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  • richardmitnick 12:22 pm on August 5, 2019 Permalink | Reply
    Tags: "ATLAS releases new search for strong supersymmetry", , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS releases new search for strong supersymmetry” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    5th August 2019

    1
    Figure 1: Distributions of observed data events, compared to the Standard Model prediction, for (left) a subset of the bins used in the multi-bin search, or (right) one of the BDT search discriminants. (Image: ATLAS Collaboration/CERN)

    New particles sensitive to the strong interaction might be produced in abundance in the proton-proton collisions generated by the LHC – provided that they aren’t too heavy. These particles could be the partners of gluons and quarks predicted by supersymmetry (SUSY), a proposed extension of the Standard Model of particle physics that would expand its predictive power to include much higher energies. In the simplest scenarios, these “gluinos” and “squarks” would be produced in pairs, and decay directly into quarks and a new stable neutral particle (the “neutralino”), which would not interact with the ATLAS detector. The neutralino could be the main constituent of dark matter.

    The ATLAS Collaboration has been searching for such processes since the early days of LHC operation. Physicists have been studying collision events featuring “jets” of hadrons, where there is a large imbalance in the momenta of these jets in the plane perpendicular to the colliding protons (“missing transverse momentum”, ETmiss). This missing momentum would be carried away by the undetectable neutralinos. So far, ATLAS searches have led to increasingly tighter constraints on the minimum possible masses of squarks and gluinos.

    Is it possible to do better, with more data? The probability of producing these heavy particles decreases exponentially with their masses, and thus repeating the previous analyses with a larger dataset only goes so far. New, sophisticated methods that help to better distinguish a SUSY signal from the background Standard Model events are needed to take these analyses further. Crucial improvements may come from increasing the efficiency for selecting signal events, improving the rejection of background processes, or looking into less-explored regions.

    Today, at the Lepton Photon Symposium in Toronto, Canada, the ATLAS Collaboration presented new results illustrating the benefits brought by more advanced analysis techniques, which were pioneered in other search channels. The sensitivity of the new analysis is significantly improved thanks to the use of two complementary approaches.

    In the first approach, referred to as the “multi-bin search”, the events are classified into bins defined by two observables: the effective mass and the ETmiss significance. These characterise the amount of energy involved in the interaction (large, if heavy particles were produced), and how unlikely the observed ETmiss is to be caused by the escaping neutralinos rather than the mismeasurement of jet energies. With up to 24 orthogonal bins defined at a time, the search is sensitive to a large variety of masses of gluinos, squarks and neutralinos (Figure 1 (left)).

    The second approach, known as the “Boosted Decision Tree (BDT) search”, uses machine learning classification algorithms to better discriminate a potential signal. The BDTs are trained with some of the kinematic properties of the jets + ETmiss final states, predicted by the Monte Carlo simulation for signal and background events. Eight such discriminants are defined, each optimised for a different region of the parameter and model space (Figure 1 (right)).

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    Figure 2: 95% confidence level exclusion limits on the masses of gluinos, squarks and neutralinos, in simplified signal scenarios assuming (left) only the pair production of gluinos, or (right) the combined pair production of gluinos and squarks for a neutralino mass of 0 GeV. (Image: ATLAS Collaboration/CERN)

    The new results made use of the full LHC Run 2 dataset, corresponding to an integrated luminosity of 139 fb-1, and did not show any significant difference between the number of observed events and the Standard Model predictions in the signal-enriched regions. Exclusion limits were therefore set on the masses of gluinos, squarks and neutralinos, assuming different scenarios. Some examples are shown in Figure 2. For the multi-bin search, the strength of all the bins can be simultaneously brought to bear, increasing the exclusion power of the analysis.

    Links

    Search for squarks and gluinos in final states with jets and missing transverse momentum using 139 fb−1 of 13 TeV proton-proton collision data with the ATLAS detector (ATLAS-CONF-2019-040, link coming soon)
    Lepton Photon 2019 plenary presentation: Overview of the ATLAS Experiment by Pierre Savard
    Search for squarks and gluinos in final states with jets and missing transverse momentum using 36 fb−1 of 13 TeV proton-proton collision data with the ATLAS detector (Phys. Rev. D 97 (2018) 112001, see figures)
    Search for squarks and gluinos using final states with jets and missing transverse momentum with the ATLAS detector in 7 TeV proton-proton collisions (ATLAS-CONF-2011-086)
    Search for top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36 fb−1 of 13 TeV proton-proton collision data with the ATLAS detector (JHEP 06 (2018) 108, see figures)
    Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in 7 TeV proton-proton collisions (Phys. Rev. Lett. 106 (2011) 131802, see figures)
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .


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  • richardmitnick 12:49 pm on July 21, 2019 Permalink | Reply
    Tags: "A golden era of exploration: ATLAS highlights from EPS-HEP 2019", , CERN ATLAS, , , , ,   

    From CERN ATLAS: “A golden era of exploration: ATLAS highlights from EPS-HEP 2019” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    20th July 2019
    Katarina Anthony

    1
    Event display of a Higgs boson candidate decaying in the four-lepton channel. (Image: ATLAS Collaboration/CERN)

    Eight years of operation. Over 10,000 trillion high-energy proton collisions. One critical new particle discovery. Countless new insights into our universe. The Large Hadron Collider (LHC) has been breaking records since data-taking began in 2010 – and yet, for ATLAS and its fellow LHC experiments, a golden era of exploration is only just beginning.

    2
    Figure 1: New ATLAS measurement of the Higgs boson decaying in the four-lepton channel, using the full LHC Run-2 dataset. The distribution of the invariant mass of the four leptons (m4l) is shown. The Higgs boson corresponds to the excess of events (blue) over the non-resonant ZZ* background (red) at 125 GeV. (Image: ATLAS Collaboration/CERN)

    This week, the ATLAS Collaboration presented 25 new results at the European Physical Society’s High-Energy Physics conference (EPS-HEP) in Ghent, Belgium. The new analyses examine the largest-ever proton–proton collision dataset from the LHC, recorded during Run 2 of the accelerator (2015–2018) at the 13 TeV energy frontier.

    The new data have been fertile ground for ATLAS. New precision measurements of the Higgs boson, observations of key electroweak processes and high-precision tests of the Standard Model are among the highlights described below; find the full list of ATLAS public results using the full Run-2 dataset here.

