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  • richardmitnick 9:12 am on August 4, 2020 Permalink | Reply
    Tags: "New ATLAS result marks milestone in the test of Standard Model properties", , CERN ATLAS, , , ,   

    From CERN ATLAS: “New ATLAS result marks milestone in the test of Standard Model properties” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    3rd August 2020
    ATLAS Collaboration

    1
    Figure 1: Diagrams of a lepton-flavour-violating Z-boson decay (left) and of the two main backgrounds to the search: a lepton-flavour-conserving Z-boson decay into a pair of tau leptons (middle) and a W-boson decay with leptons (right). The green arrows represent electrons or muons (l), the blue triangles are the visible component of hadronically-decaying tau leptons (τ had-vis) or the hadronisation of a quark or a gluon, and the dashed blue lines represent undetected neutrinos. (Image: ATLAS Collaboration/CERN)

    The ATLAS Collaboration has released a new study into a key building block of matter: leptons. This type of particle comes in three different families (flavours) and, according to the Standard Model, should follow strict rules. For instance, except for their mass, leptons of different flavours have identical properties – a feature known as lepton flavour universality. This was recently corroborated by a key measurement of the W-boson decay rates into leptons by the ATLAS Collaboration.

    Yet the Standard Model has known shortcomings. For example, it predicted that leptons could only interact with other leptons of the same flavour. Experiments observed neutrinos (neutral leptons) violate this hypothesis, transforming from one flavour into another in processes known as neutrino oscillation. This led physicists to realise that neutrinos were not, in fact, massless, as originally assumed in the Standard Model. Such discoveries show that the fundamental structure of Nature is more complex than one had thought.

    Could the violation of lepton flavour seen in neutral leptons also occur among charged leptons (with dramatic consequences)? In a new result, the ATLAS Collaboration searched for lepton-flavour-violating decays of the Z boson, where the Z boson decays into two charged leptons of different flavours. Such events are predicted from neutrino oscillation to be so rare – accounting for just one in 1054 Z-boson decays – that they should be undetectable. If they were to be observed at ATLAS, it would be an unequivocal sign of new physics beyond the Standard Model.

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    2
    Figure 2: Distribution of the neural-network output of some of the Z-boson-decay candidates analysed in the search. In the upper panel, data (black dots) are compared to the stacked expected contributions from background processes, mainly Z-boson decays to tau-lepton pairs (dark blue) and W-boson decays (yellow). The expected contribution from signal events with a decay rate of five in ten thousand is shown by the red dashed line. The lower panel shows the ratio of the data to the background prediction. No significant excess of events is seen in the data. (Image: ATLAS Collaboration/CERN)

    Physicists examined ATLAS data collected over two runs of the Large Hadron Collider (LHC, 2012–2018) to set strong constraints on lepton-flavour-violating decays involving a tau lepton (τ) and an electron (e) or a muon (μ). While there have been several low-energy experiments that specialise in lepton-flavour-violating searches, they cannot easily probe transitions involving Z bosons. This is an area where high-energy accelerators, such as CERN’s previous Large Electron Positron (LEP) collider (1989–2000) and now the LHC, play a special role.

    CERN Large Electron Positron Collider

    The ATLAS result marks a new milestone in a legacy of precision measurements established by LEP. Searching for rare Z-boson decays is a great challenge for LHC experiments. Whereas LEP produced an abundance of Z bosons in a relatively clean environment, only a small fraction of LHC collisions produce a Z boson – and always along with several background collisions. However, the LHC has one major advantage: it can produce Z bosons at a much faster rate! More than 20 years on, ATLAS’ result now supersedes those of the LEP experiments (OPAL, DELPHI).

    To analyse the enormous LHC Run 2 dataset – with its 8 billion Z bosons – ATLAS physicists developed a state-of-the-art machine-learning method using deep neural networks. The neural networks were trained to identify the kinematic properties of the signal process, where a Z boson decays into an electron or a muon and a tau lepton, which itself is unstable and decays (Figure 1). They also had to differentiate the signal process from mainly two others that produce the same particles: the lepton-flavour-conserving Z-boson decay into a pair of tau leptons, where one tau lepton decays into an electron or a muon (plus undetected neutrinos), and the W-boson decay into leptons, produced together with an additional jet of particles.

    The output distributions of the neural networks for the selected candidate event were studied to determine the presence of lepton-flavour-violating Z-boson decays (Figure 2 for the tau lepton and muon case). The classification power of the neural networks – combined with the unprecedented number of Z-boson decays studied – allowed ATLAS to set constraints on the maximum rate at which lepton-flavour-violating Z-boson decays involving a tau lepton can occur. The result excludes, at 95% confidence level, Z-boson decay rates greater than 8.1×10-6 (Z→τe) and 9.5×10-6 (Z→τμ).

    The new ATLAS result provides experimental guidance towards new theories that could explain the shortcomings of the Standard Model. The precision of the result is largely limited by the number of analysed Z-boson decays. Therefore, ATLAS is looking forward to improved sensitivity that is to be expected from the additional data of future LHC runs.

    ________________________________________________

    Links

    Lepton Flavour Violation at the LHC: a search for Z→eτ and Z→μτ decays with the ATLAS detector (ATLAS-CONF-2020-035)
    ICHEP2020 presentation by Karl Jakobs: ATLAS Highlights
    OPAL Collaboration, A search for lepton flavor violating Z0 decays , Z. Phys. C67 (1995) 555
    DELPHI Collaboration, Search for lepton flavor number violating Z0 decays, Z. Phys. C73 (1997) 243
    New ATLAS result addresses long-standing tension in the Standard Model, Physics Briefing, 28 May 2020
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .


