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  • richardmitnick 1:57 pm on July 22, 2019 Permalink | Reply
    Tags: , CERN NA64, , HEP, , ,   

    From CERN: “NA64 casts light on dark photons” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    22 July, 2019
    Ana Lopes

    The NA64 collaboration has placed new limits on the interaction between a photon and its hypothetical dark-matter counterpart.

    1
    The NA64 experiment (Image: CERN)

    Without dark matter, most galaxies in the universe would not hold together. Scientists are pretty sure about this. However, they have not been able to observe dark matter and the particles that comprise it directly. They have only been able to infer its presence through the gravitational pull it exerts on visible matter.

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

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

    Coma cluster via NASA/ESA Hubble

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


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


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

    One hypothesis is that dark matter consists of particles that interact with each other and with visible matter through a new force carried by a particle called the dark photon. In a recent study, the collaboration behind the NA64 experiment at CERN describes how it has tried to hunt down such dark photons.

    NA64 is a fixed-target experiment. A beam of particles is fired onto a fixed target to look for particles and phenomena produced by collisions between the beam particles and atomic nuclei in the target. Specifically, the experiment uses an electron beam of 100 GeV energy from the Super Proton Synchrotron accelerator.

    The Super Proton Synchrotron (SPS), CERN’s second-largest accelerator. (Image: Julien Ordan/CERN)

    In the new study, the NA64 team looked for dark photons using the missing-energy technique: although dark photons would escape through the NA64 detector unnoticed, they would carry away energy that can be identified by analysing the energy budget of the collisions.

    The team analysed data collected in 2016, 2017 and 2018, which together corresponded to a whopping hundred billion electrons hitting the target. They found no evidence of dark photons in the data but their analysis resulted in the most stringent bounds yet on the strength of the interaction between a photon and a dark photon for dark-photon masses between 1 MeV and 0.2 GeV.

    These bounds imply that a 1-MeV dark photon would interact with an electron with a force that is at least one hundred thousand times weaker than the electromagnetic force carried by a photon, whereas a 0.2-GeV dark photon would interact with an electron with a force that is at least one thousand times weaker. The collaboration anticipates obtaining even stronger limits with the upgraded detector, which is expected to be completed in 2021.

    See the full article here.


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

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

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    20th July 2019
    Katarina Anthony

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

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

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

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

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

    Studying the Higgs discovery channels

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

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

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

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

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

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

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

    Searching unseen properties of the Higgs boson

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

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

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

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

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

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

    Entering the Higgs sector

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

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

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

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

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

    Probing new physics

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

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

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

    Asymmetric top-quark production

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

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

    FNAL/Tevatron


    FNAL/Tevatron map

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

    Following the data

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

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

    See the full article here .


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  • richardmitnick 12:35 pm on July 18, 2019 Permalink | Reply
    Tags: "CMS releases open data for Machine Learning", , , HEP, , ,   

    From CERN CMS: “CMS releases open data for Machine Learning” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN CMS

    17 July, 2019

    CMS has also provided open access to 100% of its research data recorded in proton–proton collisions in 2010.

    1
    (Image: Fermilab/CERN)

    The CMS collaboration at CERN has released its fourth batch of open data to the public. With this release, which brings the volume of its open data to more than 2 PB (or two million GB), CMS has now provided open access to 100% of its research data recorded in proton–proton collisions in 2010, in line with the collaboration’s data-release policy. The release also includes several new data and simulation samples. The new release builds upon and expands the scope of the successful use of CMS open data in research and in education.

    In this release, CMS open data address the ever-growing application of machine learning (ML) to challenges in high-energy physics. According to a recent paper, collaboration with the data-science and ML community is considered a high-priority to help advance the application of state-of-the-art algorithms in particle physics. CMS has therefore also made available samples that can help foster such collaboration.

    “Modern machine learning is having a transformative impact on collider physics, from event reconstruction and detector simulation to searches for new physics,” remarks Jesse Thaler, an Associate Professor at MIT, who is working on ML using CMS open data with two doctoral students, Patrick Komiske and Eric Metodiev. “The performance of machine-learning techniques, however, is directly tied to the quality of the underlying training data. With the extra information provided in the latest data release from CMS, outside users can now investigate novel strategies on fully realistic samples, which will likely lead to exciting advances in collider data analysis.”

    The ML datasets, derived from millions of CMS simulation events for previous and future runs of the Large Hadron Collider, focus on solving a number of representative challenges for particle identification, tracking and distinguishing between multiple collisions that occur in each crossing of proton bunches. All the datasets come with extensive documentation on what they contain, how to use them and how to reproduce them with modified content.

