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  • richardmitnick 4:07 pm on September 25, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , New ATLAS result of ultra-rare B-meson decay to muon pair, , ,   

    From CERN ATLAS: “New ATLAS result of ultra-rare B-meson decay to muon pair” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    25th September 2018
    ATLAS Collaboration

    1
    Figure 1: Measured dimuon mass distributions in the selection channel with highest expected signal purity. Superimposed is the result of a fit to the data. The total fit result is shown (black continuous line), with the observed signal component (dashed red line), b→μμ X background (dashed blue), and continuum background (dashed green). Signal components are grouped in one single curve, including the B0s → μ+μ- and B0 → μ+μ- components. The peaking B0(s) → hh′ background (brown dashed line) is, for all BDT bins, very close to the x-axis. (Image: ATLAS Collaboration/CERN)

    The study of hadrons – particles that combine together quarks to form mesons or baryons – is a vital part of the ATLAS physics programme. Their analysis has not only perfected our understanding of the Standard Model, it has also provided excellent opportunities for discovery.

    Among the variety of hadrons available in nature and produced in LHC proton-proton collisions, B-mesons play a fundamental role. They are bound-states of two quarks – one a bottom quark, the other one of the lighter quarks (up, down, strange or charm) – that decay through the weak interaction to lighter hadrons and/or leptons. Over the past decades, physicists have examined rare and precisely predicted phenomena involving neutral B-mesons (i.e. B0 or Bs mesons), searching for slight discrepancies from theory predictions that could be a signal of new physics.

    On 20 September 2018, at the International Workshop on the CKM Unitarity Triangle (CKM 2018), ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons (μ). The result is a new milestone that complements analyses [Physical Review Letters] previously published by LHC experiments dedicated to the study of B-mesons in a quest spanning almost three decades.

    ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons.

    2
    Figure 2: Likelihood contours for the combination of the Run 1 and 2015-2016 Run 2 results (in black). The solid, dashed and dashed-dotted contours delimit the one, two and three standard deviation regions, respectively. The contours for the Run 2 2015-2016 result alone are overlaid in blue. The Standard Model prediction and its uncertainties are shown by the solid cross. (Image: ATLAS Collaboration/CERN)

    The rareness of this decay is due to the coincidence of two factors: first, the decay requires quantum loops with several weak interaction vertices, some of which have a low probability to occur; second, angular momentum conservation constrains the decay products of the scalar B0 or Bs meson into a highly unlikely configuration.

    According to the Standard Model, the probability of generating this decay is about 1.1 in 10 billion. The new ATLAS result gets very close, with the tightest available upper limit of 2.1 occurrences in ten billion at the 95% confidence level. The result was obtained using data collected in 2015 and 2016 combined with an analogous analysis of Run 1 data. The result also provided a 4.6 standard deviations evidence for the Bs→ μμ decay, whose branching fraction is measured to be 2.8 +0.8–0.7 x10–9. It confirms previous measurements from the LHCb and CMS collaborations.

    This new ATLAS result is the first milestone towards a more precise measurement that will be obtained with the full Run 2 dataset, which is expected to improve the current precision by about 30%. Further projections towards the high-luminosity LHC (HL-LHC) era predict that ATLAS will be able to further improve the precision of this result by about a factor 3.

    Related journal articles
    _________________________________________________
    See the full article for further references with links.

    See the full article here .


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  • richardmitnick 3:18 pm on September 7, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , , The incredible lightness of the Higgs   

    From CERN: “The incredible lightness of the Higgs” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN

    7 Sep 2018
    Ana Lopes

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    View of the ATLAS detector. The ATLAS collaboration reports results of a combination of searches for a new particle – dubbed a vector-like top quark – that could be the culprit behind the Higgs lightness.

    Why is the Higgs boson so light? That’s one of the questions that has been bothering particle physicists since the famous particle was discovered in 2012.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    This is because the theory of how the particle interacts with the most massive of all observed elementary particles, the top quark, involves corrections at a fundamental (quantum) level that could result in a Higgs mass much larger than the measured value of 125 GeV. How large? Perhaps as much as sixteen orders of magnitude larger than the measured Higgs mass. Since the Higgs mass is so light, this suggests more particles could exist that cancel the quantum corrections from the top quark (and other heavy particles).

    In a paper posted online and submitted to the journal Physical Review Letters, the ATLAS collaboration reports results of a combination of searches for a new particle – dubbed a vector-like top quark – that could help keep the Higgs boson light.

    Various proposals attempt to cancel out the large quantum corrections to the Higgs boson mass. Many of them involve vector-like top quarks, which are hypothetical particles not predicted by the Standard Model of particle physics.

    Standard Model of Particle Physics from Symmetry Magazine


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

    Unlike the Standard Model top quark, which always decays to a bottom quark and a W boson, vector-like top quarks would decay in one of three different ways, if they decayed to Standard Model particles. Specifically, a vector-like top quark would decay to a bottom quark and a W boson, or to a Z boson and a top quark, or still to a Higgs boson and a top quark.

    To maximise the chances of finding vector-like top quarks, the ATLAS collaboration conducted several different types of search using data from proton–proton collisions collected at the Large Hadron Collider (LHC) in 2015 and 2016 at an energy of 13 TeV; each individual search is sensitive to a particular set of particle decays. They then combined the results to increase the sensitivity to vector-like top quarks, yet found no sign of them.

    Despite this, their analysis allowed them to expand the reach of individual searches and place the most stringent lower bounds on the mass of vector-like top quarks to date. The analysis excludes vector-like top quarks with masses below about 1300 GeV for any combination of the three top-quark decays into Standard Model particles. The previous best lower limit from an individual search was 1190 GeV.

    It will now get more challenging: for masses heavier than 1300 GeV a single vector-like top quark is created more often than a pair. But with a wealth of data coming from the LHC, the search continues.

    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 New

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    CMS
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    LHCb
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    OTHER PROJECTS AT CERN

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    CERN CAST Axion Solar Telescope

    CERN CLOUD

    CERN CLOUD

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    CERN ISOLDE

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  • richardmitnick 1:17 pm on September 5, 2018 Permalink | Reply
    Tags: , CERN ATLAS, , , , ,   

    From CERN ATLAS: “ATLAS searches for double Higgs production” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    5th September 2018

    1
    Upper limits at 95% confidence level on the cross-section of the non-resonant Higgs boson pair production as a function of κλ. The allowed range of κλ is derived from the interval where the theoretical prediction is found below the experimental upper limits on the cross-section. (Image: ATLAS Collaboration/CERN)

    The Brout-Englert-Higgs (BEH) mechanism is at the core of the Standard Model, the theory that describes the fundamental constituents of matter and their interactions. It introduces a new field, the Higgs field, through which the weak bosons (W+/- and Z) become massive while the photon remains massless. The excitation of this field is a physical particle, the Higgs boson, which was discovered by the ATLAS and CMS collaborations in 2012.

