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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
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    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

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    CERN ATLAS New

    ALICE
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    CMS
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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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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

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    CERN CMS New

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    CERN LHCb New II

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  • richardmitnick 11:35 am on May 8, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , , ,   

    From CERN ATLAS- “Charming SUSY: running out of places to hide” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    7th May 2018
    ATLAS Collaboration

    1
    Figure 1: Limits on pair production of stop and scharm particles. The horizontal axis shows the mass of the stop or scharm, while the vertical axis corresponds to the mass of the lightest superpartner. The red line shows the observed limit, while the blue line and the yellow band show the expected limit and its uncertainty. The filled blue region shows models excluded by previous ATLAS searches. (Image: ATLAS Collaboration/CERN)

    Why is gravity so much weaker than the other forces of nature? This fundamental discrepancy, known as the “hierarchy problem”, has long been a source of puzzlement. Since the discovery of a scalar particle, the Higgs boson, with a mass of 125 GeV near that of the W and Z bosons mediating the weak force, the hierarchy problem is more acute than ever. Due to large quantum corrections the most natural mass of the Higgs boson should be many orders of magnitude above the one observed. Potentially as large as the Planck mass of order 1019 GeV, the energy at which gravity is expected to become as strong as the other forces.

    Supersymmetry addresses the hierarchy problem by introducing partners of the known elementary particles that cancel these detrimental quantum corrections to the Higgs mass. For this solution to work, however, the supersymmetric partner of the top quark (known as the “stop”) must have a mass not too different from that of the top quark itself. When this is the case, supersymmetry “stabilises” the mass of the Higgs boson because the top and stop contribute with opposite signs to the quantum corrections.

    Similar to the top quark itself, the stop is generally expected to decay via a bottom quark. Many attempts have been made to discover the stop in such decays. However, an intriguing theoretical possibility is that the stop might instead preferentially decay to final states containing a charm instead of a bottom quark.

    2
    Figure 2: Summary of the current status of searches for pair production of stops in ATLAS, not including the latest results described in Figure 1. The horizontal axis shows the mass of the stop, while the vertical axis corresponds to the mass of the lightest superpartner. The filled regions show observed limits on the superpartner masses. Different colours correspond to different search strategies, while the light blue area shows the results obtained with the 8 TeV dataset. Dashed lines correspond to expected limits. (Image: ATLAS Collaboration/CERN)

    ATLAS has released a new search for stops decaying to charm quarks, significantly improving previous results that used 8 TeV proton-proton collision data. One of the main challenges in the search is identifying the presence of charm quarks in proton-proton collisions events. Particles containing a charm quark travel just a fraction of a millimetre before decaying. This is unlike most particles composed of lighter quarks, which are either nearly stable or decay almost instantly after they are produced.

    Thanks to ATLAS’ superb particle tracking capabilities, physicists were able to pick out small displacements from charm quarks amongst hundreds of other tracks in the collision. Performing this feat is more challenging for charm quarks than bottom quarks, since particles containing bottom quarks travel longer on average before decaying, making then easier to identify. The new ATLAS search is also able to detect signals from the supersymmetric partner of the charm quark (the “s-charm” or “scharm”). Such particles could be lighter than other quark superpartners and thus may be more copiously produced at the LHC. Their signal in the detector would be identical to that of stops considered by the search.

    Though the ATLAS search found no signals of stops or scharms, new limits were set on the masses of these hypothetical particles, as shown in Figure 1. These results complement those from other ATLAS searches for the stop, summarised in Figure 2. There appears to be little room left for stops of similar mass to the top quark, and the hierarchy puzzle remains unsolved.

    See the full article here .

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  • richardmitnick 9:37 pm on May 5, 2018 Permalink | Reply
    Tags: "blinding", , CERN ATLAS, , , , ,   

    From particlebites: “Going Rogue: The Search for Anything (and Everything) with ATLAS” 

    particlebites bloc

    From particlebites

    May 5, 2018
    Julia Gonski

    Title: “A model-independent general search for new phenomena with the ATLAS detector at √s=13 TeV”

    Author: The ATLAS Collaboration

    Reference: ATLAS-PHYS-CONF-2017-001

    CERN/ATLAS detector

    When a single experimental collaboration has a few thousand contributors (and even more opinions), there are a lot of rules. These rules dictate everything from how you get authorship rights to how you get chosen to give a conference talk. In fact, this rulebook is so thorough that it could be the topic of a whole other post. But for now, I want to focus on one rule in particular, a rule that has only been around for a few decades in particle physics but is considered one of the most important practices of good science: blinding.