    Studying the Higgs discovery channels

    Just over seven years ago, the Higgs boson was an elusive particle, out of reach from physicists for nearly five decades. Today, not only is the Higgs boson frequently observed, it is studied with such precision as to become a powerful tool for exploration.

    Key to these accomplishments are the so-called “Higgs discovery channels”: H→γγ, where the Higgs boson decays into two photons, and H→ZZ*→4l, where it decays via two Z bosons into four leptons. Though rare, these decays are easily identified in the ATLAS detector, making them essential to both the particle’s discovery and study.

    ATLAS presented new explorations of the Higgs boson in these channels (Figures 1 and 2), yielding greater insight into its behaviour. The new results benefit from the large full Run-2 dataset, as well as a number of new improvements to the analysis techniques. For example, ATLAS physicists now utilise Deep-Learning Neural Networks to assign the Higgs-boson events to specific production modes.

    All four Higgs-boson production modes can now be clearly identified in a single decay channel. ATLAS’ studies of the Higgs boson have advanced so quickly, in fact, that rare processes – such as its production in association with a top-quark pair, observed only just last year – can now been seen in just a single decay channel. The new sensitivity allowed physicists to measure kinematic properties of the Higgs boson with unprecedented precision (Figure 3). These are sensitive to new physics processes, making their exploration of particular interest to the collaboration.

    All four Higgs-boson production modes can now be clearly identified in a single decay channel. ATLAS’ studies of the Higgs boson have advanced so quickly, in fact, that rare processes – such as its production in association with a top-quark pair, observed only just last year – can now been seen in just a single decay channel. The new sensitivity allowed physicists to measure kinematic properties of the Higgs boson with unprecedented precision (Figure 3). These are sensitive to new physics processes, making their exploration of particular interest to the collaboration.

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    Figure 2: Distribution of the invariant mass of the two photons in the ATLAS measurement of H→γγ using the full Run-2 dataset. The Higgs boson corresponds to the excess of events observed at 125 GeV with respect to the non-resonant background (dashed line). (Image: ATLAS Collaboration/CERN)

    4
    Figure 3: Differential cross section for the transverse momentum (pT,H) of the Higgs boson from the two individual channels (H→ZZ*→4ℓ, H→γγ) and their combination. (Image: ATLAS Collaboration/CERN)

    Searching unseen properties of the Higgs boson

    Having accomplished the observation of Higgs boson interactions with third-generation quarks and leptons, ATLAS physicists are turning their focus to the lighter, second-generation of fermions: muons, charm quarks and strange quarks. While their interactions with the Higgs boson are described by the Standard Model, they have – so far – remained relegated to theory. Results from the ATLAS Collaboration are backing up these theories with real data.

    At EPS-HEP, ATLAS presented a new search for the Higgs boson decaying into muon pairs. This already-rare process is made all the more difficult to detect by background Standard Model processes, which produce muon pairs in abundance.

    5
    Figure 4: ATLAS search for the Higgs boson decaying to two muons. The plot shows the weighted muon pair invariant mass spectrum (muu) summed over all categories. (Image: ATLAS Collaboration/CERN)

    The new result utilised novel machine learning techniques to provide ATLAS’ most sensitive result yet, with a moderate excess of 1.5 standard deviations expected for the predicted signal. In agreement with this prediction, only a small excess of 0.8 standard deviations is present around the Higgs-boson mass in the data (Figure 4).

    “This result shows that we are now close to the sensitivity required to test the Standard Model’s predictions for this very rare decay of the Higgs boson,” said ATLAS spokesperson Karl Jakobs from the University of Freiburg, Germany. “However, a definitive statement on the second generation will require the larger datasets that will be provided by the LHC in Run 3 and by the High-Luminosity LHC.”

    ATLAS’ growing sensitivity was also clearly on display in the collaboration’s new “di-Higgs” search, where two Higgs bosons are formed via the fusion of two vector bosons. Though one of the rarest Standard Model processes explored by ATLAS, its study gives unique insight into the previously-untested relationship between vector boson and Higgs-boson pairs. A small variation of this coupling relative to the Standard Model value would result in a dramatic rise in the measured cross section. The new search, despite being negative, successfully sets the first constraints on this relationship.

    Entering the Higgs sector

    The Higgs mechanism, giving mass to all elementary particles, is directly connected with profound questions about our universe, including the stability and energy of the vacuum, the “naturalness” of a world described by the Standard Model, and more. As such, the exploration of the Higgs sector is not limited to direct measurements of the Higgs boson – it instead requires a broad experimental programme that will extend over decades.

    A perfect example of this came in ATLAS’ new observation of the electroweak production of two jets in association with a pair of Z bosons. The Z and W bosons are the force carriers of weak interactions and, as they both have a spin of 1, are known as “vector bosons”. The Higgs boson is a vital mediator in “vector-boson scattering”, an electroweak process that contributes to the pair production of vector bosons (WW, WZ and ZZ) with jets. Measurements of these production processes are key for the study of electroweak symmetry breaking via the Higgs mechanism.

    The new ATLAS result – with a statistical significance of 5.5 standard deviations (Figure 5) – completes the experiment’s observation of vector-boson scattering in these critical processes, and sparks new ways to test the Standard Model.

    6
    Figure 5: Observed and predicted distributions (BDT) in the signal regions of Z-boson pairs decaying to four leptons. The electroweak production of the Z-boson pair is shown in red; the error bars on the data points (black) show the statistical uncertainty on data. (Image: ATLAS Collaboration/CERN)

    7
    Figure 6: Summary of the mass limits on supersymmetry models set by the ATLAS searches for Supersymmetry. Results are quoted for the nominal cross section in both a region of near-maximal mass reach and a demonstrative alternative scenario, in order to display the range in model space of search sensitivity. (Image: ATLAS Collaboration/CERN)

    Probing new physics

    As the community enters the tenth year of supersymmetry searches at the LHC, the ATLAS Collaboration continues to take a broad approach to the hunt. ATLAS is committed to providing results that are theory-independent as well as signature-based searches, in addition to the highly-targeted, model-dependent ones.

    Along with new, updated limits on various supersymmetry searches using the full Run-2 dataset (Figure 6), ATLAS once again highlighted new searches (first presented at the LHCP2019 conference) for superpartners produced through the electroweak interaction. Generated at extremely low rates at the LHC and decaying into Standard Model particles that are themselves difficult to reconstruct, such supersymmetry searches can only be described by the iconic quote: “not because it is easy, but because it is hard”.

