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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.

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  • 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).

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    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.

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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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  • richardmitnick 12:56 pm on June 10, 2018 Permalink | Reply
    Tags: , ATLAS Trigger, , CERN ATLAS, , , ,   

    From CERN ATLAS: “In conversation with Nick Ellis, one of the architects of the ATLAS Trigger” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    10th June 2018
    Kate Shaw

    1
    Nick Ellis with the ATLAS trigger system. (Image: K. Anthony/ATLAS Collaboration)

    A long-standing member of the ATLAS Collaboration, CERN physicist Nick Ellis was one of the original architects of the ATLAS Trigger. Working in the 1980s and 1990s, Nick led groups developing innovative ways to move and process huge quantities of data for the next generation of colliders. It was a challenge some thought was impossible to meet. Nick currently leads the CERN ATLAS Trigger and Data Acquisition Group and shared his wealth of experience as a key part of the ATLAS Collaboration.

    I first became involved in what was to become the ATLAS Collaboration in the mid- to late-1980s. I had been working on the UA1 experiment at CERN’s SPS proton–antiproton collider for several years on various physics analyses and also playing a leading role on the UA1 trigger.

    People were starting to think about experiments for higher-energy machines, such as the Large Hadron Collider (LHC) and the never-completed Superconducting Super Collider (SSC). Of course, at this point there was no ATLAS or CMS or even the precursors. There were just groups of people getting together to discuss ideas.

    I remember a first discussion I had about possibilities for the trigger in LHC experiments was over a coffee in CERN’s Restaurant 1 with Peter Jenni. He was on the UA2 experiment at the time and, together with a number of colleagues, was developing ideas for an LHC experiment. Peter later went on to lead the ATLAS Collaboration for over a decade. He told me that nobody was looking at how the trigger system might be designed, and he asked if I would like to develop something. So I did.

    The ATLAS trigger is a multilevel system that selects events that are potentially interesting for physics studies from a much larger number of events. It is very challenging since we start off with an interaction rate of the order of a billion per second. In the first stage of the selection, that has to be done within a few millionths of a second, the event rate must be reduced to about 100 kHz, four orders of magnitude below to the interaction rate, i.e. only one in ten thousand collisions can give rise to a first-level trigger. Note that each event, corresponding to a given bunch crossing, contains many tens of interactions. The rate must then be brought down by a further two orders of magnitude before the data is recorded for offline analysis.

    When I start working on such a complex technical problem, I sit down with a pen and paper and draw diagrams. It’s important to visualise the system. A trigger and data-acquisition system is complicated – you have data being produced, data being processed, data being moved. So, I make a sketch with arrows, writing down order of magnitude numbers, what has to talk to what, what signals have to be sent. These are very rough notes! I doubt anyone other than me would be able to read my sketches that fed into the early designs of ATLAS’ trigger.

    Though I was specifically looking at the first-level calorimeter trigger, which was what I was working on at UA1, I was interested in the trigger more generally. At the time, we did not know that the future held so much possibility in terms of programmable logic. The early ideas for the first-level trigger were based on relatively primitive electronics: modules with discrete logic, memories and some custom integrated circuits.

    There was also concern that the second-level trigger processing would be hard to implement, because those triggers would require too much data to move and too much data to process. Here, the first thing I had to do was to demonstrate that it could be done at all! I carried out an intellectual exercise to try and factorise the problem, to the maximal extent possible. I was driven to do this because it was so interesting, and it was virgin territory. There were no constraints on ideas that could be explored.

    My initial studies were on a maximally-factorised model, the so-called “local–global scheme”. It was never my objective that one would necessarily implement this exact scheme, but I used it as the basis for brainstorming a region-of-interest (ROI) strategy for the trigger. The triggers would look at specified regions of the detector, identified by the first-level trigger, for features of interest, rather than trying to search for features everywhere in the event. This exercise demonstrated that, at any given point in the system, you could get the data movement and computation down to a manageable level.

    2
    The ATLAS Level-1 Calorimeter Trigger, located underground in a cavern adjacent to the experiment. (Image: K. Anthony/ATLAS Collaboration)

    I, along with a few colleagues, developed this exercise into a study that we presented at the 1990 Large Hadron Collider workshop in Aachen, Germany. In the end, thanks to technological progress, it was not necessary to exploit all the ingredients used in the study. In more specific words, instead of separating the processing for each ROI and for each detector, we were able to use a single processor to process fully all of the ROIs in an event. The use of the first-level trigger to guide the second-level data access and processing became a key part of the ATLAS trigger philosophy.