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  • richardmitnick 3:27 pm on August 3, 2020 Permalink | Reply
    Tags: "CERN experiments announce first indications of a rare Higgs boson process", , CERN ATLAS, , , , , , , The ATLAS and CMS experiments at CERN have announced new results which show that the Higgs boson decays into two muons.   

    From CERN: “CERN experiments announce first indications of a rare Higgs boson process” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    3 August, 2020

    The ATLAS [below] and CMS [below] experiments at CERN have announced new results which show that the Higgs boson decays into two muons.

    1
    Candidate event displays of a Higgs boson decaying into two muons as recorded by CMS (left) and ATLAS (right). (Image: CERN)

    Geneva. At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

    Physicists at CERN have been studying the Higgs boson since its discovery in 2012 in order to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analysing how it decays into the various fundamental particles and the rate of disintegration.

    CMS achieved evidence of this decay with 3σ, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s 2σ result means the chances are one in 40 [strange, lower statistical signifance but greater probability, never saw that before] . The combination of both results would increase the significance well above 3σ and provides strong evidence for the Higgs boson decay to two muons.

    “CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.

    The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.

    Standard Model of Particle Physics, Quantum Diaries

    “This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics programme. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.

    What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimised to categorise physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.

    The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.

    Scientific materials

    Papers:
    CMS physics analysis summary: https://cds.cern.ch/record/2725423
    ATLAS paper on arXiv: https://arxiv.org/abs/2007.07830

    Physics briefings:
    CMS: https://cmsexperiment.web.cern.ch/news/cms-sees-evidence-higgs-boson-decaying-muons
    ATLAS: https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons

    Event displays and plots:
    CMS: https://cds.cern.ch/record/2720665?ln=en
    http://cds.cern.ch/record/2725728
    ATLAS: https://cds.cern.ch/record/2725717?ln=en
    https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-14

    Images:
    CMS muon system:

    ATLAS muon spectrometer:

    See the full article here.


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    Meet CERN in a variety of places:

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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

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  • richardmitnick 11:42 am on August 1, 2020 Permalink | Reply
    Tags: "New measurements of the Higgs boson find strength in unity", , , CERN ATLAS, , , , , The Standard Model remains unperturbed.   

    From CERN ATLAS: “New measurements of the Higgs boson find strength in unity” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    31st July 2020
    ATLAS Collaboration

    ATLAS reports an important boost in the precision of combined measurements of Higgs-boson couplings, as analyses of the full Run-2 dataset proceed.

    1
    Figure 1: Event display of a Higgs-boson candidate produced in association with a Z boson (ZH production), with the Higgs boson decaying to four leptons (H → ZZ*→ 2e2μ), and the Z boson to a pair of muons (Z→μμ). The Higgs boson is reconstructed from the two electrons (two green tracks and bars representing energy deposited in the calorimeter) and two muons (two red tracks on the left passing through the blue muon chambers). The associated Z boson recoils against the Higgs boson to produce two additional muons (two red tracks on the right). (Image: ATLAS Collaboration/CERN)

    The Higgs boson, first predicted in the 1960s and discovered by the ATLAS [above] and CMS experiments in 2012, is a unique elementary particle arising from the mass-generating Higgs mechanism of the Standard Model.

    CERN CMS Higgs Event May 27, 2012

    Standard Model of Particle Physics, Quantum Diaries

    It thus has a peculiar affinity to mass: the larger the mass of an elementary particle, the stronger its interaction (or coupling) with the Higgs boson. Any deviation from this pattern would reveal new physics.

    Physicists can study Higgs-boson couplings in several ways: by measuring the rates of different Higgs boson production mechanisms and decays, and also by studying the particle’s kinematic properties. The ATLAS Collaboration has just presented precise new measurements of these key quantities. Several of these measurements were updated to use the full LHC Run 2 dataset (2015–2018), to provide the best precision to date.

    When combined, ATLAS’ new measurements give detailed insight into this one-of-a-kind particle. They significantly outperform previous measurements, with the overall production rate of the Higgs boson found to be in good agreement with the Standard Model, within a measurement precision of 5% and about 4% uncertainty in the Standard Model prediction.

    2
    Figure 2: Cross sections for ggF, VBF, WH, ZH and ttH+tH normalized to their Standard Model predictions, measured assuming Standard Model values for the decay branching fractions. The black error bars, blue boxes and yellow boxes show the total, systematic, and statistical uncertainties in the measurements, respectively. The gray bands indicate the theory uncertainties in the Standard Model cross-section predictions. The compatibility level between the measurement and the Standard Model prediction is 86%. (Image: ATLAS/CERN)

    Channel surfing with Higgs boson decays

    ATLAS physicists began by measuring all of the main decay “channels” of the Higgs boson: into a pair of photons, W or Z bosons, tau leptons, bottom quarks – and even muons. Though the coupling to muons is difficult to probe, ATLAS physicists recently reported a first hint of the Higgs boson decay to muons. ATLAS researchers also searched for Higgs bosons decaying to “invisible” particles, leaving only missing transverse energy in the detector – a possible portent of dark matter, for example. Their new result sets the strongest limits yet on this process, establishing that less than 13% of Higgs boson decays could be into “invisible” particles.

    These measurements could then be broken down into the major production modes of the Higgs boson: gluon fusion (ggF), vector-boson fusion (VBF), the associated production with a W or Z boson (WH, ZH), and the associated production with top quarks (ttH, tH), as shown in Figure 2. All of these are now observed and precisely measured, with the experimental sensitivity of some modes nearing the precision of state-of-the-art theory predictions. ATLAS has furthermore established for the first time the separate observation of the associated production of the Higgs boson with, respectively, a W boson and a Z boson.