    Overall, the results place strong constraints on important supersymmetric scenarios, which will inform theory developments and future ATLAS searches. Further, they provide examples of how advanced reconstruction techniques can help improve the ATLAS’ sensitivity of new physics searches.

    Asymmetric top-quark production

    The Standard Model continued to show its strength in ATLAS’ new precision measurement of charge asymmetry in top-quark pairs (Figure 7). This intriguing imbalance – where top and antitop quarks are not produced equally at all angles with respect to the proton beam direction – is among the most subtle, difficult and yet vital properties to measure in the study of top quarks.

    The effect of this asymmetry is predicted to be extremely small, however new physics processes interfering with the known production modes can lead to larger (or even smaller) values. ATLAS found evidence of this imbalance, with a significance of four standard deviations, with a value compatible with the Standard Model. The result marks an important milestone for the field, following decades of measurements which began at the Tevatron proton–antiproton collider, the predecessor of the LHC in the USA.

    FNAL/Tevatron


    FNAL/Tevatron map

    8
    Figure 7: Measured values of the charge asymmetry (Ac) as a function of the invariant mass of the top quark pair system (mtt) in data. (Image: ATLAS Collaboration/CERN)

    Following the data

    As EPS-HEP 2019 drew to a close, it was clear that exploration of the high-energy frontier remains far from complete. With the LHC – and its upcoming HL-­LHC upgrade – set to continue apace, the future of high-energy physics will be guided by the results of ATLAS and its fellow experiments at the energy frontier.

    “Our community is living through data-driven times,” said ATLAS Deputy Spokesperson Andreas Hoecker from CERN. “Experimental results must guide the high-energy physics community to the next stage of exploration. This requires a broad and diverse particle physics research programme. The ATLAS Collaboration is up to taking this challenge!”

    See the full article here .


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  • richardmitnick 12:10 pm on July 15, 2019 Permalink | Reply
    Tags: , , CERN ATLAS, , , , , , ,   

    From CERN: “Exploring the Higgs boson “discovery channels” 

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    From CERN

    12th July 2019
    ATLAS Collaboration

    1
    Event display of a two-electron two-muon ZH candidate. The Higgs candidate can be seen on the left with the two leading electrons represented by green tracks and green EM calorimeter deposits (pT = 22 and 120 GeV), and two subleading muons indicated by two red tracks (pT = 34 and 43 GeV). Recoiling against the four lepton candidate in the left hemisphere is a dimuon pair in the right hemisphere indicated by two red tracks (pT = 139 and 42 GeV) and an invariant mass of 91.5 GeV, which agrees well with the mass of the Z boson. (Image: ATLAS Collaboration/CERN)

    At the 2019 European Physical Society’s High-Energy Physics conference (EPS-HEP) taking place in Ghent, Belgium, the ATLAS and CMS collaborations presented a suite of new results. These include several analyses using the full dataset from the second run of CERN’s Large Hadron Collider (LHC), recorded at a collision energy of 13 TeV between 2015 and 2018. Among the highlights are the latest precision measurements involving the Higgs boson. In only seven years since its discovery, scientists have carefully studied several of the properties of this unique particle, which is increasingly becoming a powerful tool in the search for new physics.

    The results include new searches for transformations (or “decays”) of the Higgs boson into pairs of muons and into pairs of charm quarks. Both ATLAS and CMS also measured previously unexplored properties of decays of the Higgs boson that involve electroweak bosons (the W, the Z and the photon) and compared these with the predictions of the Standard Model (SM) of particle physics. ATLAS and CMS will continue these studies over the course of the LHC’s Run 3 (2021 to 2023) and in the era of the High-Luminosity LHC (from 2026 onwards).

    The Higgs boson is the quantum manifestation of the all-pervading Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. Scientists look for such interactions between the Higgs boson and elementary particles, either by studying specific decays of the Higgs boson or by searching for instances where the Higgs boson is produced along with other particles. The Higgs boson decays almost instantly after being produced in the LHC and it is by looking through its decay products that scientists can probe its behaviour.

    In the LHC’s Run 1 (2010 to 2012), decays of the Higgs boson involving pairs of electroweak bosons were observed. Now, the complete Run 2 dataset – around 140 inverse femtobarns each, the equivalent of over 10 000 trillion collisions – provides a much larger sample of Higgs bosons to study, allowing measurements of the particle’s properties to be made with unprecedented precision. ATLAS and CMS have measured the so-called “differential cross-sections” of the bosonic decay processes, which look at not just the production rate of Higgs bosons but also the distribution and orientation of the decay products relative to the colliding proton beams. These measurements provide insight into the underlying mechanism that produces the Higgs bosons. Both collaborations determined that the observed rates and distributions are compatible with those predicted by the Standard Model, at the current rate of statistical uncertainty.

    Since the strength of the Higgs boson’s interaction is proportional to the mass of elementary particles, it interacts most strongly with the heaviest generation of fermions, the third. Previously, ATLAS and CMS had each observed these interactions. However, interactions with the lighter second-generation fermions – muons, charm quarks and strange quarks – are considerably rarer. At EPS-HEP, both collaborations reported on their searches for the elusive second-generation interactions.
    ATLAS presented their first result from searches for Higgs bosons decaying to pairs of muons (H→μμ) with the full Run 2 dataset. This search is complicated by the large background of more typical SM processes that produce pairs of muons. “This result shows that we are now close to the sensitivity required to test the Standard Model’s predictions for this very rare decay of the Higgs boson,” says Karl Jakobs, the ATLAS spokesperson. “However, a definitive statement on the second generation will require the larger datasets that will be provided by the LHC in Run 3 and by the High-Luminosity LHC.”
    CMS presented their first result on searches for decays of Higgs bosons to pairs of charm quarks (H→cc). When a Higgs boson decays into quarks, these elementary particles immediately produce jets of particles. “Identifying jets formed by charm quarks and isolating them from other types of jets is a huge challenge,” says Roberto Carlin, spokesperson for CMS. “We’re very happy to have shown that we can tackle this difficult decay channel. We have developed novel machine-learning techniques to help with this task.”