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


    Standard Model of Particle Physics from Symmetry Magazine

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    The BEH mechanism also predicts that the Higgs field can interact with itself; in other words, a single (virtual) Higgs boson can decay into two Higgs bosons. Observing and measuring this self-interaction, or “Higgs self-coupling”, would be the ultimate validation of the theory of mass generation, while any deviation from Standard Model predictions would open a window on new physics.

    Unfortunately, Higgs boson pairs are predicted to be very rare in proton–proton collisions, with a production rate roughly a thousand times smaller than the single Higgs boson. To make matters worse, not all di-Higgs boson production occurs through Higgs self-coupling. Vast amounts of data are therefore needed for this to be probed, making it a flagship analysis for the high-luminosity upgrade of the LHC (HL-LHC).

    It is nevertheless important to explore di-Higgs production also with smaller datasets as new physics beyond the Standard Model might enhance the production rate.

    The ATLAS collaboration has searched for Higgs boson pairs (HH) in the dataset collected in 2015 and 2016 using various decay channels. The most sensitive of these involve one Higgs boson decaying into a pair of b-quarks and one decaying into either another pair of b-quarks (HH→bbbb), two tau-leptons (HH→bbττ) or two photons (HH→bbγγ). These three searches were recently statistically combined and, as a result, the production rate of HH pairs could be excluded beyond 6.7 times the Standard Model prediction, at a 95% confidence level.

    New physics could be indicated by a Higgs self-coupling which differs from the Standard Model prediction by a factor κλ. This would affect the production rate and the kinematic distributions of the Higgs boson pairs and, as such, is an excellent probe for new physics. The recent statistical combination of the HH searches in ATLAS constrains the value of κλ to be between –5.0 and +12.1, at a 95% confidence level (see figure). It is the world’s most stringent constraint on the anomalous Higgs self-coupling to date.

    Higgs boson pairs are also a key signature of heavy new particle decays in several scenarios beyond the Standard Model. These might include an additional Higgs boson in models that extend the Higgs sector of the Standard Model, or the excitation of a graviton in models with extra spatial dimensions. The combined HH searches performed by ATLAS with the 2015-2016 dataset impose stringent constraints on the production rates of such resonances at the LHC.

    See the full article here .


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  • richardmitnick 10:33 am on August 28, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , Long-sought decay of Higgs boson observed, , ,   

    From CERN via phys.org: “Long-sought decay of Higgs boson observed” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN

    via

    phys.org

    August 28, 2018, CERN

    1
    A candidate event display for the production of a Higgs boson decaying to two b-quarks (blue cones), in association with a W boson decaying to a muon (red) and a neutrino. The neutrino leaves the detector unseen, and is reconstructed through the missing transverse energy (dashed line). Credit: ATLAS Collaboration/CERN

    CERN ATLAS Higgs Event


    CERN ATLAS

    Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented today at CERN1 by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC), is consistent with the hypothesis that the all-pervading quantum field behind the Higgs boson also gives mass to the bottom quark. Both teams have submitted their results for publication today.

    CERN CMS Higgs Event


    CERN/CMS Detector

    The Standard Model of particle physics predicts that about 60% of the time a Higgs boson will decay to a pair of bottom quarks, the second-heaviest of the six flavours of quarks.

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


    Standard Model of Particle Physics from Symmetry Magazine

    Testing this prediction is crucial because the result would either lend support to the Standard Model – which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass – or rock its foundations and point to new physics.

    Spotting this common Higgs-boson decay channel is anything but easy, as the six-year period since the discovery of the boson has shown. The reason for the difficulty is that there are many other ways of producing bottom quarks in proton–proton collisions. This makes it hard to isolate the Higgs-boson decay signal from the background “noise” associated with such processes. By contrast, the less-common Higgs-boson decay channels that were observed at the time of discovery of the particle, such as the decay to a pair of photons, are much easier to extract from the background.

    To extract the signal, the ATLAS and CMS collaborations each combined data from the first and second runs of the LHC, which involved collisions at energies of 7, 8 and 13 TeV. They then applied complex analysis methods to the data. The upshot, for both ATLAS and CMS, was the detection of the decay of the Higgs boson to a pair of bottom quarks with a significance that exceeds 5 standard deviations. Furthermore, both teams measured a rate for the decay that is consistent with the Standard Model prediction, within the current precision of the measurement.

    2
    Candidate event display for the production of a Higgs boson decaying to two b-quarks. A 2 b-tag, 2-jet, 2-electron event within the signal-like portion of the high pTV and high BDTVH output distribution is shown (Run 337215, Event 1906922941). Electrons are shown as blue tracks with a large energy deposit in the electromagnetic calorimeter, corresponding to light green bars. Two of them form an invariant mass of 93.6 GeV, compatible with a Z boson. The two central high-pT b-tagged jets are represented by light blue cones. They contain the green and yellow bars corresponding to the energy deposition in the electromagnetic and hadronic calorimeters respectively, and they have an invariant mass of 128.1 GeV. The value of pTV is 246.7 GeV, and BDTVH output value is 0.47. Credit: ATLAS Collaboration/CERN

    “This observation is a milestone in the exploration of the Higgs boson. It shows that the ATLAS and CMS experiments have achieved deep understanding of their data and a control of backgrounds that surpasses expectations. ATLAS has now observed all couplings of the Higgs boson to the heavy quarks and leptons of the third generation as well as all major production modes,” said Karl Jakobs, spokesperson of the ATLAS collaboration.

    “Since the first single-experiment observation of the Higgs boson decay to tau-leptons one year ago, CMS, along with our colleagues in ATLAS, has observed the coupling of the Higgs boson to the heaviest fermions: the tau, the top quark, and now the bottom quark. The superb LHC performance and modern machine-learning techniques allowed us to achieve this result earlier than expected,” said Joel Butler, spokesperson of the CMS collaboration.