    In brief, blinding is the notion that it’s experimentally compromising for a scientist to look at the data before finalizing the analysis. As much as we like to think of ourselves as perfectly objective observers, the truth is, when we really really want a particular result (let’s say a SUSY discovery), that desire can bias our work. For instance, imagine you were looking at actual collision data while you were designing a signal region. You might unconsciously craft your selection in such a way to force an excess of data over background prediction. To avoid such human influences, particle physics experiments “blind” their analyses while they are under construction, and only look at the data once everything else is in place and validated.

    1
    Figure 1: “Blind analysis: Hide results to seek the truth”, R. MacCounor & S. Perlmutter for Nature.com

    See the full article here .

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    What is ParticleBites?

    ParticleBites is an online particle physics journal club written by graduate students and postdocs. Each post presents an interesting paper in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

    The papers are accessible on the arXiv preprint server. Most of our posts are based on papers from hep-ph (high energy phenomenology) and hep-ex (high energy experiment).

    Why read ParticleBites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. With each brief ParticleBite, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in particle physics.

    Who writes ParticleBites?

    ParticleBites is written and edited by graduate students and postdocs working in high energy physics. Feel free to contact us if you’re interested in applying to write for ParticleBites.

    ParticleBites was founded in 2013 by Flip Tanedo following the Communicating Science (ComSciCon) 2013 workshop.

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    Flip Tanedo UCI Chancellor’s ADVANCE postdoctoral scholar in theoretical physics. As of July 2016, I will be an assistant professor of physics at the University of California, Riverside

    It is now organized and directed by Flip and Julia Gonski, with ongoing guidance from Nathan Sanders.

     
  • richardmitnick 2:09 pm on April 30, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS starts new year of data-taking” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    30th April 2018
    Katarina Anthony

    On 28 April, the ATLAS Experiment began recording the first data for physics of 2018. This will be the final year of Run 2 operation of the Large Hadron Collider and will mark the conclusion of the rich 13 TeV data harvest. Starting in 2019, the accelerator and its experiments will enter a long upgrade and maintenance period.

    Ensuring the efficient operation of the detector is the highest priority for ATLAS. Operations teams are set to collect a record amount of high-quality data for analysis, potentially doubling the amount of data recorded at 13 TeV. When combined with data taken in previous years, it will form the largest dataset of its kind – keeping ATLAS physicists busy for years to come.

    ATLAS’ extensive physics programme will benefit significantly from a large pool of data, which may contain sightings of the rarest and most complex signatures. “The size and scope of the Run 2 dataset gives it extraordinary potential,” says ATLAS spokesperson Karl Jakobs. “Many searches for new particles have – so far – been limited by statistics. But the scale of this full dataset means the potential for discovery is there, though it will take time to carry out in-depth searches for the most complex scenarios.”

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    Event display from the first stable beam proton-proton collision run of 2018. (Image: ATLAS Experiment/CERN)

    Also high on the list of priorities is the study of the Higgs boson. While physicists have spent over half a century on its discovery, the study of the particle itself remains a very new field of research. Precision measurements of Higgs properties are important to test the predictions of the Standard Model and probe possible effects of new physics. In addition, the search for rarer Higgs boson decays can be addressed.

    “The excellent performance of ATLAS has allowed us to make great strides in the area of precision physics,” says Jakobs. “Take, for example, the fantastic measurement of the W boson mass published earlier this year. This success is indicative of the LHC’s potential not just for discovery, but for extraordinary precision. Future analyses will continue to focus on measurements of such fundamental Standard Model parameters.”

    At the end of 2018, ATLAS will shut down for a long maintenance period to carry out upgrades to the detector. “One of our primary goals will be upgrading our trigger and data acquisition system, which allows us to select the collisions we want to record and analyse,” says Lauren Tompkins, ATLAS physicist with Stanford University (USA). “During the shutdown, we will be adding new capabilities that will allow us to include more criteria in the trigger’s decision-making processes. By improving the selection process of our data, ATLAS will be able to save more Higgs bosons and study more new physics scenarios.”