    Further, the kinematic properties of the Higgs boson were assessed with unprecedented precision. Physicists introduced finer partitions of the various production modes – studying, for example, the Higgs boson transverse momentum or the number of jets in an event – to uncover potential hints of new physics. For the first time, ATLAS has also measured the differential distribution of the Higgs boson transverse momentum in ttH production, shedding new light on the boson’s interaction with the top quark.

    With these measurements in hand, physicists were able to decipher the Higgs-boson couplings to other elementary particles. As shown in Figure 3, the strength of the coupling increases with the mass of the elementary particle, in good agreement with the Standard Model. This holds true across a wide range in masses, from the top quark (the heaviest particle in the Standard Model) down to the muon (1600 times lighter than the top quark).

    3
    Figure 3: The coupling-strength for fermions (t, b, τ, μ) and weak gauge bosons (W, Z) on the y-axis vs their mass on the x-axis. The Standard Model prediction is also shown (dotted line). The lower inset shows the ratios of the values to their Standard Model predictions. The level of compatibility between the combined measurement and the Standard Model prediction is 84%. (Image: ATLAS Collaboration/CERN)

    4
    Figure 4: The measured coupling to photons on the y-axis vs the coupling to gluons on the x-axis. The best-fit value for the two measurements is shown by a cross and the Standard Model hypothesis by a star. Ellipses show the 68% and 95% confidence-level contours from a combined fit. The compatibility level between the combined measurement and the Standard Model prediction is 51%, well within the one standard deviation (68%) level. (Image: ATLAS Collaboration/CERN)

    Exploration through combination

    ATLAS physicists paid particular attention to processes such as gluon-fusion production of the Higgs boson and Higgs-boson decays to a pair of photons. Both the gluon and photon are massless, and thus cannot directly interact with the Higgs boson. These processes are therefore mediated by other massive particles via loop interactions, which could be hideouts for new particles.

    Though experiments cannot directly see these loop interactions, there are still ways to infer their content. The presence of new particles would change the rate for ggF production or the Higgs boson decaying into photons. In Figure 4, the measured gluon and the photon couplings are compared to theoretical predictions. A deviation of the measured values from unity, if established, would be a smoking gun for new physics lurking in loop interactions. Instead, ATLAS physicists observed a good agreement with the Standard Model, with measured uncertainties on the measured gluon and photon couplings as low as 5%, and an overall agreement with expectations at the 51% confidence level.

    Finally, by combining together the various Higgs-boson decay measurements and including these loop interactions, ATLAS physicists set another limit on new physics. Showing the value of combined studies, this result sets a new limit of 9% for Higgs boson decays to “invisible” particles – an improvement from the 13% of the measurement quoted above.

    The Standard Model remains unperturbed

    Thanks to the excellent performance of the LHC and the ATLAS detector during Run 2, several ATLAS results have been combined to probe the couplings of the Higgs boson at unprecedented levels. Though the Standard Model remains unperturbed, the exploration is just beginning! Some important but difficult analysis channels are still to use the full Run-2 dataset – offering additional insight into the Higgs boson’s secrets.

    See the full article here .


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  • richardmitnick 7:28 am on July 31, 2020 Permalink | Reply
    Tags: "Looking forward: ATLAS measures proton scattering when light turns into matter", , ATLAS Forward Proton (AFP) spectrometer (fig No.1 in post), , CERN ATLAS, , , , , Physicists studied data recorded by the AFP spectrometer throughout 2017 to establish direct evidence of these scattered protons when matter – electron–positron or muon–antimuon pairs – are cr,   

    From CERN ATLAS: “Looking forward: ATLAS measures proton scattering when light turns into matter” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    30th July 2020
    ATLAS Collaboration

    Today, at the International Conference for High Energy Physics (ICHEP 2020), the ATLAS Collaboration announced first results using the ATLAS Forward Proton (AFP) spectrometer (Figure 1). With this instrument, physicists directly observed and measured the long sought-after prediction of proton scattering when particles of light turn into matter.

    1
    Figure 1: Schematic diagram of ATLAS Forward Proton (AFP) spectrometer relative to the main ATLAS detector (not to scale). After the incident proton beams intersect, the leptons are detected by the main ATLAS detector and the scattered proton is detected by AFP. (Image: ATLAS Collaboration/CERN)

    In 1928, theoretical physicist Paul Dirac predicted the existence of the positron, the positively-charged antimatter partner to the electron. When brought together, this matter–antimatter pair annihilates into two particles of light (photons). Remarkably, quantum mechanics predicts that the reverse can also occur. Two photons with sufficient energy can turn into a matter–antimatter pair, as shown in Figure 2.

    2
    Figure 2: Diagram of a pair of photons (γ) turning into a pair of leptons (electrons or muons) (ℓ). The scattered protons (p) can remain intact in such interactions, but deflected from their paths along the beam so that they can be measured in a proton spectrometer. (Image: ATLAS Collaboration/CERN)

    To observe this phenomenon, physicists can use the LHC, a proton–proton collider, as a photon–photon collider. Usually, particles are created by protons colliding head-on which break apart. However, if two protons pass very close to each other, they can scatter via the electromagnetic force to produce photons that turn into a matter–antimatter pair. The two protons remain intact, continuing their path in the LHC beam pipe, which the AFP spectrometer can detect. Observing these intact scattered protons is a hallmark of a photon–photon collision.