    In the years following the Aachen workshop, the ATLAS and CMS experiments began to take shape. It was a really exciting time, and the number of people involved was tiny in comparison to today. You could do anything and everything; you could come with completely new ideas!

    When first beams and first collisions finally came, things went more smoothly than I had ever dared to hope.

    3
    First collisions in ATLAS. A collision event. (Image: Claudia Marcelloni/ATLAS Experiment)

    We had spent a lot of time planning for what would come when the first single beam came, when the first collisions came, what would we do, in what order, what might go wrong and how could we mitigate it. It has always been in my nature to think ahead about all the potential problems and make plans that let us avoid future issues, ensuring that systems are robust so that a local problem does not become a global problem. Thanks to the work of excellent, dedicated colleagues, everything went really well for first collisions!

    Clearly ATLAS has a long future ahead of it, although we will always face challenges: the upgrades we have planned are by no means trivial! Even with our existing infrastructure and experience, there will no doubt be obstacles that we will have to overcome.

    And, of course, in the even longer term, CERN itself could change, depending on what happens in physics and on the global stage. It wouldn’t be the first laboratory to do so – just look at DESY and SLAC.


    DESY Helmholtz Centres & Networks


    SLAC Campus


    SLAC SSRL


    SLAC LCLS

    Even Fermilab has changed from a collider to a neutrino facility. We never know where the next big discovery will lead us!


    FNAL Short baseline neutrino detector

    FNAL LBNF/DUNE from FNAL to SURF, Lead, South Dakota, USA

    See the full article here .


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  • richardmitnick 7:38 pm on June 5, 2018 Permalink | Reply
    Tags: , , Catching hadronic vector boson decays with a finer net, CERN ATLAS, , , ,   

    From CERN ATLAS: “Catching hadronic vector boson decays with a finer net” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    5th June 2018
    ATLAS Collaboration

    1
    Figure 1: ATLAS event display showing two electroweak boson candidates with an invariant mass of 5 TeV, the highest observed in the analysis. Energy deposits in the ATLAS calorimeters are shown in green and yellow rectangles. The angular resolution limits the reconstruction of substructure of highly collimated jets. The view in the top left corner shows the higher angular resolution of the first calorimeter layer and the tracker (orange lines), revealing the striking two-prong substructure in the energy flow. (Image: ATLAS Collaboration/CERN)

    ATLAS has been collecting increasing amounts of data at a centre-of-mass energy of 13 TeV to unravel some of the big mysteries in physics today. For instance, why is the mass of the Higgs boson so much lighter than one would expect? Why is gravity so weak?

    Many theoretical models predict that new physics, which could provide answers to these questions, could manifest itself as yet-undiscovered massive particles. These include massive new particles that would decay to much lighter high-momentum electroweak bosons (W and Z). These in turn decay, and the most common signature would be pairs of highly collimated bundles of particles, known as jets. So far, no evidence of such new particles has been uncovered.

    The ability to distinguish jets initiated by decays of W or Z bosons from those initiated by other processes is critical for the success of these searches. While the energy flow from the bosons exhibits a distinct two-prong structure from the two-body decay of the boson, no such feature exists for jets from a single quark or gluon – the latter being the most frequent scattering products when colliding protons.

    In the past, ATLAS identified this two-prong structure using its fine-grained calorimeter, which measures the energy of the particles inside jets with good resolution. However, in very energetic jets from decays of particles with masses of multiple TeV, the average separation of these prongs is comparable to the segmentation of the ATLAS calorimeter. This creates confusion within the algorithms responsible for identifying the bosons, limiting our sensitivity to new physics at high masses. In contrast to the calorimeter, the ATLAS inner tracking detector reconstructs charged particles with excellent angular resolution, but it lacks sufficient momentum resolution.

    2
    Figure 2 Comparison between the current and previous limits on the cross section times branching ratio for a hypothetical particle V versus its mass. Lower vertical values represent higher sensitivity to new physics. Due to the improvements on the analysis techniques, the current result improves our reach for new physics far beyond what we get from only increasing the size of the data set (middle blue line). (Image: ATLAS Collaboration/CERN)

    A new ATLAS analysis combines the angular information of charged particles reconstructed by the inner detector with the energy information from the calorimeter. This lets ATLAS physicists eliminate the limitations in identifying very energetic jets from bosons. Similar to increasing the magnification of a microscope, this improvement to the ATLAS event reconstruction software allows it to better resolve the energy flow in very energetic jets. This improved magnification allows physicists to also optimize the analyses techniques.