    While 2018 will mark the end of Run 2, the exploration of the highest-energy frontier at the Large Hadron Collider has only just begun. Will the road ahead lead to a new discovery? Or will future measurements continue to agree with the Standard Model? Analyses of the full 13 TeV dataset should hold the clues to which direction high-energy physics will take.

    See the full article here .

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  • richardmitnick 6:10 pm on February 13, 2018 Permalink | Reply
    Tags: , CERN ATLAS, , , , , ,   

    From CERN Courier: “ATLAS extends searches for natural supersymmetry” 


    CERN Courier

    Jan 15, 2018

    1
    Exclusion limits

    Despite many negative searches during the last decade and more, supersymmetry (SUSY) remains a popular extension of the Standard Model (SM). Not only can SUSY accommodate dark matter and gauge–force unification at high energy, it offers a natural explanation for why the Higgs boson is so light compared to the Planck scale. In the SM, the Higgs boson mass can be decomposed into a “bare” mass and a modification due to quantum corrections. Without SUSY, but in the presence of a high-energy new physics scale, these two numbers are extremely large and thus must almost exactly oppose one another – a peculiar coincidence called the hierarchy problem. SUSY introduces a set of new particles that each balances the mass correction of its SM partner, providing a “natural” explanation for the Higgs boson mass.

    Thanks to searches at the LHC and previous colliders, we know that SUSY particles must be heavier than their SM counterparts. But if this difference in mass becomes too large, particularly for the particles that produce the largest corrections to the Higgs boson mass, SUSY would not provide a natural solution of the hierarchy problem.

    New SUSY searches from ATLAS using data recorded at an energy of 13 TeV in 2015 and 2016 (some of which were shown for the first time at SUSY 2017 in Mumbai from 11–15 December) have extended existing bounds on the masses of the top squark and higgsinos, the SUSY partners of the top quark and Higgs bosons, respectively, that are critical for natural SUSY. For SUSY to remain natural, the mass of the top squark should be below around 1 TeV and that of the higgsinos below a few hundred GeV.

    ATLAS has now completed a set of searches for the top squark that push the mass limits up to 1 TeV. With no sign of SUSY yet, these searches have begun to focus on more difficult to detect scenarios in which SUSY could hide amongst the SM background. Sophisticated techniques including machine learning are employed to ensure no signal is missed.

    First ATLAS results have also been released for higgsino searches. If the lightest SUSY particles are higgsino-like, their masses will often be close together and such “compressed” scenarios lead to the production of low-momentum particles. One new search at ATLAS targets scenarios with leptons reconstructed at the lowest momenta still detectable. If the SUSY mass spectrum is extremely compressed, the lightest charged SUSY particle will have an extended lifetime, decay invisibly, and leave an unusual detector signature known as a “disappearing track”.

    Such a scenario is targeted by another new ATLAS analysis. These searches extend for the first time the limits on the lightest higgsino set by the Large Electron Positron (LEP) collider 15 years ago. The search for higgsinos remains among the most challenging and important for natural SUSY. With more data and new ideas, it may well be possible to discover, or exclude, natural SUSY in the coming years.

    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 9:54 am on February 12, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, First high-precision measurement of the mass of the W boson at the LHC, , , , ,   

    From CERN ATLAS : “First high-precision measurement of the mass of the W boson at the LHC” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    12th February 2018

    1
    Display of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions recorded by ATLAS with LHC stable beams at a collision energy of 7 TeV. (Image: ATLAS Collaboration/CERN).

    In a paper published today in the European Physical Journal C, the ATLAS Collaboration reports the first high-precision measurement at the Large Hadron Collider (LHC) of the mass of the W boson. This is one of two elementary particles that mediate the weak interaction – one of the forces that govern the behaviour of matter in our universe. The reported result gives a value of 80370±19 MeV for the W mass, which is consistent with the expectation from the Standard Model of Particle Physics, the theory that describes known particles and their interactions.

    The measurement is based on around 14 million W bosons recorded in a single year (2011), when the LHC was running at the energy of 7 TeV. It matches previous measurements obtained at Large Electron-Positron Collider[LEP] , the ancestor of the LHC at CERN, and at the Tevatron , a former accelerator at Fermilab [FNAL] in the United States, whose data made it possible to continuously refine this measurement over the last 20 years.