    With more data, the collaborations will improve the precision of these and other measurements and probe the decay of the Higgs boson into a pair of much-less-massive fermions called muons, always watching for deviations in the data that could point to physics beyond the Standard Model.

    3
    Candidate event display for the production of a Higgs boson decaying to two b-quarks. A 2-tag, 2-jet, 0-lepton event within the signal-like portion of the high pTV and high BDTVH output (Run 339500, Event 694513952) is shown. The ETMiss, shown as a white dashed line, has a magnitude of 479.1 GeV. The two central high-pT b-tagged jets are represented by light blue cones. They contain the green and yellow bars corresponding to the energy deposition in the electromagnetic and hadronic calorimeters respectively. The dijet invariant mass of 128.1 GeV. The BDTVH output value is 0.74. Credit: ATLAS Collaboration/CERN

    “The experiments continue to home in on the Higgs particle, which is often considered a portal to new physics. These beautiful and early achievements also underscore our plans for upgrading the LHC to substantially increase the statistics. The analysis methods have now been shown to reach the precision required for exploration of the full physics landscape, including hopefully new physics that so far hides so subtly,” said CERN Director for Research and Computing Eckhard Elsen.

    Science paper:
    Observation of Higgs boson decay to bottom quarks

    See the full article here.


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  • richardmitnick 1:04 pm on August 14, 2018 Permalink | Reply
    Tags: , , Brute-force approach to particle hunt, CERN ATLAS, , , , , ,   

    From Nature: “LHC physicists embrace brute-force approach to particle hunt” 

    Nature Mag
    From Nature

    14 August 2018
    Davide Castelvecchi

    The world’s most powerful particle collider has yet to turn up new physics [since Higgs] — now some physicists are turning to a different strategy.

    1
    The ATLAS detector at the Large Hadron Collider near Geneva, Switzerland.Credit: Stefano Dal Pozzolo/Contrasto /eyevine

    A once-controversial approach to particle physics has entered the mainstream at the Large Hadron Collider (LHC).

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    The LHC’s major ATLAS experiment has officially thrown its weight behind the method — an alternative way to hunt through the reams of data created by the machine — as the collider’s best hope for detecting behaviour that goes beyond the standard model of particle physics. Conventional techniques have so far come up empty-handed.

    So far, almost all studies at the LHC — at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland — have involved ‘targeted searches’ for signatures of favoured theories. The ATLAS collaboration now describes its first all-out ‘general’ search of the detector’s data, in a preprint posted on the arXiv server last month and submitted to European Physics Journal C. Another major LHC experiment, CMS, is working on a similar project.

    “My goal is to try to come up with a really new way to look for new physics” — one driven by the data rather than by theory, says Sascha Caron of Radboud University Nijmegen in the Netherlands, who has led the push for the approach at ATLAS. General searches are to the targeted ones what spell checking an entire text is to searching that text for a particular word. These broad searches could realize their full potential in the near future, when combined with increasingly sophisticated artificial-intelligence (AI) methods.

    LHC researchers hope that the methods will lead them to their next big discovery — something that hasn’t happened since the detection of the Higgs boson in 2012, which put in place the final piece of the standard model. Developed in the 1960s and 1970s, the model describes all known subatomic particles, but physicists suspect that there is more to the story — the theory doesn’t account for dark matter, for instance. But big experiments such as the LHC have yet to find evidence for such behaviour. That means it’s important to try new things, including general searches, says Gian Giudice, who heads CERN’s theory department and is not involved in any of the experiments. “This is the right approach, at this point.”

    Collision course

    The LHC smashes together millions of protons per second at colossal energies to produce a profusion of decay particles, which are recorded by detectors such as ATLAS and CMS. Many different types of particle interaction can produce the same debris. For example, the decay of a Higgs might produce a pair of photons, but so do other, more common, processes. So, to search for the Higgs, physicists first ran simulations to predict how many of those ‘impostor’ pairs to expect. They then counted all photon pairs recorded in the detector and compared them to their simulations. The difference — a slight excess of photon pairs within a narrow range of energies — was evidence that the Higgs existed.

    ATLAS and CMS have run hundreds more of these targeted searches to look for particles that do not appear in the standard model.

    CERN/ATLAS detector


    CERN/CMS Detector

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


    Standard Model of Particle Physics from Symmetry Magazine

    Many searches have looked for various flavours of supersymmetry, a theorized extension of the model that includes hypothesized particles such as the neutralino, a candidate for dark matter. But these searches have come up empty so far.

    Standard model of Supersymmetry DESY

    This leaves open the possibility that there are exotic particles that produce signatures no one has thought of — something that general searches have a better chance of finding. Physicists have yet to look, for example, events that produced three photons instead of two, Caron says. “We have hundreds of people looking at Higgs decay and supersymmetry, but maybe we are missing something nobody thought of,” says Arnd Meyer, a CMS member at Aachen University in Germany.

    Whereas targeted searches typically look at only a handful of the many types of decay product, the latest study looked at more than 700 types at once. The study analysed data collected in 2015, the first year after an LHC upgrade raised the energy of proton collisions in the collider from 8 teraelectronvolts (TeV) to 13 TeV. At CMS, Meyer and a few collaborators have conducted a proof-of-principle study, which hasn’t been published, on a smaller set of data from the 8 TeV run.

    Neither experiment has found significant deviations so far. This was not surprising, the teams say, because the data sets were relatively small. Both ATLAS and CMS are now searching the data collected in 2016 and 2017, a trove tens of times larger.

    Statistical cons

    The approach “has clear advantages, but also clear shortcomings”, says Markus Klute, a physicist at the Massachusetts Institute of Technology in Cambridge. Klute is part of CMS and has worked on general searches in at previous experiments, but he was not directly involved in the more recent studies. One limitation is statistical power. If a targeted search finds a positive result, there are standard procedures for calculating its significance; when casting a wide net, however, some false positives are bound to arise. That was one reason that general searches had not been favoured in the past: many physicists feared that they could lead down too many blind alleys. But the teams say they have put a lot of work into making their methods more solid. “I am excited this came forward,” says Klute.

    Most of the people power and resources at the LHC experiments still go into targeted searches, and that might not change anytime soon. “Some people doubt the usefulness of such general searches, given that we have so many searches that exhaustively cover much of the parameter space,” says Tulika Bose of Boston University in Massachusetts, who helps to coordinate the research programme at CMS.