    Making such improvements while collecting more data is necessary to maximize the potential for discovery when exploring new kinematic regimes. This time no new physics was seen, but the technique can be applied to many more searches – and still larger datasets.

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

    See the full article here .


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  • richardmitnick 9:23 am on June 4, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , , ,   

    From CERN Courier: “Higgs boson reaches the top” 


    From CERN Courier

    Jun 1, 2018
    No writer credit

    The CMS collaboration has published the first direct observation of the coupling between the Higgs boson and the top quark, offering an important probe of the consistency of the Standard Model (SM). In the SM, the Higgs boson interacts with fermions via a Yukawa coupling, the strength of which is proportional to the fermion mass. Since the top quark is the heaviest particle in the SM, its coupling to the Higgs boson is expected to be the largest and thus the dominant contribution to many loop processes, making it a sensitive probe of hypothetical new physics.

    1
    Combined likelihood analysis

    The associated production of a Higgs boson with a top quark–antiquark pair (ttH) is the best direct probe of the top-Higgs Yukawa coupling with minimal model dependence, and thus a crucial element to verify the SM nature of the Higgs boson. However, its small production rate – constituting only about 1% of the total Higgs production cross-section – makes the ttH measurement a considerable challenge.

    The CMS and ATLAS collaborations reported first evidence for the process last year, based on LHC data collected at a centre-of-mass energy of 13 TeV (CERN Courier May 2017 p49 and December 2017 p12). The first observation, constituting statistical significance above five standard deviations, is based on an analysis of the full 2016 CMS dataset recorded at an energy of 13 TeV and by combining these results with those collected at lower energies.

    The ttH process gives rise to a wide variety of final states, and the new CMS analysis combines results from a number of them. Top quarks decay almost exclusively to a bottom quark (b) and a W boson, the latter subsequently decaying either to a quark and an antiquark or to a charged lepton and its associated neutrino. The Higgs-boson decay channels include the decay to a bb quark pair, a τ+τ– lepton pair, a photon pair, and combinations of quarks and leptons from the decay of intermediate on- or off-shell W and Z bosons. These five Higgs-boson decay channels were analysed by CMS using sophisticated methods, such as multivariate techniques, to separate signal from background events. Each channel poses different experimental challenges: the bb channel has the largest rate but suffers from a large background of events containing a top-quark pair and jets, while the photon and Z-boson pair channels offer the highest signal-to-background ratio at a very small rate.

    CMS observed an excess of events with respect to the background-only hypothesis at a significance of 5.2 standard deviations. The measured values of the signal strength in the considered channels are consistent with each other, and a combined value of 1.26 +0.31/–0.26 times the SM expectation is obtained (see figure). The measured production rate is thus consistent with the SM prediction within one standard deviation. The result establishes the direct Yukawa coupling of the Higgs boson to the top quark, marking an important milestone in our understanding of the properties of the Higgs boson.

    Further reading

    https://arxiv.org/abs/1804.02610
    https://arxiv.org/abs/1803.05485
    https://journals.aps.org/prd/abstract/10.1103/PhysRevD.97.072003

    See the full article here .


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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN LHC Map
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  • richardmitnick 8:58 am on June 4, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , ,   

    From CERN CMS and ATLAS: “The Higgs boson reveals its affinity for the top quark” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    New results from the ATLAS and CMS experiments at the LHC reveal how strongly the Higgs boson interacts with the heaviest known elementary particle, the top quark, corroborating our understanding of the Higgs and setting constraints on new physics.

    CERN CMS Event NOV 2010

    From CERN CMS

    By CMS

    The first observation of the simultaneous production of a Higgs boson with a top quark-antiquark pair is being published today in the journal Physical Review Letters. This major milestone, first reported by the CMS Collaboration in early April 2018, unambiguously demonstrates the interaction of the Higgs boson and top quarks, which are the heaviest known subatomic particles. It is an important step forward in our understanding of the origin of mass. The paper features as a PRL Editors’ Suggestion and also has a Physics Viewpoint article published about it.