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    CERN LEP

    3
    FNAL Tevatron

    FNAL/Tevatron

    The W boson is one of the heaviest known particles in the universe. Its discovery in 1983 crowned the success of CERN’s Super Proton Synchrotron , leading to the Nobel Prize in physics in 1984. Although the properties of the W boson have been studied for more than 30 years, measuring its mass to high precision remains a major challenge.

    4
    Super Proton Synchrotron

    “Achieving such a precise measurement despite the demanding conditions present in a hadron collider such as the LHC is a great challenge,” said the physics coordinator of the ATLAS Collaboration, Tancredi Carli. “Reaching similar precision, as previously obtained at other colliders, with only one year of Run 1 data is remarkable. It is an extremely promising indication of our ability to improve our knowledge of the Standard Model and look for signs of new physics through highly accurate measurements.”

    The Standard Model is very powerful in predicting the behaviour and certain characteristics of the elementary particles and makes it possible to deduce certain parameters from other well-known quantities. The masses of the W boson, the top quark and the Higgs boson for example, are linked by quantum physics relations. It is therefore very important to improve the precision of the W boson mass measurements to better understand the Higgs boson, refine the Standard Model and test its overall consistency.

    Remarkably, the mass of the W boson can be predicted today with a precision exceeding that of direct measurements. This is why it is a key ingredient in the search for new physics, as any deviation of the measured mass from the prediction could reveal new phenomena conflicting with the Standard Model.

    The measurement relies on a thorough calibration of the detector and of the theoretical modelling of the W boson production. These were achieved through the study of Z boson events and several other ancillary measurements. The complexity of the analysis meant it took almost five years for the ATLAS team to achieve this new result. Further analysis with the huge sample of now-available LHC data, will allow even greater accuracy in the near future.

    See the full article here .

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  • richardmitnick 12:16 pm on January 18, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , Measurements of weak top quark processes gain strength, ,   

    From ATLAS at CERN: “Measurements of weak top quark processes gain strength” 

    This post is dedicated to L.Z. from H.P. and Rutgers Physics. I hope that he sees it.

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    18th January 2018
    ATLAS Collaboration

    1
    Normalised differential cross-sections as a function of the mass of the two charged leptons and the b-jet unfolded from data, compared with selected Monte Carlo models. (Image: ATLAS Collaboration/CERN)

    The production of top quarks in association with vector bosons is a hot topic at the LHC. ATLAS first reported strong evidence for the production of a top quark in association with a Z boson at the EPS 2017 conference. In a paper submitted to the Journal of High-Energy Physics, the ATLAS experiment describes the measurement of top-quark production in association with a W boson in 13 TeV collisions.

    The new ATLAS result using the full 2015 and 2016 dataset extracts differential cross-sections for the production of a top quark in association with a W boson for the first time. This is particularly complex as top quarks almost always decay into a b quark and a W boson, and thus there are two W bosons in final state that decay very quickly. Events are selected that contain two charged leptons (electrons or muons), a jet that is identified as containing a hadron with a b quark, and missing transverse momentum due to the presence of neutrinos.

    Multivariate techniques are used to suppress large background contributions, especially from the production of a top quark with a top antiquark that occurs with much larger rate. They achieve a signal to background ratio of about 1:2, which allows the signal cross-section to be extracted as a function of kinematic observables. The measured background-subtracted distributions are corrected to remove the effects of experimental resolution so that they can be directly compared with theoretical predictions.

    Differential cross-sections as a function of several variables related to both the event and top quark or W boson kinematic properties have been measured and compared to theory predictions, implemented in different Monte Carlo programmes. The figure shows one out of the six extracted cross-sections.

    The uncertainty on the measurements is at the 20­-50% level, dominated by statistical effects. While this does not allow to draw firm conclusions, the data tend to have more events with high-momentum final-state objects than predicted. This effect can be seen in the figure. A quantitative analysis reveals, however, that the tested Monte Carlo models are all statistically compatible with the data. As ATLAS continues to study this channel, the increased size of the data sample and improvements in the predictions should make such comparisons more significant.

    Links:

    Measurement of differential cross-sections of a single top quark produced in association with a W boson at 13 TeV with ATLAS (arXiv: 1712.01602, see figures ).
    Measurement of the cross-section for producing a W boson in association with a single top quark in pp collisions at 13 TeV with ATLAS (arXiv: 1612.07231).
    Measurement of the production cross-section of a single top quark in association with a Z boson in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2017-052).
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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