    Many researchers who work on general searches say that they eventually want to use AI to do away with standard-model simulations altogether. Proponents of this approach hope to use machine learning to find patterns in the data without any theoretical bias. “We want to reverse the strategy — let the data tell us where to look next,” Caron says. Computer scientists are also pushing towards this type of ‘unsupervised’ machine learning — compared with the supervised type, in which the machine ‘learns’ from going through data that have been tagged previously by humans.

    See the full article here .

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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

     
  • richardmitnick 12:00 pm on August 8, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, Could a new type of quark fix the “unnaturalness” of the Standard Model?, , , , , ,   

    From CERN ATLAS: “Could a new type of quark fix the “unnaturalness” of the Standard Model?” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    8th August 2018
    ATLAS Collaboration

    1
    Figure 1: One of the Feynman diagrams for T pair production at the LHC. (Image: ATLAS Collaboration © CERN 2018)

    While the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 confirmed many Standard Model predictions, it has raised as many questions as it has answered. For example, interactions at the quantum level between the Higgs boson and the top quark ought to lead to a huge Higgs boson mass, possibly as large as the Planck mass (>1018 GeV). So why is it only 125 GeV? Is there a mechanism at play to cancel these large quantum corrections caused by the top quark (t)? Finding a way to explain the lightness of the Higgs boson is one of the top (no pun intended) questions in particle physics.

    A wide range of solutions have been proposed and a common feature in many of them is the existence of vector-like quarks – in particular, a vector-like top quark (T). Like other quarks, vector-like quarks would be spin-½ particles that interact via the strong force. While all spin-½ particles have left- and right-handed components, the weak force only interacts with the left-handed components of Standard Model particles. However, vector-like quarks would have “ambidextrous” interactions with the weak force, giving them a bit more leeway in how they decay. While the Standard Model top quark always decays to a bottom quark (b) by emitting a W boson (t→Wb), a vector-like top can decay three different ways: T→Wb, T→Zt or T→Ht (Figure 1).

    2
    Figure 2: Lower limit (scale on right axis) on the mass of a vector-like top as a function of the branching ratio to Wb and Ht (bottom and left axes). (Image: ATLAS Collaboration © CERN 2018)

    The ATLAS collaboration uses a custom-built programme to search for vector-like top pairs in LHC data. It utilizes data from several dedicated analyses, each of them sensitive to various experimental signatures (involving leptons, boosted objects and/or large missing transverse momentum). This allows ATLAS to look for all of possible decays, increasing the chance of discovery.

    ATLAS has now gone one step further by performing a combination of all of the individual searches. While individual analyses are designed to study a particular sets of decays, combined results provide sensitivity to all possible sets of decays. These have allowed ATLAS to search for vector-like tops with masses over 1200 GeV. It appears, however, that vector-like tops are so far nowhere to be found. On the bright side, the combination allows ATLAS to set the most stringent lower limits on the mass of a vector-like top for arbitrary sets of branching ratios to the three decay modes (Figure 2).

    Between these limits on vector-like top quarks and those on other theories that could offer a solution (like supersymmetry), the case for a naturally light Higgs boson is not looking good… but Nature probably still has a few tricks up its sleeve for us to uncover.

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


    Standard Model of Particle Physics from Symmetry Magazine

    See the full article here .


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  • richardmitnick 9:23 am on July 10, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , Higgs boson observed decaying to b quarks – at last!, , ,   

    From CERN ATLAS: “Higgs boson observed decaying to b quarks – at last!” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    9th July 2018

    The Brout-Englert-Higgs mechanism solves the apparent theoretical impossibility of weak vector bosons (W and Z) to have mass. The discovery of the Higgs boson in 2012 via its decays into photon, Z and W pairs was a triumph of the Standard Model built upon this mechanism. The Higgs field can also be used in an elegant way to provide mass to charged fermions (quarks and leptons) through interactions involving “Yukawa couplings”, with strength proportional to the particle mass. The observation of the Higgs boson decaying into pairs of τ leptons provided the first direct evidence of this type of interaction.

    Six years after its discovery, ATLAS has observed about 30% of the Higgs boson decays predicted in the Standard Model. However, the favoured decay of the Higgs boson into a pair of b quarks (H→bb), which is expected to account for almost 60% of all possible decays, had remained elusive up to now. Observing this decay mode and measuring its rate is a mandatory step to confirm (or not…) the mass generation for fermions via Yukawa interactions, as predicted in the Standard Model.

    Today, at the 2018 International Conference on High Energy Physics (ICHEP) in Seoul, the ATLAS experiment reported a preliminary result establishing the observation of the Higgs boson decaying into pairs of b quarks, furthermore at a rate consistent with the Standard Model prediction. In the community of particle physics (and beyond), for the detection of a process to be qualified as an “observation”, it is necessary to exclude at a level of one in three million the probability that it arises from a fluctuation of the background that could mimic the process in question. When such a probability is at the level of only one in a thousand, the detection is qualified as an “evidence”. Evidence of the H→bb decay was first provided at the Tevatron in 2012, and a year ago by the ATLAS and CMS Collaborations, independently.

    FNAL/Tevatron map

    FNAL/Tevatron

    CERN/CMS Detector


    CERN CMS Higgs Event

    Combing through the haystack of b quarks

    Given the abundance of the H→bb decay, and how much rarer decay modes such as H→γγ had already been observed at the time of discovery, why did it take so long to achieve this observation?

    The main reason: the most copious production process for the Higgs boson in proton-proton interactions leads to just a pair of particle jets originating from the fragmentation of b quarks (b-jets). These are almost impossible to distinguish from the overwhelming background of b-quark pairs produced via the strong interaction (quantum chromodynamics or QCD). To overcome this challenge, it was necessary to consider production processes that are less copious, but exhibit features not present in QCD. The most effective of these is the associated production of the Higgs boson with a vector boson, W or Z. The leptonic decays, W→ℓν, Z→ℓℓ and Z→νν (where ℓ stands for an electron or a muon) provide signatures that allow for efficient triggering and powerful QCD background reduction.

    However, the Higgs boson signal remains orders of magnitude smaller than the remaining backgrounds arising from top quark or vector boson production, which lead to similar signatures. For instance, a top quark pair can decay as tt→[(W→ℓν)b][(W→qq)b] with a final state containing an electron or a muon and two b quarks, exactly as the (W→ℓν)(H→bb) signal.

    The main handle to discriminate the signal from such backgrounds is the invariant mass, mbb, of pairs of b-jets identified by sophisticated “b-tagging” algorithms. An example of such a mass distribution is shown in Figure 1, where the sum of the signal and background components is confronted to the data.