    ________________________________________________________
    From CMS – first reported by the CMS Collaboration in early April 2018

    The observation of a Higgs boson in 2012 at the Large Hadron Collider marked the starting point of a broad experimental program to determine the properties of the newly discovered particle. In the standard model, the Higgs boson couples to fermions in a Yukawa-type interaction, with a coupling strength proportional to the fermion mass. While decays into γγ, ZZ, WW, and ττ final states have been observed and there is evidence for the direct decay of the particle to the bb (down-type quarks) final state, the decay to the tt (up-type quarks) final state is not kinematically possible. Therefore, it is of paramount importance to probe the coupling of the Higgs boson to the top quark, the heaviest known fermion, by producing the Higgs in the fusion of a top quark-antiquark pair (left diagram) or through radiation from a top quark (right diagram).

    1

    The associated production of a Higgs boson and a top quark-antiquark pair (ttH production) is a direct probe of the top–Higgs coupling. Hence the observation of this production mechanism is one of the primary objectives of the the Higgs physics program at the LHC.

    The CMS experiment has searched for ttH production in the data collected at the center-of-mass energies of 7, 8, and 13 TeV with the Higgs boson decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets. The results have been combined to maximize the sensitivity to this challenging and yet fundamental process.

    3
    An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance for the standard model Higgs boson with a mass of 125.09 GeV is 4.2 standard deviations. The measured production rate is consistent with the standard model prediction within one standard deviation.

    In addition to comprising the first observation of a new Higgs boson production mechanism, this measurement establishes the tree-level coupling of the Higgs boson to the top quark, and hence to an up-type quark, and is another milestone towards the measurement of the Higgs boson coupling to fermions.
    ________________________________________________________

    4

    An event candidate for the production of a top quark and top anti-quark pair in conjunction with a Higgs Boson in the CMS detector. The Higgs decays into a tau+ lepton and a tau- lepton; the tau+ in turn decays into hadrons and the tau- decays into an electron. The decay product symbols are in blue. The top quark decays into three jets (sprays of lighter particles) whose names are given in purple. One of these is initiated by a b-quark. The top anti-quark decays into a muon and b-jet, whose names appear in red.

    Further reading:

    [1] CMS ttH observation journal article: Physical Review Letters, June 4, 2018

    See the full CMS article here.

    From CERN ATLAS

    6
    CERN ATLAS Event 2012

    New ATLAS result establishes production of Higgs boson in association with top quarks.

    This rare process is one of the most sensitive tests of the Higgs mechanism.

    By ATLAS Collaboration, 4th June 2018

    According to the Standard Model, quarks, charged leptons, and W and Z bosons obtain their mass through interactions with the Higgs field, a quantum fluctuation of which gives rise to the Higgs boson. To test this theory, ATLAS takes high-precision measurements of the interactions between the Higgs boson and these particles. While the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) had observed and measured the Higgs boson decaying to pairs of W or Z bosons, photons or tau leptons, the Higgs coupling to quarks had not – despite evidence – been observed.

    In results presented today at the LHCP2018 conference, the ATLAS Collaboration has observed the production of the Higgs boson together with a top-quark pair (known as “ttH” production). Only about 1% of all Higgs bosons are produced through this rare process. This result establishes a direct measurement of the interaction between the top quark and the Higgs boson (known as the “top quark Yukawa coupling”). As the top quark is the heaviest particle in the Standard Model, this measurement is one of the most sensitive tests of the Higgs mechanism.

    Previous ATLAS measurements using 2015 and 2016 data provided the first evidence for ttH production from a combination of channels where the Higgs boson decayed to two W or Z bosons (WW* or ZZ*), to a pair of tau leptons, to a pair of b-quarks, or to a pair of photons (“diphoton”). Those results have now been updated with the measurements of the diphoton and ZZ* decay modes that use the larger 2015-2017 dataset, and where improved reconstruction algorithms and new analysis techniques have increased the sensitivity of the measurements. The CMS Collaboration recently reported the observation of ttH production by combining 2015 and 2016 data with data taken at lower collision energies in earlier LHC runs.
    Evidence for ttH production in the diphoton channel in the 2015-2017 dataset

    The probability of a Higgs boson decaying to a diphoton pair is only about 0.2%, making the predicted rate for ttH production in this channel quite small. However, because the energy and direction of photons can be well measured with the ATLAS detector, the reconstructed mass peak obtained with this decay mode is narrow. It is therefore possible to observe a signal even when the number of events is low. Furthermore, regions with lower and higher reconstructed mass (called the “sidebands”) can be used to estimate the background under the signal peak using the data themselves, rendering this channel particularly robust.