    The AFP spectrometer is unique in many ways. Installed in 2017, it is one of the newest additions to the ATLAS experiment. It sits either side of the main ATLAS cavern, just over 200 metres downstream from the collision point as shown in Figure 1. Its detectors are based on silicon technology, which reach directly into the LHC beam pipe to only two millimetres from the proton beam itself. If a scattered proton emits a photon and loses a few percentage points of energy, the LHC magnets deflect the proton into the AFP spectrometer. These scattered protons are among the highest-energy particles measured at the LHC.

    Physicists studied data recorded by the AFP spectrometer throughout 2017 to establish direct evidence of these scattered protons when matter – electron–positron or muon–antimuon pairs – are created from the interaction of two photons. This was achieved by comparing the proton energy loss measured by the AFP spectrometer from the proton deflection angle to the produced matter–antimatter pair recorded in the central ATLAS experiment, as shown in Figure 3. If the scattered proton arose while photons turned into matter, the measurements from both locations are predicted to be equal (within the measurement precision).

    The ATLAS Collaboration has observed this striking phenomenon, recording 180 events that have an intact proton detected by the AFP spectrometer and a matching electron–positron or muon–antimuon pair measured in the main ATLAS detector. The expected background from accidentally matching forward protons amounts to about 20 events. The statistical significance of this result thus exceeds 9 standard deviations for each electron and muon channels.

    This landmark measurement using the AFP spectrometer provides valuable information about how often the protons stay intact, which is challenging to calculate from theory. These measurements are important tests of how light interacts with matter at the highest laboratory energies. Certain theories predict such interactions are modified by new particles that could explain the mysterious dark matter in our universe. With more data, physicists can use the AFP to search for these phenomena in new ways.

    3
    Figure 3: The fractional proton energy loss measured by the AFP spectrometer (ξAFP) is compared to that measured from the electron or muon pairs in the central ATLAS detector (ξll). A signal peak is observed when these two quantities are approximately equal, indicating that the scattered proton emitted a photon that produced the lepton pair. The labels A and C denote opposite sides of the collision point along the beam line. (Image: ATLAS Collaboration/CERN)

    See the full article here .


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  • richardmitnick 8:39 am on July 30, 2020 Permalink | Reply
    Tags: "Jetting into the dark side: a precision search for dark matter", , , CERN ATLAS, , , Momentum conservation in the transverse detector plane – that is perpendicular to the beam direction ., , ,   

    From CERN ATLAS: “Jetting into the dark side: a precision search for dark matter” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    27th July 2020
    ATLAS Collaboration

    1
    Figure 1: A monojet event recorded by the ATLAS experiment in 2017, with a single jet of 1.9 TeV transverse momentum recoiling against corresponding missing transverse momentum (MET). The green and yellow bars show the energy deposits in the electromagnetic and hadronic calorimeters, respectively. The MET is shown as the red dashed line on the opposite side of the detector. (Image: ATLAS Collaboration/CERN)

    The nature of Dark Matter remains one of the great unsolved puzzles of fundamental physics. Unexplained by the Standard Model, dark matter has led scientists to probe new physics models to understand its existence.

    Standard Model of Particle Physics, Quantum Diaries

    Many such theoretical scenarios postulate that dark matter particles could be produced in the intense high-energy proton–proton collisions of the LHC. While the dark matter would escape the ATLAS detector unseen, it could occasionally be accompanied by a visible jet of particles radiated from the interaction point, thus providing a detectable signal.

    The ATLAS Collaboration set out to find just that. Today, at the International Conference in High-Energy Physics (ICHEP 2020), ATLAS presented a new search for novel phenomena in collision events with jets and high missing transverse momentum (MET). The search was designed to uncover events that could indicate the existence of physics processes that lie outside the Standard Model and, in doing so, open a window to the cosmos.

    To identify such events, physicists exploited the principle of momentum conservation in the transverse detector plane – that is, perpendicular to the beam direction – looking for visible jets recoiling from something invisible. As events with jets are common at the LHC, physicists further refined their parameters: the events had to have at least one highly energetic jet and significant MET, generated by the momentum imbalance of the “invisible” particles. This is known as a monojet event – a spectacular example of which can be seen in Figure 1, a 2017 event display featuring the highest-momentum (1.9 TeV) monojet recorded so far by ATLAS.

    A plethora of exotic phenomena, not directly detectable by collider experiments, could also have yielded this characteristic monojet signature. ATLAS physicists thus set out to make their study inclusive of several new physics models, including those featuring supersymmetry, dark energy, large extra spatial dimensions, or axion-like particles.

    2
    Figure 2: Missing transverse momentum distribution after the monojet selection in data and in the Standard Model predictions. The different background processes are shown with colours. The expected distributions of dark energy, supersymmetric and weakly-interacting massive particle scenarios are illustrated with dashed lines. (Image: ATLAS Collaboration/CERN).

    Evidence of new phenomena would be seen in an excess of collision events with large MET when compared to the Standard Model expectation. Accurately predicting the different background contributions was a key challenge, as several abundant Standard Model processes could exactly mimic the signal topology – such as the production of a jet plus a Z boson, which then decays to two neutrinos that also leave ATLAS without being directly detected.

    Physicists used a combination of data-driven techniques and high-precision theoretical calculations to estimate the Standard Model background. The total background uncertainty in the signal region ranges from about 1% to 4% in the range of MET between 200 GeV and 1.2 TeV. The shape of the MET spectrum was used to enhance the discrimination power between signals and backgrounds, thus increasing the discovery potential. Figure 2 shows a comparison of the MET spectrum observed in the entire dataset collected from the ATLAS experiment during Run 2 (2015–2018), and the Standard Model expectation.