    1
    Figure 1: Distribution of mbb in the (W→ℓν)(H→bb) search channel. The signal is shown in red, the different backgrounds in various colours. The data are shown as points with error bars. (Image: ATLAS Collaboration/CERN)

    When all WH and ZH channels are combined and the backgrounds (apart from WZ and ZZ production) subtracted from the data, the distribution shown in Figure 2 exhibits a clear peak arising from Z boson decays to b-quark pairs, which validates the analysis procedure. The shoulder on the upper side is consistent in shape and rate with the expectation from Higgs boson production.

    2
    Figure 2: Distribution of mbb from all search channels combined after subtraction of all backgrounds except for WZ and ZZ production. The data (points with error bars) are compared to the expectations from the production of WZ and ZZ (in grey) and of WH and ZH (in red). (Image: ATLAS Collaboration/CERN)

    When all WH and ZH channels are combined and the backgrounds (apart from WZ and ZZ production) subtracted from the data, the distribution shown in Figure 2 exhibits a clear peak arising from Z boson decays to b-quark pairs, which validates the analysis procedure. The shoulder on the upper side is consistent in shape and rate with the expectation from Higgs boson production.

    This is, however, not sufficient to reach the level of detection that can be qualified as observation. To this end, the mass of the b-jet pair is combined with other kinematic variables that show distinct differences between the signal and the various backgrounds, for instance the angular separation between the two b-jets, or the transverse momentum of the associated vector boson. This combination of multiple variables is performed using the technique of boosted decision trees (BDTs). A combination of the BDT outputs from all channels, reordered in terms of signal-to-background ratio, is shown in Figure 3. It can be seen that the signal closely follows the distribution expected from the Standard Model. The BDT outputs are subjected to a sophisticated statistical analysis to extract the “significance” of the signal. This is another way to measure the probability of a fake observation in terms of standard deviations, σ, of a Gaussian distribution. The magic number corresponding to the observation of a signal is 5σ.

    3
    Figure 3: Distribution showing the combination of all BDT outputs reordered in terms of log(S/B), where S and B are the signal and background yields, respectively. The signal is shown in red, and the different backgrounds in various colours. The data are shown as points with error bars. The lower panel shows the “pull”, i.e. the ratio of data minus background to the statistical uncertainty of the background. (Image: ATLAS Collaboration/CERN)

    Observation achieved!

    The analysis of 13 TeV data collected by ATLAS during Run 2 of the LHC in 2015, 2016 and 2017 leads to a significance of 4.9σ – alone almost sufficient to claim observation. This result was combined with those from a similar analysis of Run 1 data and from other searches by ATLAS for the H→bb decay mode, namely where the Higgs boson is produced in association with a top quark pair or via a process known as vector boson fusion (VBF). The significance achieved by this combination is 5.4σ.

    Furthermore, combining the present analysis with others that target Higgs boson decays to pairs of photons and Z bosons measured at 13 TeV provides the observation at 5.3σ of associated VH (V = Z or W) production, in agreement with the Standard Model prediction. All four primary Higgs boson production modes at hadron colliders have now been observed, of which two only this year. In order of discovery: (1) fusion of gluons to a Higgs boson, (2) fusion of weak bosons to a Higgs boson, (3) associated production of a Higgs boson with two top quarks, and (4) associated production of a Higgs boson with a weak boson.

    With these observations, a new era of detailed measurements in the Higgs sector opens up, through which the Standard Model will be further challenged.

    See the full article here .


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  • richardmitnick 8:42 am on July 10, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, Combined measurements of Higgs boson couplings reach new level of precision, , , ,   

    From CERN ATLAS: “Combined measurements of Higgs boson couplings reach new level of precision” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    9th July 2018

    1
    Figure 1: Measured cross-sections of main Higgs boson production modes at the LHC, namely gluon-gluon fusion (ggF), weak boson fusion (VBF), associated production with a weak vector boson W or Z (WH and ZH), and associated production with top quarks (ttH and tH), normalized to Standard Model predictions. The uncertainty of each measurement (indicated by the error bar) is broken down into statistical (yellow box) and systematic (blue box) parts. The theory uncertainty (grey box) on the Standard Model prediction (vertical red line at unity) is also shown. (Image: ATLAS Collaboration/CERN)

    The Higgs boson, discovered at the LHC in 2012, has a singular role in the Standard Model of particle physics.

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

    Standard Model of Particle Physics from Symmetry Magazine

    Most notable is the Higgs boson’s affinity to mass, which can be likened to the electric charge for an electric field: the larger the mass of a fundamental particle, the larger the strength of its interaction, or “coupling”, with the Higgs boson. Deviations from these predictions could be a hallmark of new physics in this as-yet little-explored part of the Standard Model.

    Higgs boson couplings manifest themselves in the rate of production of the Higgs boson at the LHC, and its decay branching ratios into various final states. These rates have been precisely measured by the ATLAS experiment, using up to 80 fb–1 of data collected at a proton-proton collision energy of 13 TeV from 2015 to 2017. Measurements were performed in all of the main decay channels of the Higgs boson: to pairs of photons, W and Z bosons, bottom quarks, taus, and muons. The overall production rate of the Higgs boson was measured to be in agreement with Standard Model predictions, with an uncertainty of 8%. The uncertainty is reduced from 11% in the previous combined measurements released last year.

    The measurements are broken down into production modes (assuming Standard Model decay branching ratios), as shown in Figure 1. All four main production modes have now been observed at ATLAS with a significance of more than 5 standard deviations: the long-established gluon-gluon fusion mode, the recently-observed associated production with top-quark pair, and last-remaining weak boson fusion mode, presented today by ATLAS. Together with the observation of production in association with a weak boson and of the H→bb decay in a separate measurement, these results paint a complete picture of Higgs boson production and decay.

    Physicists can use these new results to study the couplings of the Higgs boson to other fundamental particles. As shown in Figure 2, these couplings are in excellent agreement with the Standard Model prediction over a range covering 3 orders of magnitude in mass, from the top quark (the heaviest particle in the Standard Model and thus with the strongest interaction with the Higgs boson) to the much lighter muons (for which only an upper limit of the coupling with the Higgs boson has been obtained so far).