    3
    An event recorded by CMS showing a candidate for a Higgs boson produced in association with two top quarks. The Higgs boson and top quarks decay leading to a final state with seven jets (orange cones), an electron (green line), a muon (red line) and missing transverse energy (pink line) (Image: CMS/CERN)

    The Higgs boson also acts as a mediator of physics processes in which electroweak bosons scatter or bounce off each other. Studies of these processes with very high statistics serve as powerful tests of the Standard Model. ATLAS presented the first-ever measurement of the scattering of two Z bosons. Observing this scattering completes the picture for the W and Z bosons as ATLAS has previously observed the WZ scattering process and both collaborations the WW processes. CMS presented the first observation of electroweak-boson scattering that results in the production of a Z boson and a photon.
    “The experiments are making big strides in the monumental task of understanding the Higgs boson,” says Eckhard Elsen, CERN’s Director of Research and Computing. “After observation of its coupling to the third-generation fermions, the experiments have now shown that they have the tools at hand to address the even more challenging second generation. The LHC’s precision physics programme is in full swing.”

    See the full article here .


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    THE FOUR MAJOR PROJECT COLLABORATIONS

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  • richardmitnick 9:00 am on July 12, 2019 Permalink | Reply
    Tags: "ATLAS searches for rare Higgs boson decays into muon pairs", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS searches for rare Higgs boson decays into muon pairs” 

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    From CERN ATLAS

    11th July 2019
    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)

    Could the Higgs boson still surprise us? Since its discovery in 2012, the ATLAS and CMS collaborations at CERN have been actively studying the properties of this latest and most mysterious addition to the Standard Model of particle physics.

    In the Standard Model, the Brout-Englert-Higgs mechanism predicts the Higgs boson will interact with matter particles (quarks and leptons, known as fermions) with a strength proportional to the particle’s mass. It also predicts the Higgs boson will interact with the force carrier particles (W and Z bosons) with a strength proportional to the square of the particle’s mass. Therefore, by measuring the Higgs boson decay and production rates, which depend on the interaction strength to these other particles, ATLAS physicists can perform a fundamental test of the Standard Model.

    Today, at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, the ATLAS Collaboration released a new preliminary result searching for Higgs boson decays to a muon and antimuon pair (H → μμ). The new, more sensitive result uses the full Run 2 dataset, analysing almost twice as many Higgs boson events as the previous ATLAS result (released in 2018, for the ICHEP conference).

    Both the ATLAS and CMS Collaborations have already observed the Higgs boson decaying to tau lepton – the muon’s heavier cousin, belonging to the third “generation” of fermions. Since muons are much lighter than tau leptons, the Higgs boson decay to a muon pair is expected to occur about 300 times less often than that to a tau-lepton pair. Despite this scarceness, the H → μμ decay offers the best opportunity to measure the Higgs interaction with second-generation fermions at the LHC, providing new insights into the origin of mass for different fermion generations.

    2
    Figure 2: The muon pair invariant mass spectrum summed over all categories. Each event is weighted by log(1+S/B), where S and B are the number of signal and background events between 120 and 130 GeV of a given category determined by the simultaneous fit. The weighting visualizes the effect of the categorization on the analysis. The curves show the results of the fit. (Image: ATLAS Collaboration/CERN)

    Experimentally, ATLAS is well-equipped to identify and reconstruct muon pairs. By combining measurements from the ATLAS inner detector and muon spectrometer, physicists can achieve a good muon momentum resolution. However, they must also account for muons being created by a common background: the abundant “Drell-Yan process”, where a muon pair is produced via the exchange of a virtual Z boson or a photon. To help differentiate the H → μμ signal from this background, ATLAS teams use multivariate discriminants (boosted decision trees), which exploit the different production and decay properties of each event. For example, H → μμ signal events are characterised by a more central muon pair system and a larger momentum in the plane transverse to the colliding protons.

    To further enhance the sensitivity of the search, physicists separate the potential H → μμ events into multiple categories, each with different expected signal-to-background ratios. They examine each category separately, studying the distribution of the mass of the muon pair of the selected events. The signal and background abundances could then be determined simultaneously by a fit to the mass spectrum, exploiting the different shapes of the signal and background processes. Figure 2 shows the resulting muon pair mass distribution combined over all the categories.

    In the new ATLAS result, no significant excess of events above the measured background was observed in the signal region around the Higgs boson mass of 125 GeV. The observed signal significance is 0.8 standard deviations for 1.5 standard deviations expected from the Standard Model. An upper limit on the Higgs boson production cross section times branching fraction to muons was set at 1.7 times the Standard Model prediction at 95% confidence level. This new result represents an improvement of about 50% with respect to previous ATLAS results.

    See the full article here .


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  • richardmitnick 8:42 am on July 12, 2019 Permalink | Reply
    Tags: "ATLAS finds evidence of charge asymmetry in top quark pairs", , CERN ATLAS, , ,   

    From CERN ATLAS: “ATLAS finds evidence of charge asymmetry in top quark pairs” 

    CERN/ATLAS detector

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    From CERN ATLAS

    11th July 2019
    ATLAS Collaboration

    1
    Figure 1: Measured values of the charge asymmetry (Ac) as a function of the invariant mass of the top quark pair system (mtt) in data. The green hatched regions show new state-of-the-art Standard Model predictions, while red hatched regions show the asymmetry as implemented in simulated ‘Monte Carlo’ events. Vertical bars correspond to the total uncertainties. (Image: ATLAS Collaboration/CERN)

    Among the most intriguing particles studied by the ATLAS collaboration is the top quark. As the heaviest known fundamental particle, it plays a unique role in the Standard Model of particle physics and – perhaps – in yet unseen physics beyond the Standard Model.

    During Run 2 of the Large Hadron Collider (LHC), proton beams were collided with high luminosity at a centre-of-mass energy of 13 TeV. This allowed ATLAS to detect and measure an unprecedented number of events involving top-antitop quark pairs, providing ATLAS physicists with a unique opportunity to gain insight into the top quark’s properties.

    Due to sneaky interference between particles involved in the production, top and antitop quarks are not produced equally with respect to the proton beam direction in the ATLAS detector. Instead, top quarks are produced preferentially in the centre of the LHC’s collisions, while antitop quarks are produced preferentially at larger angles. This is known as a ‘charge asymmetry’.

    Charge asymmetry is similar to a phenomenon measured at the Tevatron collider at Fermilab, known as a ‘forward-backward’ asymmetry. At Tevatron, colliding beams were made of protons and anti-protons, respectively, which led to top and antitop quarks each being produced at non-central angles, but in opposite directions. A forward-backward asymmetry, compatible with improved Standard Model predictions, was observed.