    In its policy on data preservation and open access, CMS commits to releasing 100% of its analysable data within ten years of collecting them. Around half of proton-proton collision data collected at 7 TeV center-of-mass in 2010 were released in the first CMS release in 2014, and the remaining data are included in this new release. In addition, a small sample of unprocessed raw data from LHC’s Run 1 (2010 to 2012) are also released. These samples will help test the chain for processing CMS data using the legacy software environment.

    Reconstructed data and simulations from the CASTOR calorimeter, which was used by CMS in 2010, are also available and represent the first release of data from the very-forward region of CMS. Finally, CMS has released instructions and examples on how to generate simulated events and how to analyse data in isolated “containers”, within which one has access to the CMS software environment required for specific datasets. It is also easier to search through the simulated data and to discover the provenance of datasets.

    As before, the data are released into the public domain under the Creative Commons CC0 waiver via the CERN Open Data portal. The portal is openly developed by the CERN Information Technology department, in cooperation with the experimental collaborations who release open data on it.

    See the full article here.


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  • richardmitnick 7:59 am on July 17, 2019 Permalink | Reply
    Tags: "Bottomonium particles don’t go with the flow", , , , HEP, , ,   

    From CERN: “Bottomonium particles don’t go with the flow” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    16 July, 2019
    Ana Lopes

    The first measurement, by the ALICE [below] collaboration, of an elliptic-shaped flow for bottomonium particles could help shed light on the early universe.

    A few millionths of a second after the Big Bang, the universe was so dense and hot that the quarks and gluons that make up protons, neutrons and other hadrons existed freely in what is known as the quark–gluon plasma. The ALICE experiment at the Large Hadron Collider (LHC) can recreate this plasma in high-energy collisions of beams of heavy ions of lead. However, ALICE, as well as any other collision experiments that can recreate the plasma, cannot observe this state of matter directly. The presence and properties of the plasma can only be deduced from the signatures it leaves on the particles that are produced in the collisions.

    In a new article, presented at the ongoing European Physical Society conference on High-Energy Physics, the ALICE collaboration reports the first measurement of one such signature – the elliptic flow – for upsilon particles produced in lead–lead LHC collisions.

    The upsilon is a bottomonium particle, consisting of a bottom (often also called beauty) quark and its antiquark. Bottomonia and their charm-quark counterparts, charmonium particles, are excellent probes of the quark–gluon plasma. They are created in the initial stages of a heavy-ion collision and therefore experience the entire evolution of the plasma, from the moment it is produced to the moment it cools down and gives way to a state in which hadrons can form.

    One indication that the quark–gluon plasma forms is the collective motion, or flow, of the produced particles. This flow is generated by the expansion of the hot plasma after the collision, and its magnitude depends on several factors, including: the particle type and mass; how central, or “head on”, the collision is; and the momenta of the particles at right angles to the collision line. One type of flow, called elliptic flow, results from the initial elliptic shape of non-central collisions.

    In their new study, the ALICE team determined the elliptic flow of the upsilons by observing the pairs of muons (heavier cousins of the electron) into which they transform, or “decay”. They found that the magnitude of the upsilon elliptic flow for a range of momenta and collision centralities is small, making the upsilons the first hadrons that don’t seem to exhibit a significant elliptic flow.

    The results are consistent with the prediction that the upsilons are largely split up into their constituent quarks in the early stages of their interaction with the plasma, and they pave the way to higher-precision measurements using data from ALICE’s upgraded detector, which will be able to record ten times more upsilons. Such data should also cast light on the curious case of the J/psi flow. This lighter charmonium particle has a larger flow and is believed to re-form after being split up by the plasma.

    See the full article here.


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  • richardmitnick 12:10 pm on July 15, 2019 Permalink | Reply
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    From CERN: “Exploring the Higgs boson “discovery channels” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    12th July 2019
    ATLAS Collaboration

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

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

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

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

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

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

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

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

    See the full article here .


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

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

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    11th July 2019
    ATLAS Collaboration

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

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

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

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

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

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

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

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

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

    See the full article here .


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  • richardmitnick 5:49 pm on June 20, 2019 Permalink | Reply
    Tags: , , FNAL and Spain, HEP, , ,   

    From Fermi National Accelerator Lab: “The enduring collaboration between Spain and Fermilab” 

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

    June 20, 2019
    Caitlyn Buongiorno

    In 2015, 700 scientists from around the world came together and established a collaboration on neutrino research that has grown to include more than 1,000 scientists from over 30 countries. And Spain was involved from the beginning.