    2
    Figure 2: Higgs boson coupling strength to each particle (error bars) as a function of particle mass compared with Standard Model prediction (blue dotted line). (Image: ATLAS Collaboration/CERN)

    The measurements also probe the coupling of the Higgs boson to gluons in the gluon-gluon fusion production process, which proceeds through a loop diagram and is thus particularly sensitive to new physics. In the Standard Model, the loop is mediated mainly by top quarks. Therefore, possible new physics contributions can be tested by comparing the gluon coupling with the direct measurement of the top quark coupling in Higgs boson production in association with top quarks, as shown in Figure 3.

    3
    Figure 3: Ratios of coupling strengths to each particle. By taking ratios, model assumptions (such as on the total width of the Higgs boson) can be significantly reduced. Among all the interesting tests performed, the one comparing the gluon-gluon fusion and Higgs boson production in association with top quarks is represented by λtg in the plot. (Image: ATLAS Collaboration/CERN)

    The excellent agreement with the Standard Model, which is observed throughout, can be used to set stringent limits on new physics models. These are based on possible modifications to Higgs couplings and complement direct searches performed at the LHC.

    Links:

    Combined measurements of Higgs boson production and decay using up to 80 fb−1 of proton-proton collision data at 13 TeV collected with the ATLAS experiment (ATLAS-CONF-2018-031)
    ICHEP2018 presentation by Nicolas Morange: Measurements of Higgs boson properties using a combination of different Higgs decay channels
    ICHEP2018 presentation by Tancredi Carli: Highlights from the ATLAS and ALICE Experiments
    ICHEP2018 presentation by Giacinto Piacquadio (coming Tuesday 9 July)
    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 July 5, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , , Quarks observed to interact via minuscule 'weak lightsabers'   

    From CERN ATLAS: “Quarks observed to interact via minuscule ‘weak lightsabers'” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    5th July 2018
    1
    Left: Especially at invariant jet-jet masses, mjj, > 1000 GeV the yellow signal of W±W± W±W± scattering can be clearly seen above the background from other processes. Right: The orange signal of W±Z W±Z scattering is evident as the white contribution at large values of the score value of a multivariate boosted decision tree (BDT). (Image: ATLAS Collaboration/CERN)

    Two among the rarest processes probed so far at the LHC, the scattering between W and Z bosons emitted by quarks in proton-proton collisions, have been established by the ATLAS experiment at CERN.

    W and Z bosons play the same mediating role for the weak nuclear interaction as photons do for electromagnetism. As light beams of photons from torches or lasers unaffectedly penetrate each other, electromagnetic “lightsabers” will forever stay science fiction. However, beams of W and Z bosons – or “weak light rays” – can scatter from one another.

    One of the key motivations for building the Large Hadron Collider (LHC) at CERN was to study exactly this process, called weak “vector boson scattering” (VBS). One quark in each of two colliding protons has to radiate a W or a Z boson. These extremely short-lived particles are only able to fly a distance of 0.1×10-15m before transforming into other particles, and their interaction with other particles is limited to a range of 0.002×10-15m. In other words, these extremely short “weak lightsabers” extend only about 1/10th of a proton’s radius and have to approach each other by 1/500th of a proton’s radius! Such an extremely improbable coincidence happens only about once in 20,000 billion proton-proton interactions, recorded typically in one day of LHC operation.

    Using 2016 data, ATLAS has now doubtlessly observed WZ and WW electroweak production, with the dominant part of it being the weak vector boson scattering: W±W± → W±W± and W±Z → W±Z. This continues the experiment’s long journey to scrutinize the VBS process: using 8 TeV data from 2012, ATLAS had obtained the first evidence for the W±W± → W±W± process with 18 candidate events. Such a yield would occur with a probability of less than 1:3000 as a pure statistical fluctuation. Now, at a higher center-of-mass energy of 13 TeV, ATLAS has identified 60 W±W± → W±W± events, which only would happen less than once in 200 billion cases as a fluctuation from pure background processes. This corresponds to a statistical significance of 6.9 standard deviations (σ) above background. Besides the decay products of the scattered W or Z bosons, the signature of the process are two high-energetic particle jets originating from the two quarks that initially radiated the W or Z.

    ATLAS has also combined 2015 and 2016 data to establish the scattering of W±Z → W±Z with a statistical significance of 5.6 σ above background. In this channel, the lower-energy data of 2012 had revealed a significance of only 1.9σ, not sufficient to claim any evidence for the process. This time, thanks to a multivariate “BDT” analysis technique implemented in 2016, ATLAS was able to isolate 44 signal candidate events, of which about half reveal “BDT score” values above 0.4, where only little background is present.

    For this scattering process of vector bosons, three basic Standard Model “vertices” contribute: the interaction via the well-known “triple-boson-coupling” (green) is drastically reduced by the contributions of “quartic-boson-couplings” (red) and the “boson-Higgs-couplings” (orange). Only the latter ensures that the rate of this scattering for large centre-of-mass energies obeys the basic “unitarity” law, that a probability cannot be bigger than 100%. With the discovery of VBS, a new chapter of Standard Model tests has started, allowing ATLAS to scrutinize the so far experimentally inaccessible quartic-boson-couplings and properties of the Higgs boson.

    Related journal articles
    _________________________________________________
    See the full article for further references with links.

    See the full article here .


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  • richardmitnick 1:02 pm on July 4, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , ,   

    From CERN ATLAS: “The Higgs boson: the hunt, the discovery, the study and some future perspectives” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    1
    Figure 1: A candidate Higgs to ZZ to 4-lepton event as seen in the ATLAS detector. The four reconstructed muons are visualised as red lines. The green and blue boxes show where the muons passed through the muon detectors. (Image: ATLAS Collaboration/CERN)


    The origins of the Higgs boson

    Many questions in particle physics are related to the existence of particle mass. The “Higgs mechanism,” which consists of the Higgs field and its corresponding Higgs boson, is said to give mass to elementary particles. By “mass” we mean the inertial mass, which resists when we try to accelerate an object, rather than the gravitational mass, which is sensitive to gravity. In Einstein’s celebrated formula E = mc2, the “m” is the inertial mass of the particle. In a sense, this mass is the essential quantity, which defines that at this place there is a particle rather than nothing.

    In the early 1960s, physicists had a powerful theory of electromagnetic interactions and a descriptive model of the weak nuclear interaction – the force that is at play in many radioactive decays and in the reactions which make the Sun shine. They had identified deep similarities between the structure of these two interactions, but a unified theory at the deeper level seemed to require that particles be massless even though real particles in nature have mass.