    FNAL/Tevatron map

    FNAL/Tevatron

    2
    Figure 2: Confidence limits on the linear combination C−/Λ2 of Wilson coefficients of dimension-six EFT operators. The bounds are derived from a comparison of the charge-asymmetry measurements presented in this paper with the state-of-the-art Standard Model predictions. Also shown are bounds derived from the forward-backward asymmetry measurements at the Tevatron using collisions at a centre-of-mass energy 1.96TeV, at Run 1 LHC charge-asymmetry measurements in proton-proton collisions at a centre-of-mass energy of 8 TeV. (Image: ATLAS Collaboration/CERN)

    The effect of charge asymmetry at the LHC is predicted to be extremely small (< 1%), as the dominant production mode of top-quark pairs via the scattering of gluons (the carriers of the strong force) emerging from the protons does not exhibit a charge asymmetry. A residual asymmetry can only be generated by more complicated scattering processes involving also quarks. However, new physics processes interfering with the known production modes can lead to much larger (or even smaller) values. Therefore, a precision measurement of the charge asymmetry is a stringent test of the Standard Model. It is among the most subtle, difficult, and yet important properties to measure in the study of top quarks.

    A new ATLAS result, presented today at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, examines the full Run 2 dataset to measure top-antitop production in a channel where one top quark decays to one charged lepton, a neutrino and one hadronic "jet" (a spray of hadrons); and the other decays to three hadronic jets. The analysis fully includes events where the hadronic jets are merged together (so-called "boosted topology").

    ATLAS finds evidence of charge asymmetry in top-quark pair events, with a significance of four standard deviations. The measured charge asymmetry of 0.0060 ± 0.0015 (stat+syst.) is compatible with the latest Standard Model prediction, and the measurement confidently states that the observed asymmetry is non-zero. It is the first ATLAS top physics measurement to utilise the full Run 2 dataset.

    The new ATLAS result marks a very important milestone following decades of measurements. Figure 1 shows that the dataset allows ATLAS to measure the charge asymmetry as a function of the mass of the top-antitop system. Figure 2 shows the resulting bounds on anomalous effective field theory (EFT) couplings that parametrise effects from new physics which would be beyond the reach of being directly produced at the LHC.

    This new result is yet another demonstration of ATLAS’ ability to study subtle Standard Model effects with great precision. The observed agreement with Standard Model predictions provides one more piece to the puzzle in our understanding of particle physics at the energy frontier.

    See the full article here .


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  • richardmitnick 4:10 pm on July 5, 2019 Permalink | Reply
    Tags: CERN ATLAS, Simulation and supersymmetry two things that have defined Zachary Marshall’s career. Zach is a researcher with Lawrence Berkeley National Lab., Zach is currently the co-convener of the ATLAS Supersymmetry group., Zachary Marshall- a leading voice in the search for new physics   

    From CERN ATLAS: “In conversation with Zachary Marshall, a leading voice in the search for new physics” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    1
    Zachary Marshall in CERN’s Building 40. (Image: E. Ward/ATLAS Collaboration)

    He is currently the co-convener of the ATLAS Supersymmetry group, leading the team searching for supersymmetry and all its various manifestations, building on his previous work as convenor of the ATLAS Simulation. group.

    “Back in 2005, there were two big (though certainly not equal-sized) new experimental physics facilities whose start-up was on the horizon: IceCube and the LHC. I was completing my undergraduate studies at Berkeley at the time, and found that the most interesting people I knew were high-energy physicists. As one of my former supervisors was joining the CMS collaboration, picking LHC research for my PhD seemed like the obvious choice.

    I joined the ATLAS collaboration and, in 2007, moved to CERN for what I thought would be 18 months. After all, the LHC would be starting soon, I would get my hands on some data, then head home and write up my thesis. But when the 2008 “incident” occurred, a lot of us PhD students found ourselves with a real dilemma. ATLAS wouldn’t collect any collision data until 2009, which we needed in order to graduate. I was ready to book a flight home to work on an experiment at Jefferson Lab, before my supervisor, Emlyn Hughes (now with Columbia University), convinced me to stay. We had a really useful discussion, where we went through and re-examined the situation to see how I could graduate working on ATLAS. We picked modelling jet shapes, which is one of the earliest things that you can do with data. That decision made a massive difference in my life as, ten years later, I am still working here at ATLAS.

    I began my career in ATLAS working on detector simulations, which I loved doing. I worked on improving our computing performance – in other words, getting every drop of physics possible out of the CPU. For example, at one point, we were spending 5% of our performance time simulating neutrinos. Why would we do that? We can’t see them – so why simulate them at all? Finding issues like these that can be cut is essential to improving the physics performance of ATLAS.

    A lot of people, when they look at code performance, focus on which specific lines of code are taking up more time. In simulation, that is helpful, but it is not the whole story. The same line of code could be applied to an electron or it could be applied to a supersymmetric particle – and it matters much more in one case to get the simulation correct. So, you have to have a few different dimensions in view, and take them all into account when looking at and improving the code.

    Working in simulation gave me a good understanding of the entire detector, and therefore a unique perspective on analysis once I moved over to the ATLAS Supersymmetry group. But, of course, there were teething problems. Happily, we learn from our mistakes. With my first analysis, I had to overcome some simple barriers like accessing the data and figuring out how the software works. For my second search, the difficulty was with working out what the fit was doing, and controlling the background in various ways.

    I later inherited a role in Z+jet search for supersymmetry and found a new challenge learning to manage under intense scrutiny. Before a result is published publicly, analysis groups go through intense scrutiny internal to the collaboration. Colleagues who weren’t involved in the result review your team’s work, and try to think of things that may have gone wrong along the way. The Z+jet search was one that a lot of people paid attention to, because there was a potential 3 sigma deviation from the Standard Model. It was our job to defend it, tooth-and-nail.

    Going through one of these reviews feels a lot like a PhD thesis defence. No matter what issue your colleagues highlight, you have to have already considered it in your analysis and be ready to explain its impact in great detail. When you’ve worked on a result for a couple of years, pouring your life into it, this kind of scrutiny can often feel quite critical. As though you’ve screwed up.

    Learning not to take this critique as a personal attack can be hard, and it’s one of the first things I try to teach my students. I remind them that no one is out to “get them” – rather, our colleagues are out to get the best possible results for ATLAS. They raise questions and concerns in order to ensure no stone went unturned, and the analysis meets the high standards of the ATLAS collaboration.

    Now, as convenor of the Supersymmetry group, I often find myself on the other side of the equation. Making sure that every result is water tight is the most important job of a convenor. When I am tasked with critiquing a result, I try to do so as nicely as I can, as I remember the other side very well.