    As no significant excess was observed, physicists used the level of agreement between data and the prediction to set limits on the parameters of new physics models. In the context of weakly-interacting massive particles (a popular dark matter candidate), ATLAS physicists were able to exclude dark matter particle masses up to about 500 GeV and interaction axial-vector mediators up to 2 TeV, both at the 95% confidence level. These results provide the most stringent dark matter limits in collider experiments so far, and a milestone of the ATLAS search programme.

    See the full article here .

    _________________________________________________

    Dark Matter Background
    Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

    Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

    Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova


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  • richardmitnick 6:39 pm on July 29, 2020 Permalink | Reply
    Tags: "ATLAS result addresses long-standing tension in the Standard Model", , , CERN ATLAS, Each lepton flavour is equally likely to interact with a W boson., , Lepton flavour universality, , ,   

    From CERN ATLAS: “ATLAS result addresses long-standing tension in the Standard Model” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    29 July, 2020

    A new ATLAS measurement of a key feature of the Standard Model known as lepton flavour universality suggests that a previous discrepancy measured by the LEP collider in W boson decays may be due to a fluctuation.


    Researchers from the ATLAS collaboration explain their new measurement of “lepton flavour universality” – a unique property of the Standard Model of particle physics. (Video: CERN)

    The best-known particle in the lepton family is the electron, a key building block of matter and central to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, the muon and the tau lepton, and together they are known as the three lepton flavours. According to the Standard Model of particle physics, the only difference between the siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lepton is about 17 times heavier than the muon. It is a remarkable feature of the Standard Model that each flavour is equally likely to interact with a W boson, which results from the so-called lepton flavour universality. Lepton flavour universality has been probed in different processes and energy regimes to high precision.

    Standard Model of Particle Physics, Quantum Diaries

    In a new study, described in a paper posted today on the arXiv [ “Test of the universality of τ and μ lepton couplings in W-boson decays from tt¯ events with the ATLAS detector” ( https://arxiv.org/abs/2007.14040 )] and first presented at the LHCP 2020 conference, the ATLAS collaboration presents a precise measurement of lepton flavour universality using a brand-new technique.

    ATLAS physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. “The LHC is a top-quark factory, and produced 100 million top-quark pairs during Run 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a large unbiased sample of W bosons decaying to muons and tau leptons, which was essential for this high-precision measurement.”

    They then measured the relative probability that the lepton resulting from a W-boson decay is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity, as the strength of the interaction with a W boson should be the same for a tau-lepton and a muon. But there has been tension about this ever since the 1990s when experiments at the Large Electron-Positron (LEP) collider measured R(τ/μ) to be 1.070 ± 0.026, deviating from the Standard Model expectation by 2.7 standard deviations.

    CERN Large Electron Positron Collider

    The new ATLAS measurement gives a value of R(τ/μ) = 0.992 ± 0.013. This is the most precise measurement of the ratio to date, with an uncertainty half the size of that from the combination of LEP results. The ATLAS measurement is in agreement with the Standard Model expectation and suggests that the previous LEP discrepancy may be due to a fluctuation.

    “The LHC was designed as a discovery machine for the Higgs boson and heavy new physics,” says ATLAS Spokesperson Karl Jakobs. “But this result further demonstrates that the ATLAS experiment is also capable of measurements at the precision frontier. Our capacity for these types of precision measurements will only improve as we take more data in Run 3 and beyond.”

    Although it has survived this latest test, the principle of lepton flavour universality will not be completely out of the woods until the anomalies in B-meson decays recorded by the LHCb experiment have also been definitively probed.

    See the full article here .


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  • richardmitnick 12:00 pm on July 26, 2020 Permalink | Reply
    Tags: "ATLAS one step closer in the search for rare Higgs boson decays to muons", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS one step closer in the search for rare Higgs boson decays to muons” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    23rd July 2020
    ATLAS Collaboration

    1
    Figure 1: A Run 2 Higgs boson candidate event containing two muons (red) and two jets (yellow cones). (Image: ATLAS Collaboration/CERN)

    “Who ordered that?” commented physicist Isidor Isaac Rabi when the muon was discovered in 1936. In the 80 years since, scientists have learnt a lot about the muon’s role in our Universe and have studied its properties with extreme precision. Muons have even been used via decays of intermediate weak bosons in the detection of new particles, such as the Higgs boson – now the centerpiece of its own extremely rich field of research.

    In the Standard Model, elementary particles acquire mass through interaction with the Higgs field: the stronger the interaction, the larger the mass of the particle.

    Standard Model of Particle Physics, Quantum Diaries

    So far, physicists have collected conclusive evidence of the Higgs boson interacting with bosons and the heaviest elementary fermions belonging to the third fermion generation (tau-lepton as well as top and bottom quarks). Yet to date, there is no indication whether the Higgs boson interacts with the next lighter fermions, muon or charm quark, belonging to the second fermion generation.

    The ATLAS Collaboration has released a new paper [ https://arxiv.org/abs/2007.07830 ] on the search for the Higgs-boson decay to a pair of muons. The new study uses the entire dataset collected by the ATLAS experiment during Run 2 of the LHC (2015–2018) to give a first hint of this elusive process.

    The wheat from the chaff

    Despite a simple experimental signature, spotting this rare decay continues to be a challenge. This is due to the low probability of the Higgs boson decaying to muons (predicted to be just 0.02%) and the large number of events from similar Standard Model background processes that can dominate the search. Only 0.2% of selected muon-pair events with masses between 120 and 130 GeV from proton–proton collisions are expected to come from a Higgs-boson decay.