    In 1964, theorists proposed a solution to this puzzle. Independent efforts by Robert Brout and François Englert in Brussels, Peter Higgs at the University of Edinburgh, and others lead to a concrete model known as the Brout-Englert-Higgs (BEH) mechanism. The peculiarity of this mechanism is that it can give mass to elementary particles while retaining the nice structure of their original interactions. Importantly, this structure ensures that the theory remains predictive at very high energy. Particles that carry the weak interaction would acquire masses through their interaction with the Higgs field, as would all matter particles. The photon, which carries the electromagnetic interaction, would remain massless.

    In the history of the universe, particles interacted with the Higgs field just 10-12 seconds after the Big Bang. Before this phase transition, all particles were massless and travelled at the speed of light. After the universe expanded and cooled, particles interacted with the Higgs field and this interaction gave them mass. The BEH mechanism implies that the values of the elementary particle masses are linked to how strongly each particle couples to the Higgs field. These values are not predicted by current theories. However, once the mass of a particle is measured, its interaction with the Higgs boson can be determined.

    The BEH mechanism had several implications: first, that the weak interaction was mediated by heavy particles, namely the W and Z bosons, which were discovered at CERN in 1983. Second, the new field itself would materialize in another particle. The mass of this particle was unknown, but researchers knew it should be lower than 1 TeV – a value well beyond the then conceivable limits of accelerators. This particle was later called the Higgs boson and would become the most sought-after particle in all of particle physics.

    The accelerator, the experiments and the Higgs

    The Large Electron-Positron collider (LEP), which operated at CERN from 1989 to 2000, was the first accelerator to have significant reach into the potential mass range of the Higgs boson.

    CERN LEP Collider

    Though LEP did not find the Higgs boson, it made significant headway in the search, determining that the mass should be larger than 114 GeV.

    In 1984, a few physicists and engineers at CERN were exploring the possibility of installing a proton-proton accelerator with a very high collision energy of 10-20 TeV in the same tunnel as LEP. This accelerator would probe the full possible mass range for the Higgs, provided that the luminosity[1] was very high. However, this high luminosity would mean that each interesting collision would be accompanied by tens of background collisions. Given the state of detector technology of the time, this seemed a formidable challenge. CERN wisely launched a strong R&D programme, which enabled fast progress on the detectors. This seeded the early collaborations, which would later become ATLAS, CMS and the other LHC experiments.

    On the theory side, the 1990s saw much progress: physicists studied the production of the Higgs boson in proton-proton collisions and all its different decay modes. As each of these decay modes depends strongly on the unknown Higgs boson mass, future detectors would need to measure all possible kinds of particles to cover the wide mass range. Each decay mode was studied using intensive simulations and the important Higgs decay modes were amongst the benchmarks used to design the detector.

    Meanwhile, at the Fermi National Accelerator Laboratory (Fermilab) outside of Chicago, Illinois, the Tevatron collider was beginning to have some discovery potential for a Higgs boson with mass around 160 GeV. Tevatron, the scientific predecessor of the LHC, collided protons with antiprotons from 1986 to 2011.

    Tevatron Accelerator


    FNAL/Tevatron CDF detector


    FNAL/Tevatron DZero detector

    In 2008, after a long and intense period of construction, the LHC and its detectors were ready for the first beams. On 10 September 2008, the first injection of beams into the LHC was a big event at CERN, with the international press and authorities invited. The machine worked beautifully and we had very high hopes. Alas, ten days later, a problem in the superconducting magnets significantly damaged the LHC. A full year was necessary for repairs and to install a better protection system. The incident revealed a weakness in the magnets, which limited the collision energy to 7 TeV.

    When restarting, we faced a difficult decision: should we take another year to repair the weaknesses all around the ring, enabling operation at 13 TeV? Or should we immediately start and operate the LHC at 7 TeV, even though a factor of three fewer Higgs bosons would be produced? Detailed simulations showed that there was a chance of discovering the Higgs boson at the reduced energy, in particular in the range where the competition of the Tevatron was the most pressing, so we decided that starting immediately at 7 TeV was worth the chance.

    The LHC restarted in 2010 at 7 TeV with a modest luminosity – a luminosity that would increase in 2011. The ATLAS Collaboration had made good use of the forced stop of 2009 to better understand the detector and prepare the analyses. In 2010, Higgs experts from experiments and theory created the LHC Higgs Cross-Section[2] Working Group (LHCHXSWG), which proved invaluable as a forum to accompany the best calculations and to discuss the difficult aspects about Higgs production and decay. These results have since been regularly documented in the “LHCHXSWG Yellow Reports,” famous in the community.

    2
    Figure 2: The invariant mass from pairs of photons selected in the Higgs to γγ analysis, as shown at the seminar at CERN on 4 July 2012. The excess of events over the background prediction around 125 GeV is consistent with predictions for the Standard Model Higgs boson. (Image: ATLAS Collaboration/CERN)

    The discovery of the Higgs boson

    As Higgs bosons are extremely rare, sophisticated analysis techniques are required to spot the signal events within the large backgrounds from other processes. After signal-like events have been identified, powerful statistical methods are used to quantify how significant the signal is. As statistical fluctuations in the background can also look like signals, stringent statistical requirements are made before a new signal is claimed to have been discovered. The significance is typically quoted as σ, or a number of standard deviations of the normal distribution. In particle physics, a significance of 3σ is referred to as evidence, while 5σ is referred to as an observation, corresponding to the probability of a statistical fluctuation from the background of less than 1 in a million.

    Eager physicists analysed the data as soon as it arrived. In the summer of 2011, there was a small excess in the Higgs decay to two W bosons for a mass around 140 GeV. Things got more interesting as an excess at a similar mass was also seen in the diphoton channel. However, as the dataset increased, the size of this excess first increased and then decreased.

    By the end of 2011, ATLAS had collected and analysed 5 fb-1 of data at a centre-of-mass energy of 7 TeV. After combining all the channels, it was found that the Standard Model Higgs boson could be excluded for all masses except for a small window around 125 GeV, where an excess with a significance of around 3σ was observed, largely driven by the diphoton and four lepton decay channels. The results were shown at a special seminar at CERN on 13 December 2011. Although neither experiment had strong enough results to claim observation, what was particularly telling was the fact that both ATLAS and CMS had excesses at the same mass.

