe the smallest bits of matter and how they interact. Experimental results have time and again validated the model’s other predictions. But finding the Higgs boson would not close the book on particle physics. While the Standard Model accounts for fundamental forces such as electromagnetism and the strong nuclear force, it cannot make sense of gravity, which is disproportionately weak compared to the other forces. One possible explanation is that we experience only a fraction of the force of gravity because most of it acts in hidden extra dimensions.

    8. If the Higgs particle exists, it may have relatives.

    Many theorists have tried to explain the known particles and their masses without a Higgs boson, but no one has yet come up with a successful model. In fact, a popular theory known as supersymmetry predicts at least five Higgs particles, and others predict many more.

    Standard model of Supersymmetry DESY

    It is up to experiments at the Large Hadron Collider in Europe and at the Tevatron collider in the United States – where experiments have concluded, but data are still being analyzed – to discover whether the Higgs boson exists and, if so, whether it is the particle we expected.

    FNAL Tevatron

    FNAL/Tevatron map


    FNAL/Tevatron DZero detector


    FNAL/Tevatron CDF detector

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    9. Scientists may have first glimpsed the Higgs boson more than a decade ago.

    In 2000, CERN’s flagship accelerator, the Large Electron Positron Collider, was scheduled to close after 11 years of successful operation when something curious happened.

    CERN LEP


    CERN LEP Collider

    The LEP experiments began to find signs of something that looked rather like the Higgs particle with a mass around 115 GeV/c^2, about the mass of an iodine atom. Excited scientists convinced CERN management to keep LEP running for six weeks beyond the original shut-down date to see if the observation would grow more convincing with additional data. During the machine’s stay of execution, even more candidate Higgs events appeared. Physicists requested a second extension to see if their observation might blossom into a discovery, but the machine was dismantled to make way for a higher-energy Higgs hunter, the LHC. The latest LHC results, made public in December 2011, indicate that the Higgs particle, if it exists, must have a mass between 115-130 GeV/c^2. The ATLAS [result above] and CMS [result above] experiments reported intriguing hints of a Higgs boson with a mass in the region of 124-126 GeV

    10. Finding the new particle would be only the beginning.

    Just because something looks like the Higgs particle does not mean it is the Higgs particle. If physicists do discover a new particle, they will need to measure its numerous properties before they can determine whether it is the Higgs boson described by the Standard Model of particle physics. Theory predicts in great detail how a Standard Model Higgs particle would interact with other particles. Only after carefully measuring and testing these interactions — like a biologist examining the genetic makeup of a new plant species — would scientists be certain that they had indeed found the Standard Model Higgs boson. A new particle that did not act as expected would give physicists a whole new set of mysteries to explore.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
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