    Fortunately, a signal can be distinguished from background processes by looking at the shape of the mass distribution of the precisely measured muon pairs. Higgs-boson events will cluster around the Higgs-boson mass of 125 GeV, producing a narrow peak that can be distinguished from the smoothly-falling distribution of background events. By fitting the invariant-mass spectrum, ATLAS physicists are able to directly constrain the background and extract a possible signal.

    2
    Figure 2: The invariant-mass spectrum of the reconstructed muon-pairs in ATLAS data. Events are weighted according to the expected signal-to-background ratio of their category. In the top panel, the signal-plus-background fit is visible in blue, while in the lower panel the fitted signal (in red) is compared to the difference between the data and the background model. (Image: ATLAS Collaboration/CERN)

    Divide et impera

    To further increase the sensitivity of their analysis, ATLAS physicists divided their events into 20 mutually-exclusive “categories”. These categories focussed on the features of an event – such as the number and properties of its additional jets or leptons – to target specific production modes of the Higgs boson, including the scattering of two gluons or two weak bosons, and the associated production with a weak W or Z boson or a top-quark pair. Inside these categories, events were further split using dedicated multivariate discriminants (Boosted Decision Trees). As a result of this complex division, ATLAS physicists could separate out the few Higgs-boson-like events from the more common, but less Higgs-boson-like, events.

    In addition, ATLAS physicists developed a robust (and ambitious) background-modelling strategy using a variety of simulation techniques to create more than 10 billion simulated events. Detailed ATLAS detector simulations (totalling about five times the Run-2 dataset) were complemented by dedicated fast simulation samples (more than 100 times the dataset). The fast simulation samples were crucial to ensure that the overwhelming backgrounds could not mimic a false signal, while maximising the analysis sensitivity to a real signal.

    Let the die be cast

    The new ATLAS result gives a first hint of the Higgs boson decaying to a muon pair; the significance of the observed signal amounts to 2.0 standard deviations and the ratio of the observed signal yield to the one expected in the Standard Model is 1.2 ± 0.6. The data, together with the signal-plus-background fit, are shown in Figure 2, where data events are weighted to reflect the signal-to-background ratio of their respective categories. More data to be collected in Run 3 (2022–2024) and during the operation of the High-Luminosity LHC will help close in on this first hint.

    See the full article here .


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  • richardmitnick 1:12 pm on June 19, 2020 Permalink | Reply
    Tags: "ATLAS Experiment measures light scattering on light and constrains axion-like particles", , , , CERN ATLAS, , , , ,   

    From CERN ATLAS via phys.org: “ATLAS Experiment measures light scattering on light and constrains axion-like particles” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    via


    phys.org

    June 19, 2020

    1
    Figure 1: Differential cross section of γγ→γγ production in lead–lead collisions at 5.02 TeV as a function of the invariant mass of the diphoton system and the cosine of the scattering angle in the photon-photon centre-of-mass frame, as measured by ATLAS. The measurements are compared to the theoretical prediction. Credit: ATLAS Collaboration/CERN.

    Light-by-light scattering is a rare phenomenon in which two photons—particles of light—interact, producing another pair of photons. Direct observation of this process at high energy had proven elusive for decades, until it was first seen by the ATLAS Experiment in 2016 and established in 2019. In a new measurement, ATLAS physicists are using light-by-light scattering to search for a hyped phenomenon beyond the Standard Model of particle physics: axion-like particles.

    Collisions of heavy lead ions in the Large Hadron Collider (LHC) provide the ideal environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated corresponding to an electrical field with strength of up to 1025 volt per metre. When ions from opposite beams pass next to each other at the centre of the ATLAS detector, their surrounding photons can interact and scatter off one another. Because the lead ions lose only a tiny fraction of their energy in this process, the outgoing ions continue their path around the LHC ring, unseen by the ATLAS detector. These interactions are known as ultra-peripheral collisions. This leads to a distinct event signature, very unlike typical lead ion collision events, with two back-to-back photons and no further activity in the detector.

    Based on lead-lead collision data recorded in 2015, the ATLAS Collaboration found the first direct evidence of high-energy light-by-light scattering. More recently the ATLAS Collaboration reported the observation of light-by-light scattering with a significance of 8.2 standard deviations, using a large data sample taken in 2018.

    2
    Figure 2: Compilation of exclusion limits at 95% confidence level in the photon–a (axion-like particle) coupling (1/Λa) versus a mass (ma) plane obtained by different experiments. The existing limits are compared to the limits extracted from this measurement. Credit: ATLAS Collaboration/CERN

    The ATLAS Collaboration has studied the full LHC Run-2 dataset of heavy-ion collisions to measure light-by-light scattering with improved precision and more detail. Out of the more than hundred billion ultra-peripheral collisions probed, ATLAS observed a total of 97 candidate events while 27 events are expected from background processes. In addition to the production rate (cross section), ATLAS measured the energies and angular distributions of the produced photons (i.e. their kinematics). The result explores a broader range of diphoton masses, increasing the expected signal yield by about 50% in comparison to the previous ATLAS measurements.

    The measurement of light-by-light scattering is sensitive to processes beyond the Standard Model, such as axion-like particles. These are hypothetical spin-less (scalar) particles with an odd parity quantum number (the Higgs boson, for example, is a scalar with even parity) and typically weak interactions with Standard Model particles. In the new ATLAS result, physicists considered whether the pairs of interacting photons produce axion-like particles (a) as they scatter off each other (γγ → a → γγ), which would lead to an excess of scattering events with diphoton mass equal to the mass of a. They examined the diphoton mass distribution for a mass range for a between 6 and 100 GeV. No significant excess of events over the expected background was found in the analysis. ATLAS physicists were able to derive, at a 95% confidence level, an exclusion bound of the axion-like particles coupling to photons (Figure 2). Assuming 100% of the putative particles decay to photons, this new analysis places the strongest existing limits on the production of axion-like particles in the examined mass range to date.