    “It was an exciting time,” said Inés Gil-Botella, member of Fermilab’s Physics Advisory Committee and senior scientist at Spain’s Center for Energy, Environment and Technology Research (CIEMAT). “It was a huge step toward a neutrino-based partnership between Fermilab and Spain, and the rest of the world.”

    The early collaboration meetings set the stage for what would quickly become the Deep Underground Neutrino Experiment, a name Gil-Botella remembers the early collaboration voting on. Already involved in the development of similar technology at the European laboratory CERN, Spanish institutions brought technical expertise of photon detectors and liquid-argon cryostats to the collaboration.

    “Spanish scientists and institutions have a long history of working with Fermilab and have made countless important contributions over the years,” said Fermilab Director Nigel Lockyer. “Projects such as the development of the photon detection system are critical to the success of DUNE, and we benefit from the expertise of our colleagues from Spain.”

    1
    A CIEMAT technician installs the photon detection system in one of the ProtoDUNE detectors. Photo: Enrique Calvo, CIEMAT

    Currently, Spain is developing a photon detection system for DUNE’s giant particle detector. This is key to identifying and recreating a particle interaction. The system will allow scientists to understand when an interaction took place inside the detector and to determine the energy of that interaction. Knowing this information helps scientists narrow down when and where the neutrinos came from – a supernova for instance. The system is currently being tested in one of the aptly named ProtoDUNE detectors at CERN. Scientists from Spain are also working on controls and instrumentation for the DUNE detector that let researchers adjust voltages and monitor temperatures, for example.

    Well before DUNE was even an idea, Spain was participating in research at Fermilab – most notably with the CDF collaboration, which, along with the DZero collaboration, discovered the top quark in 1995 using the Tevatron particle collider.

    FNAL/Tevatron map

    FNAL/Tevatron CDF detector

    FNAL/Tevatron DZero detector

    During this era, Spanish institutions contributed the expertise they gained at CERN in collider physics to Fermilab experiments. One link between the Tevatron era and the current DUNE era was scientist Mario Martinez of the Spanish Institute for High Energy Physics in Barcelona. A leader in the CDF experiment, he also previously served as a representative for Spain in the early days of LBNF/DUNE.

    2
    Collaborators at the Institute of Corpuscular Physics in Valencia, Spain, constructed this 25-foot thermometer for one of the ProtoDUNE detectors at CERN. Photo: CERN

    Today, more than 60 scientists at 15 Spanish institutions contribute their expertise to more than a dozen experiments at Fermilab. In addition to their research at the Large Hadron Collider, Spanish scientists bring knowledge about neutrino physics, liquid-argon cryostats, computing, cosmology and more to the Fermilab research program. CIEMAT and the University of Granada, for example, are helping build the Short-Baseline Near Detector, which is part of Fermilab’s Short-Baseline Neutrino Program and relies on the liquid-argon detector technology that is also used by DUNE.

    FNAL Short-Baseline Near Detector under construction

    FNAL NOvA Near Detector

    Neutrinos have three “flavors,” or types: muon, electron and tau. The SBN program will measure how the neutrinos’ flavors change as they go through the three detectors, which allows scientists to look for the existence of a fourth type of neutrino. The Short-Baseline Near Detector, the SBN detector closest to the neutrino production point, will record over a million neutrino interactions per year, providing scientists with an enormous treasure trove of data for analysis.

    Spain is contributing to the simulation-side of the photon detection system of SBND. The two institutions hope to expand their involvement when the SBN program begins taking data.

    “We are open to help with what they need,” said Gil-Botella. “Physics is the goal, and I think the world has a lot to gain.”

    3
    CIEMAT collaborators mount the fibers of one of the ProtoDUNE light calibration system. Photo: Enrique Calvo, CIEMAT

    Spain also collaborates with Fermilab on theoretical physics. Scientists are searching for ways experiments like DUNE can go beyond the Standard Model of physics, the framework that describes nature’s fundamental forces and particles at the subatomic scale. Scientists from the Institute of Corpuscular Physics (IFIC) in Valencia, the Autonomous University of Madrid (UAM) and the Institute for High Energy Physics in Barcelona are frequent collaborators with Fermilab’s theoretical physics group. Fermilab has also established formal staff and student exchange programs with both IFIC and UAM and plans to establish a similar program with the University of Barcelona. Every year the Fermilab Theoretical Physics Department receives many students, postdocs and scientists from Spanish institutions who come to Fermilab for several weeks to perform their theoretical work and interact with Fermilab theorists and experimentalists.