    In 2012, the energy of the LHC was increased from 7 to 8 TeV, which increased the cross-sections for Higgs boson production. The data arrived quickly: by the summer of 2012, ATLAS had collected 5 fb-1 at 8 TeV, doubling the dataset. As quickly as the data arrived it was analysed and, sure enough, the significance of that small bump around 125 GeV increased further. Rumours were flying around CERN when a joint seminar between ATLAS and CMS was announced for 4 July 2012. Seats at the seminar were so highly sought after that only the people who queued all night were able to get into the room. The presence of François Englert and Peter Higgs at the seminar increased the excitement even further.

    At the famous seminar, the spokespeople of the ATLAS and CMS Collaborations showed their results consecutively, each finding an excess around 5σ at a mass of 125 GeV. To conclude the session, CERN Director-General Rolf Heuer declared, “I think we have it.”

    The ATLAS Collaboration celebrated the discovery with champagne and by giving each member of the collaboration a t-shirt with the famous plots. Incidentally, only once they were printed was it discovered that there was a typo in the plot. No matter, these t-shirts would go on to become collector’s items.

    ATLAS and CMS published the results in Physics Letters B a few weeks later in a paper titled “Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC.” The Nobel Prize in Physics was later awarded to Peter Higgs and François Englert in 2013.

    What we have learned since discovery

    After discovery, we began to study the properties of the newly-discovered particle to understand if it was the Standard Model Higgs boson or something else. In fact, we initially called it a Higgs-like boson as we did not want to claim it was the Higgs boson until we were certain. The mass, the final unknown parameter in the Standard Model, was one of the first parameters measured and found to be approximately 125 GeV (roughly 130 times larger than the mass of the proton). It turned out that we were very lucky – with this mass, the largest number of decay modes are possible.

    Standard Model of Particle Physics from Symmetry Magazine

    In the Standard Model, the Higgs boson is unique: it has zero spin, no electric charge and no strong force interaction. The spin and parity were measured through angular correlations between the particles it decayed to. Sure enough, these properties were found to be as predicted. At this point, we began to call it “the Higgs boson.” Of course, it still remains to be seen if it is the only Higgs boson or one of many, such as those predicted by supersymmetry.

    The discovery of the Higgs boson relied on measurements of its decay to vector bosons. In the Standard Model, different couplings determine its interactions to fermions and bosons, so new physics might impact them differently.

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

    Therefore, it is important to measure both. The first direct probe of fermionic couplings was to tau particles, which was observed in the combination of ATLAS and CMS results performed at the end of Run 1. During Run 2, the increase in the centre-of-mass energy to 13 TeV and the larger dataset allowed further channels to be probed. Over the past year, the evidence has been obtained for the Higgs decay to bottom quarks and the production of the Higgs boson together with top quarks has been observed. This means that the interaction of the Higgs boson to fermions has been clearly established.

    Perhaps one of the neatest ways to summarise what we currently know about the interaction of the Higgs boson with other Standard Model particles is to compare the interaction strength to the mass of each particle, as shown in Figure 4. This clearly shows that the interaction strength depends on the particle mass: the heavier the particle, the stronger its interaction with the Higgs field. This is one of the main predictions of the BEH mechanism in the Standard Model.

    We don’t only do tests to verify that the properties of the Higgs boson agree with those predicted by the Standard Model – we specifically look for properties that would provide evidence for new physics. For example, constraining the rate that the Higgs boson decays to invisible or unobserved particles provides stringent limits on the existence of new particles with masses below that of the Higgs boson. We also look for decays to combinations of particles forbidden in the Standard Model. So far, none of these searches have found anything unexpected, but that doesn’t mean that we’re going to stop looking anytime soon!

    Outlook

    2018 is the last year that ATLAS will take data as part of the LHC’s Run 2. During this run, 13 TeV proton-proton collisions have been producing approximately 30 times more Higgs bosons than those used in the 2012 Higgs boson discovery. As a result, more and more results have been obtained to study the Higgs boson in greater detail.

    Over the next few years, analysis of the large Run 2 dataset will not only be an opportunity to reach a new level of precision in previous measurements, but also to investigate new methods to probe Standard Model predictions and to test for the presence of new physics in as model-independent a way as possible. This new level of precision will rely on obtaining a deeper level of understanding of the performance of the detector, as well as the simulations and algorithms used to identify particles passing through it. It also poses new challenges for theorists to keep up with the improving experimental precision.

    In the longer term, another big step in performance will be brought by the High-Luminosity LHC (HL-LHC), planned to begin operation in 2024. The HL-LHC will increase the number of collisions by another factor of 10. Among other measurements, this will open the possibility to investigate a very peculiar property of the Higgs boson: that it couples to itself. Events produced via this coupling feature two Higgs bosons in the final state, but they are exceedingly rare. Thus, they can only be studied within a very large number of collisions and using sophisticated analysis techniques. To match the increased performance of the LHC, the ATLAS and CMS detectors will undergo comprehensive upgrades during the years before HL-LHC.

    Looking more generally, the discovery of the Higgs boson with a mass of 125 GeV sets a new foundation for particle physics to build on. Many questions remain in the field, most of which have some relation to the Higgs sector. For example:

    A popular theory beyond the Standard Model is “supersymmetry”, which presents attractive features for solving current issues, such as the nature of dark matter. The minimal version of supersymmetry predicts that the Higgs boson mass should be less than 120-130 GeV, depending on some other parameters. Is it a coincidence that the observed value sits exactly at this critical value, hence still marginally allowing for this supersymmetric model?
    Several models have been recently proposed where the only link of Dark Matter with regular matter would be through the Higgs boson.
    Stability of the universe: the value of 125 GeV is almost at the critical boundary between a stable universe and a meta-stable universe. A meta-stable system possesses another baseline state, into which it can decay anytime due to quantum tunnelling.[3] Is this also a coincidence?
    The phase transition: the details of this transition may play a role in the process which led our universe to be entirely matter and not contain any anti-matter. Present calculations with the Standard Model Higgs boson alone are inconsistent with the observed matter-antimatter asymmetry. Is this a call for new physics or only incomplete calculations?
    Are fermion masses all related to the Higgs boson field? If yes, why is there such a huge hierarchy between the fermion masses spanning from fractions of electron-volts for the mysterious neutrinos up to the very heavy top quark, with a mass on the order of hundreds of billions of electron-volts?

    From what we’ve learned about it so far, the Higgs boson seems to play a very special role in nature… Can it show us the way to answer further questions?

    See the full article here .


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