    With the much larger dataset expected in the future LHC runs, physicists will continue to explore the sensitivity of light-by-light scattering to phenomena beyond the Standard Model.

    More information: Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb−1 of Pb+Pb data with the ATLAS detector (ATLAS-CONF-2020-010):
    https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-010/

    See the full article here .


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  • richardmitnick 10:18 am on April 22, 2020 Permalink | Reply
    Tags: , ATLAS Experiment measures the 'beauty' of the Higgs boson, , CERN ATLAS, , , , ,   

    From CERN ATLAS via phys.org: “ATLAS Experiment measures the ‘beauty’ of the Higgs boson” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    via


    phys.org

    April 22, 2020

    1
    Figure 1: Event display of a very boosted H→bb candidate event where particles originating from the two b-quarks (green and yellow energy deposits in the calorimeters) have been merged into a single jet (blue cone). Credit: ATLAS Collaboration/CERN

    Two years ago, the Higgs boson was observed decaying to a pair of beauty quarks (H→bb), moving its study from the “discovery era” to the “measurement era.” By measuring the properties of the Higgs boson and comparing them to theoretical predictions, physicists can better understand this unique particle, and in the process, search for deviations from predictions that would point to new physics processes beyond our current understanding of particle physics.

    One such deviation could be the rate at which Higgs bosons are produced under particular conditions. The larger the transverse momentum of the Higgs boson—that is, the momentum of the Higgs boson perpendicular to the direction of the Large Hadron Collider (LHC) proton beams—the greater we believe is the sensitivity to new physics processes from heavy, yet unseen particles.

    H→bb is the ideal search channel to search for such deviations in the production rate. As the most likely decay of the Higgs boson (accounting for ~58% of all Higgs-boson decays), its larger abundance allows physicists to probe further into the high-transverse-momentum regions, where the production rate decreases due to the composite structure of the colliding protons.

    In new results released this month, the ATLAS Collaboration at CERN studied the full LHC Run 2 dataset to give an updated measurement of H→bb, where the Higgs boson is produced in association with a vector boson (W or Z). Among several new results, ATLAS reports the observation of Higgs-boson production in association with a Z boson with a significance of 5.3 standard deviations (σ), and evidence of production with a W boson with a significance of 4.0 σ.

    2
    Figure 2. Observed and predicted distribution for one of the 14 BDTs used to separate the Higgs boson signal from the background processes. The Higgs boson signal is shown in red, the backgrounds in various colours. The data points are shown as points with error bars. Credit: ATLAS Collaboration/CERN

    The new analysis uses ~75% more data than the previous edition. Further, ATLAS physicists implemented several improvements including:

    Better Boosted Decision Tree (BDT) machine learning algorithms used to separate collisions containing a Higgs boson from those containing only background processes. Figure 2 shows the separation achieved between these processes by one of the BDTs.
    Updated selections used to identify collisions of interest enriched in the various background processes. These “control regions” allowed the physicists to gain a better understanding of and a handle on the background processes.
    Increased number of simulated collisions. Whilst crucial for predicting backgrounds in a measurement, simulating collisions throughout the ATLAS detector is a compute-intensive process. In this new analysis, teams throughout ATLAS made strong efforts to increase the number of simulated collisions by a factor of four compared to the previous analysis.

    3
    Figure 3: A comparison of the excess of collision data (black points) over the background processes (subtracted from the data). Shown are the reconstructed mass from the H→bb decays (red) and the well-understood diboson Z→bb decay (grey) used to validate the result. Credit: ATLAS Collaboration/CERN

    These improvements allowed ATLAS physicists to make more precise measurements of the Higgs-boson production rate at different transverse momenta, and to extend their reach to higher values.

    ATLAS physicists also announced an extension to the H→bb study: a new version of the analysis designed to probe the Higgs boson when it is produced with very large transverse momenta. Normally, the two b-quarks from the H→bb decay manifest themselves in the ATLAS detector as two separate sprays of highly collimated and energetic particles, called “jets.” However, when the Higgs boson is produced at very large transverse momentum, exceeding twice the Higgs-boson mass of 125 GeV, the H→bb system is “boosted.” The two b-quarks then tend to be produced close together, merging into one jet, as shown in the event display above. The new analysis used different b-jet reconstruction algorithms tuned to this boosted regime. They allowed physicists to identify boosted H→bb decays, reconstruct the mass of the Higgs boson, and identify an excess over the background processes, as shown in Figure 3.

    The new technique allowed ATLAS to explore the particularly interesting Higgs-boson phase space of large transverse momentum events with improved efficiency. It further allowed physicists to look at Higgs bosons produced at even larger transverse-momentum values—an important advancement in the search for new physics.

    These analyses are vital steps in a long journey towards measuring the properties of the Higgs boson. As physicists further enhance their algorithms, improve their understanding of background processes and collect more data, they venture ever further into uncharted territory where new physics may await.

    See the full article here .


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  • richardmitnick 1:17 pm on April 19, 2020 Permalink | Reply
    Tags: "Novel probes of the strong force: precision jet substructure and the Lund jet plane", , CERN ATLAS, , , ,   

    From CERN ATLAS: “Novel probes of the strong force: precision jet substructure and the Lund jet plane” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    19th April 2020

    A hallmark of the strong force at the Large Hadron Collider (LHC) is the dramatic production of collimated jets of particles when quarks and gluons scatter at high energies. Particle physicists have studied jets for decades to learn about the structure of quantum chromodynamics – or QCD, the theory of the strong interaction – across a wide range of energy scales.