    “If you are a neutrino physicist, Fermilab is the laboratory of reference,” said Michel Sorel, a scientist at IFIC who has been involved in Fermilab neutrino experiments for the past 20 years. “It is the neutrino capital of the world.”

    Groups at the Institute of Space Sciences, the Institute for High Energy Physics and CIEMAT also participate in the Fermilab-hosted Dark Energy Survey, which completed its sixth and final year of data-taking earlier this year.

    This survey is an international collaboration that mapped a 5,000-square-degree area of the sky, recording information from 300 million galaxies.

    The contributions go both ways. For example, Fermilab scientists are working on NEXT, the Neutrino Experiment with a Xenon TPC. Located at the Canfranc Underground Laboratory in Spain, NEXT seeks to determine whether or not neutrinos are their own antiparticles.

    “We are collaborating on a day-to-day basis,” Sorel said. “It is a very close relationship both for the institutions and the individual scientists.”

    For more information on Spain’s contributions at Fermilab, look at some of the previously published articles.

    See the full article here .


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    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 8:38 am on June 12, 2019 Permalink | Reply
    Tags: , , , HEP, , , , ,   

    From Science Magazine: “Exotic particles called pentaquarks may be less weird than previously thought” 

    AAAS
    From Science Magazine

    Jun. 5, 2019
    Adrian Cho

    1
    The Large Hadron Collider beauty experiment has discovered three new pentaquarks. Peter Ginter/CERN

    Four years ago, when experimenters spotted pentaquarks—exotic, short-lived particles made of five quarks—some physicists thought they had glimpsed the strong nuclear force, which binds the atomic nucleus, engaging in a bizarre new trick. New observations have now expanded the zoo of pentaquarks, but suggest a tamer explanation for their structure. The findings, from the Large Hadron Collider beauty experiment (LHCb), a particle detector fed by the LHC at CERN, the European particle physics laboratory near Geneva, Switzerland, suggest pentaquarks are not bags of five quarks binding in a new way, but are more like conventional atomic nuclei.

    “I’m really excited that the new data send such a clear message,” says Tomasz Skwarnicki, an LHCb physicist at Syracuse University in New York who led the study. But, he notes, “It may not be the message some people had hoped for.”

    Pentaquarks are heavier cousins of protons and neutrons, which are also made of quarks. In ordinary matter, quarks come in two types, up and down. Atom smashers can blast four heavier types of quarks into brief existence: charm, strange, top, and bottom. Quarks cling to one another through the strong force so mightily they cannot be isolated. Instead, they are almost always found in groups of three in particles known as baryons—including the proton and neutron—or in pairs called mesons, which consist of a quark and an antimatter quark.

    But for decades, some theorists have hypothesized the existence of larger bundles of quarks. In recent years, experimenters have found evidence for four-quark particles, or tetraquarks. Then, in 2015, LHCb reported signs of two pentaquarks.

    Some theorists argue that the new particles are bags of four and five quarks, bound together through the exchange of quantum particles called gluons, adding a new wrinkle to the often intractable theory of the strong force. Others argue they’re more like an atomic nucleus. In this “molecular” picture a pentaquark is a three-quark baryon stuck to a two-quark meson the same way that protons and neutrons bind in a nucleus—by exchanging short-lived pi mesons.

    LHCb’s new pentaquarks, reported today in Physical Review Letters (PRL), bolster the molecular picture. In 2015, LHCb researchers reported a pentaquark with a mass of 4450 megaelectron volts (MeV), 4.74 times the mass of the proton. With nine times more data, they now find in that mass range two nearly overlapping but separate pentaquarks with masses of 4440 MeV and 4457 MeV. They also find a lighter pentaquark at 4312 MeV. Each contains the same set of quarks: charm, anticharm, two ups, and a down. (Previous hints of a pentaquark at 4380 MeV have faded.)

    3
    Pentaquark depiction

    5
    New Large Hadron Collider data reveal that exotic quark quintets, discovered in 2016, are composites of quark-antiquark mesons and three-quark baryons.

    The lightest pentaquark has a mass just below the sum of a particular baryon and meson that together contain the correct quark ingredients. The heavier pentaquarks have masses just below the sum of the same baryon and a related meson with extra internal energy. That suggests each pentaquark is just a baryon bound to a meson, with a tiny bit of mass taken up in binding energy. “This is a no-brainer explanation,” says Marek Karliner, a theorist at Tel Aviv University in Israel.