    My convenorship will be ending at the end of the year, and I’m looking forward to getting back to more technical work. I like making things that make people’s lives better and I really love hard technical problems. If somebody comes to me and says “nobody has ever figured out how to do this”, then that is all I will be able to think about for weeks. The more difficult the problem, the more interested I am.

    Even simple things like, taking the figures and captions out of a paper automatically. This used to be a nightmare for ATLAS collaborators, as they had to do it manually and it could take hours. I developed a script to help me with this issue, and then shared it. It felt like a lifesaver to people and it developed into the figure processing script we use today throughout the collaboration.

    The best advice I can give people trying to solve complex problems is to just… start. Being a good coder requires a certain mindset: you need to see the big problem, then look past it to start attacking the smaller problems inside of it that you know how to solve. And then rely on the fact that, when you get to the next small problem, you’ll know how to solve that too.

    The next few years will be particularly busy for the ATLAS collaboration. First, we need to make absolute certain that if there are any new particles within our reach, we find them. Second, we need to consider the legacy we leave the physics community. Many of our measurements will serve the community for several decades, while we prepare for the next collider. Such was the case for LEP, whose direct stau production search we have only just recently managed to surpass. Our job now is to make sure that future colliders – be they the ILC or the FCC – will have to work just as hard to do the same to our results.”

    See the full article here .


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    Please help promote STEM in your local schools.

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  • richardmitnick 4:43 pm on May 20, 2019 Permalink | Reply
    Tags: "Searching for Electroweak SUSY: not because it is easy but because it is hard", , CERN ATLAS, , , ,   

    From CERN ATLAS: “Searching for Electroweak SUSY: not because it is easy, but because it is hard” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    20th May 2019
    ATLAS Collaboration

    The Standard Model is a remarkably successful but incomplete theory.

    Standard Model of Particle Physics

    Supersymmetry (SUSY) offers an elegant solution to the Standard Model’s limitations, extending it to give each particle a heavy “superpartner” with different “spin” properties (an important “quantum number”, distinguishing matter particles from force particles and the Higgs boson). For example, “sleptons” are the spin 0 superpartners of spin 1/2 electrons, muons and tau leptons, while “charginos” and “neutralinos” are the spin 1/2 counterparts of the spin 0 Higgs bosons (SUSY postulates a total of five Higgs bosons) and spin 1 gauge bosons.

    If these superpartners exist and are not too massive, they will be produced at CERN’s Large Hadron Collider (LHC) and could be hiding in data collected by the ATLAS detector. However, unlike most processes at the LHC, which are governed by strong force interactions, these superpartners would be created through the much weaker electroweak interaction, thus lowering their production rates. Further, most of these new SUSY particles are expected to be unstable. Physicists can only search for them by tracing their decay products – typically into a known Standard Model particle and a “lightest supersymmetric particle” (LSP), which could be stable and non-interacting, thus forming a natural dark matter candidate.

    ______________________________________________
    If sleptons, charginos and neutralinos exist, they will be produced at the LHC and could be hiding in Run 2 data. New searches from the ATLAS Collaboration look for these particles around unexplored corners.
    ______________________________________________

    Today, at the Large Hadron Collider Physics (LHCP) conference in Puebla, Mexico, and at the SUSY2019 conference in Corpus Christi, USA, the ATLAS Collaboration presented numerous new searches for SUSY based on the full Run-2 dataset (taken between 2015 and 2018), including two particularly challenging searches for electroweak SUSY. Both searches target particles that are produced at extremely low rates at the LHC, and decay into Standard Model particles that are themselves difficult to reconstruct. The large amount of data successfully collected by ATLAS in Run 2 provides a unique opportunity to explore these scenarios with new analysis techniques.

    Search for the “stau”

    Collider and astroparticle physics experiments have set limits on the mass of various SUSY particles. However, one important superpartner – the tau slepton, known as the “stau” – has yet to be searched for beyond the exclusion limit of around 90 GeV found at the LHC’s predecessor at CERN, the Large Electron-Positron collider (LEP). A light stau, if it exists, could play a role in neutralino co-annihilation, moderating the amount of dark matter in the visible universe, which otherwise would be too abundant to explain astrophysical measurements.

    The search for a light stau is experimentally challenging due to its extremely low production rate in LHC proton-proton collisions, requiring advanced techniques to reconstruct the Standard Model tau leptons it can decay into. In fact, during Run 1, only a narrow parameter region around a stau mass of 109 GeV and a massless lightest neutralino could be excluded by LHC experiments.

    2
    Figure 1: Observed (expected) limits on the combined left and right stau pair production are shown by the red line (black dashed line). The mass of stau is shown on the x-axis, while the mass of the LSP is shown on the y-axis. (Image: ATLAS Collaboration/CERN)

    3
    Figure 2: Observed (expected) limits on the stau-left pair production are shown by the red line (black dashed line). The mass of stau is shown on the x-axis, while the mass of the LSP is shown on the y-axis. (Image: ATLAS Collaboration/CERN)

    This first ATLAS Run 2 stau search targets the direct production of a pair of staus, each decaying into one tau lepton and one invisible LSP. Each tau lepton further decays into hadrons and an invisible neutrino. Signal events would thus be characterised by the presence of two sets of close-by hadrons and large missing transverse energy (ETmiss) originating from the invisible LSP and neutrinos. Events are further categorized into regions with medium and high ETmiss, to examine different stau mass scenarios.

    The ATLAS data did not reveal hints for stau pair production and thus new exclusion limits were set on the mass of staus. These limits are shown in Figures 1 and 2 using different assumptions on the presence of both possible stau types (left and right, referring to the two different spin states of the tau partner lepton). The limits obtained are the strongest obtained so far in these scenarios.

    Compressed search

    One of the reasons physicists have yet to see charginos and neutralinos may be because their masses are “compressed”. In other words, they are very close to the mass of the LSP. This is expected in scenarios where these particles are “higgsinos”, the superpartners of the Higgs bosons.

    Compressed higgsinos decay to pairs of electrons or muons with very low momenta. It is challenging to identify and reconstruct these particles in an environment with more than a billion high-energy collisions every second and a detector designed to measure high-energy particles – like trying to locate a whispering person in a very crowded and noisy room.