    To optimize the measurement ATLAS employs machine learning techniques. Events consistent with the ttH kinematics are selected using “boosted decision tree” (BDT) algorithms that allow physicists to separate the events into multiple categories with different signal-to-background abundance ratios. Depending on the top-quark decay channel considered, the inputs given to the BDT are the momenta of the “jets” (collimated groups of particles that are produced by a quark or gluon), leptons and photons observed in each event. As the decay of a top quark always produces a b-quark, identifying jets that arise from b-quarks is essential for reducing backgrounds. To achieve this, ATLAS developed a b-identification algorithm (also based on machine learning); the b-identification decision for each jet is included in the BDT inputs.

    4
    Figure 1: Time-lapse animation showing the increasing ttH signal in the diphoton mass spectrum as more data are included in the measurement. (Image: ATLAS Collaboration/CERN)

    Each category is analysed separately by studying the distribution of the invariant mass of the diphoton candidates in selected events. This distribution is fit to a combination of signal (Higgs boson decay to diphoton in events containing a top-quark pair) and background (cases where the diphoton candidate does not arise from a Higgs boson or where the event does not contain a true top-quark pair). The numbers of fitted signal events in the different categories are then statistically combined, taking into account correlated experimental and theoretical systematic uncertainties.

    The result of the above procedure, using 80 fb-1 of data recorded in 2015, 2016 and the recent 2017 run of the LHC, is summarised in Figure 1, which shows the diphoton invariant mass distribution, summed over categories weighted by their signal purity. The significance of the observed signal is 4.1 standard deviations; the expected significance for Standard Model production is 3.7 standard deviations.

    Search continues for ttH production in the ZZ* channel in the 2015-2017 dataset.

    The decay of a Higgs boson to ZZ* with the subsequent decay of the ZZ* to four leptons is another channel where the Higgs mass peak is narrow. Due to the very clean detector signature of the four-lepton decay mode, this channel is essentially free of backgrounds apart from small contributions from Higgs bosons produced through other production modes than ttH. However, this decay mode is even rarer than that of diphotons, with less than one event expected from ttH production in the 80 fb-1 of the full 2015-2017 dataset. A dedicated search for this decay was performed, but no candidate events were found in the 2015-2017 ATLAS data.

    5
    Figure 2: Combined ttH production cross section, as well as cross sections measured in the individual analyses, divided by the SM cross section prediction. ML indicates the analysis of the two and three lepton final states (multilepton). The black lines show the total uncertainties, while the bands indicate the statistical and systematic uncertainties. The red line indicates the SM cross section prediction, and the grey band represents the theoretical uncertainties on the prediction. For the γγ and ZZ* channels to full dataset at 13 GeV (collected between 2015 and 2017) have been used, whereas the results of the other channels are based on the 2015 and 2016 data. (Image: ATLAS Collaboration/CERN)

    Combination with earlier ATLAS results

    The measurements described above have been combined with the previously reported searches for ttH that used 2015 and 2016 data. Decays of the Higgs boson to a b-quark pair and to a pair of W bosons or tau leptons had observed (expected) significances of 1.4 (1.6) and 4.2 (2.8) standard deviations, respectively.

    After the combination, the observed (expected) significance of the signal over the background is 5.8 (4.9) standard deviations. The ratio of the combined ttH cross section measurement and the cross section measurements separated by Higgs boson decay modes are presented in Figure 2. The measured ratio of 1.32 ± 0.27 is slightly larger than, but consistent with the Standard Model expectation.

    Further searches for the ttH process were performed using 7 and 8 TeV data collected during Run 1. When combined with the 2015-2017 results, the observed (expected) significance is 6.3 (5.1) standard deviations.

    Summary

    ATLAS has observed the production of the Higgs boson in association with a top-quark pair with a significance of 6.3 standard deviations over the background-only hypothesis. The measured ttH production cross section is consistent with the Standard Model prediction. This measurement provides direct evidence for the coupling of the Higgs boson to the top quark and supports the Standard Model mechanism whereby the top quark obtains its mass through interaction with the Higgs field.

    Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector Physical Review D

    See the full ATLAS article here .


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