    The molecular picture also helps explain why the pentaquarks, although fleeting, appear to be more stable than expected, Karliner says. That’s because packaging the charm quark in the baryon and anticharm quark in the meson separates them, keeping them from annihilating each other.

    Other theorists rushed to a similar conclusion when LHCb researchers discussed their results at a conference in La Thuile, Italy, in March. For example, within a day, Li-Sheng Geng, a theorist at Beihang University in Beijing, and colleagues posted a paper, in press at PRL, that uses the molecular picture to predict the existence of four more pentaquarks that should be within LHCb’s reach.

    But the bag-of-quarks picture is not dead. Pentaquarks should occasionally form when protons are bombarded with gamma ray photons, as physicists at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, are trying to do. But they have yet to spot any pentaquarks. That undermines the molecular picture because it predicts higher rates for such photoproduction than the bag-of-quarks model does, says Ahmed Ali, a theorist at DESY, the German accelerator laboratory in Hamburg. “They are already almost excluding the molecular interpretation,” he says. Others say it’s too early to draw such conclusions.

    The structure of pentaquarks isn’t necessarily an either/or proposition, notes Feng-Kun Guo, a theorist at the Chinese Academy of Sciences in Beijing. Quantum mechanics allows a tiny object to be both a particle and a wave, or to be in two places at once. Similarly, a pentaquark could have both structures simultaneously. “It’s just a question of which one is dominant,” Guo says.

    Regardless of the binding mechanism, the new pentaquarks are exciting because they suggest the existence of a whole new family of such particles, Karliner says. “It’s like a whole new periodic table.”

    See the full article here .


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  • richardmitnick 12:38 pm on May 25, 2019 Permalink | Reply
    Tags: "CMS hunts for dark photons coming from the Higgs boson", , , , HEP, One idea is that dark matter comprises dark particles that interact with each other through a mediator particle called the dark photon, , ,   

    From CERN CMS: “CMS hunts for dark photons coming from the Higgs boson” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN CMS

    24 May, 2019
    Ana Lopes

    1
    A proton–proton collision event featuring a muon–antimuon pair (red), a photon (green), and large missing transverse momentum. (Image: CERN)

    They know it’s there but they don’t know what it’s made of. That pretty much sums up scientists’ knowledge of dark matter.

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

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

    Coma cluster via NASA/ESA Hubble

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


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


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

    This knowledge comes from observations of the universe, which indicate that the invisible form matter is about five to six times more abundant than visible matter.

    One idea is that dark matter comprises dark particles that interact with each other through a mediator particle called the dark photon, named in analogy with the ordinary photon that acts as a mediator between electrically charged particles. A dark photon would also interact weakly with the known particles described by the Standard Model of particle physics, including the Higgs boson.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    At the Large Hadron Collider Physics (LHCP) conference, happening this week in Puebla, Mexico, the CMS collaboration reported the results of its latest search for dark photons.

    The collaboration used a large proton–proton collision dataset, collected during the Large Hadron Collider’s second run, to search for instances in which the Higgs boson might transform, or “decay”, into a photon and a massless dark photon. They focused on cases in which the boson is produced together with a Z boson that itself decays into electrons or their heavier cousins known as muons.

    Such instances are expected to be extremely rare, and finding them requires deducing the presence of the potential dark photon, which particle detectors won’t see. For this, researchers add up the momenta of the detected particles in the transverse direction – that is, at right angles to the colliding beams of protons – and identify any missing momentum needed to reach a total value of zero. Such missing transverse momentum indicates an undetected particle.

    But there’s another step to distinguish between a possible dark photon and known particles. This entails estimating the mass of the particle that decays into the detected photon and the undetected particle. If the missing transverse momentum is carried by a dark photon produced in the decay of the Higgs boson, that mass should correspond to the Higgs-boson mass.

    The CMS collaboration followed this approach but found no signal of dark photons. However, the collaboration placed upper bounds on the likelihood that a signal would have been seen.

    Another null result? Yes, but results such as these and the ATLAS results on supersymmetry also presented this week in Puebla, while not finding new particles or ruling out their existence, are much needed to guide future work, both experimental and theoretical.

    For more details about this result, see the CMS website.

    See the full article here.


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

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

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    20th May 2019
    ATLAS Collaboration

    The Standard Model is a remarkably successful but incomplete theory.

    Standard Model of Particle Physics

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

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

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

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

    Search for the “stau”

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

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

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

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

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

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

    Compressed search

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

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

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

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

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

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

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

    See the full article for further reseach materials.

    See the full article here .


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