    3
    Figure 3: The distribution of the electron/muon and track pair mass, where the signal events tend to cluster at low mass values. The solid histogram indicates the Standard Model background process, the points with error bars indicate the data, and the dashed lines indicate hypothetical Higgsino events. The bottom plot shows the ratio of the data to the total Standard Model background. (Image: ATLAS Collaboration/CERN)

    4
    Figure 4: Observed (expected) limits on higgsino production are shown by the red line (blue dashed line). The mass of the produced higgsino is shown on the x-axis, while the mass difference to the LSP is shown on the y-axis. The grey region represents the models excluded by the LEP experiments. The blue region represents the constraint from the previous ATLAS search for higgsinos.(Image: ATLAS Collaboration/CERN)

    A new search for higgsinos utilizes muons measured with unprecedentedly low – for ATLAS, so far – momenta. It also benefits from new and unique analysis techniques that allow physicists to look for higgsinos in areas that were previously inaccessible. For example, the search uses charged particle tracks, which can be reconstructed with very low momentum, as a proxy for one of the electrons or muons in the decay pair. Because of the small mass difference between the higgsinos, the mass of the electron/muon and track pair is also expected to be small, as shown in Figure 3.

    Once again, no signs of higgsinos were found in this search. As shown in Figure 4, the results were used to extend constraints on higgsino masses set by ATLAS in 2017 and by the LEP experiments in 2004.

    Overall, both sets of results place strong constraints on important supersymmetric scenarios, which will guide future ATLAS searches. Further, they provide examples of how advanced reconstruction techniques can help improve the sensitivity of new physics searches.

    See the full article for further reseach materials.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    Stem Education Coalition

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  • richardmitnick 3:05 pm on May 19, 2019 Permalink | Reply
    Tags: CERN ATLAS,   

    From CERN ATLAS: “Exploring the scientific potential of the ATLAS experiment at the High-Luminosity LHC” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    17th May 2019

    1
    Display of a simulated HL-LHC collision event in an upgraded ATLAS detector. The event has an average of 200 collisions per particle bunch crossing. (Image: ATLAS Collaboration/CERN)

    The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) is scheduled to begin colliding protons in 2026. This major improvement to CERN’s flagship accelerator will increase the total number of collisions in the ATLAS experiment by a factor of 10. To cope with this increase, ATLAS is preparing a complex series of upgrades including the installation of new detectors using state-of-the-art technology, the replacement of ageing electronics, and the upgrade of its trigger and data acquisition system.

    What discovery opportunities will be in reach for ATLAS with the HL-LHC upgrade? How precisely will physicists be able to measure properties of the Higgs boson? How deeply will they be able to probe Standard Model processes for signs of new physics? The ATLAS Collaboration has carried out and released dozens of studies to answer these questions – the results of which have been valuable input to discussions held this week at the Symposium on the European Strategy for Particle Physics, in Granada, Spain.

    “Studying the discovery potential of the HL-LHC was a fascinating task associated with the ATLAS upgrades,” says Simone Pagan Griso, ATLAS Upgrade Physics Group co-convener­. “The results are informative not only to the ATLAS Collaboration but to the entire global particle-physics community, as they reappraise the opportunities and challenges that lie ahead of us.” Indeed, these studies set important benchmarks for forthcoming generations of particle physics experiments.

    Pagan Griso worked with Leandro Nisati, the ATLAS representative on the HL-LHC Physics Potential ‘Yellow Report’ steering committee, and fellow ATLAS Upgrade Physics Group co-convener, Sarah Demers, to coordinate these studies for the collaboration. “A CERN Yellow Report, with publication in its final form forthcoming, will combine ATLAS’ results with those from other LHC experiments, as well as input from theoretical physicists,” says Nisati.

    Estimating the performance of a machine that has not yet been built, which will operate under circumstances that have never been confronted, was a complex task for the ATLAS team. “We took two parallel approaches,” explains Demers. “For one set of analysis projections, we began with simulations of the challenging HL-LHC experimental conditions. These simulated physics events were then passed through custom software to show us how the particles would interact with an upgraded ATLAS detector. We then developed new algorithms to try to pick the physics signals from the challenging amount of background events.” Dealing with abundant background will be a common complication for HL-LHC operation.

    See the full article here .


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    Please help promote STEM in your local schools.

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  • richardmitnick 12:04 pm on May 14, 2019 Permalink | Reply
    Tags: >Model-dependent vs model-independent research, , CERN ATLAS, , , , , , , , ,   

    From Symmetry: “Casting a wide net” 

    Symmetry Mag
    From Symmetry

    05/14/19
    Jim Daley

    1
    Illustration by Sandbox Studio, Chicago

    In their quest to discover physics beyond the Standard Model, physicists weigh the pros and cons of different search strategies.

    On October 30, 1975, theorists John Ellis, Mary K. Gaillard and D.V. Nanopoulos published a paper [Science Direct] titled “A Phenomenological Profile of the Higgs Boson.” They ended their paper with a note to their fellow scientists.

    “We should perhaps finish with an apology and a caution,” it said. “We apologize to experimentalists for having no idea what is the mass of the Higgs boson… and for not being sure of its couplings to other particles, except that they are probably all very small.

    “For these reasons, we do not want to encourage big experimental searches for the Higgs boson, but we do feel that people performing experiments vulnerable to the Higgs boson should know how it may turn up.”

    What the theorists were cautioning against was a model-dependent search, a search for a particle predicted by a certain model—in this case, the Standard Model of particle physics.

    Standard Model of Particle Physics

    It shouldn’t have been too much of a worry. Around then, most particle physicists’ experiments were general searches, not based on predictions from a particular model, says Jonathan Feng, a theoretical particle physicist at the University of California, Irvine.

    Using early particle colliders, physicists smashed electrons and protons together at high energies and looked to see what came out. Samuel Ting and Burton Richter, who shared the 1976 Nobel Prize in physics for the discovery of the charm quark, for example, were not looking for the particle with any theoretical prejudice, Feng says.

    That began to change in the 1980s and ’90s. That’s when physicists began exploring elegant new theories such as supersymmetry, which could tie up many of the Standard Model’s theoretical loose ends—and which predict the existence of a whole slew of new particles for scientists to try to find.

    Of course, there was also the Higgs boson. Even though scientists didn’t have a good prediction of its mass, they had good motivations for thinking it was out there waiting to be discovered.