    Due to their ubiquity, our understanding of jet formation and QCD is one of the factors which can limit understanding of other facets of the Standard Model at the LHC. By studying the rich substructure of jets, physicists can gather new clues about the behaviour of the strong force at high energies. An improved understanding of their formation also benefits a broad range of other studies, including measurements of the top quark and Higgs boson.

    1
    Figure 1: A histogram of the logarithm of the invariant mass normalized by the jet momentum (ρ) at the point in the jet history when a quark or a gluon radiated a significant fraction of its energy. The metric for determining “significant” is the soft-drop criteria. The ATLAS data are in black and various predictions from state-of-the-art QCD theory are shown in coloured markers. (Image: ATLAS Collaboration/CERN)

    Precision jet substructure

    Dissecting jet substructure requires both precise experimental measurements and theoretical calculations – two areas that have advanced significantly during Run 2 of the LHC. On the experimental side, ATLAS developed an accurate new method for reconstructing charged particle tracks inside jets. This has traditionally been quite challenging, due to the high density of particles inside the core of jets.

    On the theory side, there has been an outburst of new techniques for representing jet substructure, including new analytic predictions for what experiments should observe in their data. A key new theoretical idea makes use of clustering algorithms to study a jet’s constituents. Jets are constructed by taking a set of particles (experimentally, tracks and calorimeter energy deposits) and sequentially clustering them in pairs until the area of the jet candidates reaches a fixed size. The steps in a jet’s clustering history can also be traversed in reverse, allowing parts of the process to be associated with various steps in a jet’s evolution.

    The ATLAS Collaboration has released new measurements [Physical Review D] using this novel declustering methodology. Physicists were able to examine specific moments in a jet’s evolution where a quark or a gluon radiates a significant fraction of its energy. The jet’s mass at this stage is amenable to precise theoretical predictions, as shown in Figure 1.

    Achieving this result was a significant endeavour, as ATLAS physicists had first to account for distortions in the data due to the measurement process and to estimate the uncertainty on these corrections. The new theoretical predictions provided an excellent model of the data, allowing physicists to perform a stringent test of the strong force in a regime that had not been previously tested with this level of experimental and theoretical precision.

    Lund jet plane

    Physicists can also look beyond a single step in the clustering history by studying a new observable: the Lund jet plane. Its name is derived from the Lund plane diagrams that have been used by the QCD community for over 30 years, after their introduction in a paper by authors from Lund University (Sweden). In 2018, theorists applied the approach to jet substructure for the first time, designing a Lund jet plane to characterize the relative energy and angle of each declustering step (or emission) during a jet’s evolution. Through its study, physicists can investigate the statistical properties of all instances where the quark or gluon that initiated the jet radiated some fraction of its energy. Different physical effects become localised in specific regions of the plane, so that if predictions do not describe the data, physicists can identify the epoch in a jet’s history that needs to be investigated.

    ATLAS has performed the first measurement of the Lund jet plane [Physical Review Letters] , which is built from the energies and angles of each step in a jet’s evolution. ATLAS studied about 30 million jets to form the plane shown in Figure 2. For this result, physicists used measurements of particle tracks, as they provide excellent angular resolution for reconstructing radiation found in the dense core of jets.

    3
    Figure 2: The average number of declustering emissions in a given bin of relative energy (y-axis) and relative angle (x-axis), after accounting for detector effects. (Image: ATLAS Collaboration/CERN)

    4
    Figure 3: The horizontal slice through Figure 2 including comparisons to QCD predictions. (Image: ATLAS Collaboration/CERN)

    The figure uses colour to describe the average number of emissions observed in that region. The angular information of the jet is described in the horizontal axis, and its energy by the vertical axis. The number of emissions is approximately constant in the lower left corner (wide angle, large energy fraction) and there is a large suppression of emissions in the top right corner (where the angle is nearly collinear, low energy fraction). The first of these observations is related to the near scale-invariance of the strong force, as the masses of most quarks are tiny compared to the relevant energies at the LHC. The suppression in the top right corner is due to hadronization, the process by which quarks form bound states.

    To truly test the strong force, physicists dug deeper into this result. Figure 3 shows a horizontal slice through the plane, compared with state-of-the-art predictions based on the parton shower method. Parton showers are numerical simulations which describe the full radiation pattern inside jets, including the number of particles in the shower, their energies, angles and type.

    The different coloured predictions in Figure 3 change one aspect of the physics modelling at a time. For example, the orange markers show one prediction where the only difference between the open and closed markers is the model used to describe hadronization. It is exciting to see that the open and closed orange markers only differ on the right side of the plot, which is exactly where hadronization effects are expected to be localized. The same is true for the other colours, for example the open and closed green markers differ only on the left side of the plot. This demonstrates the utility of the ATLAS data for learning more about the various facets of the strong force and improving parton shower models.

    A growing field of exploration

    The highly-granular ATLAS detector is well-suited to measure jet substructure in great detail, and there is still much to learn about the strong force at high energies. While extracting insights cleanly from jet substructure measurements has historically been challenging, recent theoretical advancements have resulted in better first-principles understanding than ever before. This has opened new doors to put QCD to the test with ATLAS data, which have been made publicly available, so the QCD community will be able to learn from these additions to the growing field of precision jet substructure measurements for years to come.

    See the full article here .


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