    And it was. Almost 40 years after the theorists’ tongue-in-cheek warning about searching for the Higgs, Ellis found himself sitting in the main auditorium at CERN next to experimentalist Fabiola Gianotti, the spokesperson of the ATLAS experiment at the Large Hadron Collider who, along with CMS spokesperson Joseph Incandela, had just co-announced the discovery of the particle he had once so pessimistically described.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    Model-dependent vs model-independent

    Scientists’ searches for particles predicted by certain models continue, but in recent years, searches for new physics independent of those models have begun to enjoy a resurgence as well.

    “A model-independent search is supposed to distill the essence from a whole bunch of specific models and look for something that’s independent of the details,” Feng says. The goal is to find an interesting common feature of those models, he explains. “And then I’m going to just look for that phenomenon, irrespective of the details.”

    Particle physicist Sara Alderweireldt uses model-independent searches in her work on the ATLAS experiment at the Large Hadron Collider.

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

    Alderweireldt says that while many high-energy particle physics experiments are designed to make very precise measurements of a specific aspect of the Standard Model, a model-independent search allows physicists to take a wider view and search more generally for new particles or interactions. “Instead of zooming in, we try to look in as many places as possible in a consistent way.”

    Such a search makes room for the unexpected, she says. “You’re not dependent on the prior interpretation of something you would be looking for.”

    Theorist Patrick Fox and experimentalist Anadi Canepa, both at Fermilab, collaborate on searches for new physics.


    In Canepa’s work on the CMS experiment, the other general-purpose particle detector at the LHC, many of the searches are model-independent.

    While the nature of these searches allows them to “cast a wider net,” Fox says, “they are in some sense shallower, because they don’t manage to strongly constrain any one particular model.”

    At the same time, “by combining the results from many independent searches, we are getting closer to one dedicated search,” Canepa says. “Developing both model-dependent and model-independent searches is the approach adopted by the CMS and ATLAS experiments to fully exploit the unprecedented potential of the LHC.”

    Driven by data and powered by machine learning

    Model-dependent searches focus on a single assumption or look for evidence of a specific final state following an experimental particle collision. Model-independent searches are far broader—and how broad is largely driven by the speed at which data can be processed.

    “We have better particle detectors, and more advanced algorithms and statistical tools that are enabling us to understand searches in broader terms,” Canepa says.

    One reason model-independent searches are gaining prominence is because now there is enough data to support them. Particle detectors are recording vast quantities of information, and modern computers can run simulations faster than ever before, she says. “We are able to do model-independent searches because we are able to better understand much larger amounts of data and extreme regions of parameter and phase space.”

    Machine-learning is a key part of this processing power, Canepa says. “That’s really a change of paradigm, because it really made us make a major leap forward in terms of sensitivity [to new signals]. It really allows us to benefit from understanding the correlations that we didn’t capture in a more classical approach.”

    These broader searches are an important part of modern particle physics research, Fox says.

    “At a very basic level, our job is to bequeath to our descendants a better understanding of nature than we got from our ancestors,” he says. “One way to do that is to produce lots of information that will stand the test of time, and one way of doing that is with model-independent searches.”

    Models go in and out of fashion, he adds. “But model-independent searches don’t feel like they will.”

    See the full article here .


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    Please help promote STEM in your local schools.


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:37 am on April 16, 2019 Permalink | Reply
    Tags: , CERN ATLAS, , , , , ,   

    From Symmetry: “A collision of light” 

    Symmetry Mag
    From Symmetry

    04/16/19
    Sarah Charley

    1
    Natasha Hartono

    One of the latest discoveries from the LHC takes the properties of photons beyond what your electrodynamics teacher will tell you in class.

    Professor Anne Sickles is currently teaching a laboratory class at the University of Illinois in which her students will measure what happens when two photons meet.

    What they will find is that the overlapping waves of light get brighter when two peaks align and dimmer when a peak meets a trough. She tells her students that this is process called interference, and that—unlike charged particles, which can merge, bond and interact—light waves can only add or subtract.

    “We teach undergraduates the classical theory,” Sickles says. “But there are situations where effects forbidden in the classical theory are allowed in the quantum theory.”

    Sickles is a collaborator on the ATLAS experiment at CERN and studies what happens when particles of light meet inside the Large Hadron Collider.

    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    For most of the year, the LHC collides protons, but for about a month each fall, the LHC switches things up and collides heavy atomic nuclei, such as lead ions. The main purpose of these lead collisions is to study a hot and dense subatomic fluid called the quark-gluon plasma, which is harder to create in collisions of protons. But these ion runs also enable scientists to turn the LHC into a new type of machine: a photon-photon collider.

    “This result demonstrates that photons can scatter off each other and change each other’s direction,” says Peter Steinberg, and ATLAS scientist at Brookhaven National Laboratory.

    When heavy nuclei are accelerated in the LHC, they are encased within an electromagnetic aura generated by their large positive charges.

    As the nuclei travel faster and faster, their surrounding fields are squished into disks, making them much more concentrated. When two lead ions pass closely enough that their electromagnetic fields swoosh through one another, the high-energy photons which ultimately make up these fields can interact. In rare instances, a photon from one lead ion will merge with a photon from an oncoming lead ion, and they will ricochet in different directions.

    However, according to Steinberg, it’s not as simple as two solid particles bouncing off each other. Light particles are both chargeless and massless, and must go through a quantum mechanical loophole (literally called a quantum loop) to interact with one another.

    “That’s why this process is so rare,” he says. “They have no way to bounce off of each other without help.”

    When the two photons see each other inside the LHC, they sometimes overreact with excitement and split themselves into an electron and positron pair. These electron-positron pairs are not fully formed entities, but rather unstable quantum fluctuations that scientists call virtual particles. The four virtual particles swirl into each other and recombine to form two new photons, which scatter off at weird angles into the detector.

    “It’s like a quantum-mechanical square dance,” Steinberg says.

    When ATLAS first saw hints of this process in 2017, they had only 13 candidate events with the correct characteristics (collisions that resulted in two low-energy photons inside the detector and nothing else).

    After another two years of data taking, they have now collected 59 candidate events, bumping this original observation into the statistical certainty of a full-fledged discovery.

    Steinberg sees this discovery as a big win for quantum electrodynamics, a theory about the quantum behavior of light that predicted this interaction. “This amazingly precise theory, which was developed in the first half of the 20th century, made a prediction that we are finally able to confirm many decades later.”

    Sickles says she is looking forward to exploring these kinds of light-by-light interactions and figuring out what else they could teach us about the laws of physics. “It’s one thing to see something,” she says. “It’s another thing to study it.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
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