290 research outputs found

    Graph Neural Network Flavour Tagging and Boosted Higgs Measurements at the LHC

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    This thesis presents investigations into the challenges of, and potential improvements to, b-jet identification (b-tagging) at the ATLAS experiment at the Large Hadron Collider (LHC). The presence of b-jets is a key signature of many interesting physics processes such as the production of Higgs bosons, which preferentially decay to a pair of b-quarks. In this thesis, a particular focus is placed on the high transverse momentum regime, which is a critical region in which to study the Higgs boson and the wider Standard Model, but also a region within which b-tagging becomes increasingly difficult. As b-tagging relies on the accurate reconstruction of charged particle trajectories (tracks), the tracking performance is investigated and potential improvements are assessed. Track reconstruction becomes increasingly difficult at high transverse momentum due to the in- creased multiplicity and collimation of tracks, and also due to the presence of displaced tracks from the decay of a long-flying b-hadron. The investigations reveal that the quality selections applied during track reconstruction are suboptimal for b-hadron decay tracks inside high transverse momentum b-jets, motivating future studies into the optimisation of these selections. Two novel approaches are developed to improve b-tagging performance. Firstly, an algorithm which is able to classify the origin of tracks is used to select a more optimal set of tracks for input to the b-tagging algorithms. Secondly, a graph neural network (GNN) jet flavour tagging algorithm has been developed. This algorithm directly accepts jets and tracks as inputs, making a break from previous algorithms which relied on the outputs of intermediate taggers. The model is trained to simultaneously predict the jet flavour, track origins, and the spatial track-pair compatibility, and demonstrates marked improvements in b-tagging performance both at low and high transverse momenta. The closely related task of c-jet identification also benefits from this approach. Analysis of high transverse momentum H → bb decays, where the Higgs boson is produced in association with a vector boson, was performed using 139 fb−1 of 13 TeV proton-proton collision data from Run 2 of the LHC. This analysis provided first measurements of the V H, H → bb process in two high transverse momentum regions, and is described with a particular focus on the background modelling studies performed by the author

    Search for lepton-favour violation in high-mass dilepton final states using 139 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector

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    A search is performed for a heavy particle decaying into different-flavour, dilepton final states, using 139 fb−1 of proton-proton collision data at √s = 13 TeV collected in 2015–2018 by the ATLAS detector at the Large Hadron Collider. Final states with electrons, muons and hadronically decaying tau leptons are considered (eμ, eτ or μτ). No significant excess over the Standard Model predictions is observed. Upper limits on the production cross-section are set as a function of the mass of a Z′ boson, a supersymmetric τ-sneutrino, and a quantum black-hole. The observed 95% CL lower mass limits obtained on a typical benchmark model Z′ boson are 5.0 TeV (eμ), 4.0 TeV (eτ), and 3.9 TeV (μτ), respectively

    Search for quantum black hole production in lepton+jet final states using proton-proton collisions at √s = 13 TeV with the ATLAS detector

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    A search for quantum black holes in electron+jet and muon+jet invariant mass spectra is performed with 140 fb−1 of data collected by the ATLAS detector in proton-proton collisions at √s = 13 TeV at the Large Hadron Collider. The observed invariant mass spectrum of lepton+jet pairs is consistent with Standard Model expectations. Upper limits are set at 95% confidence level on the production cross-sections times branching fractions for quantum black holes decaying into a lepton and a quark in a search region with invariant mass above 2.0 TeV. The resulting quantum black hole lower mass threshold limit is 9.2 TeV in the Arkani-Hamed-Dimopoulos-Dvali model, and 6.8 TeV in the Randall-Sundrum model

    Physics at ATLAS from top to bottom: jet substructure in boosted tt¯ events at 13 TeV and development of pixel detector modules for the inner tracker upgrade

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    Operating at the energy frontier, between 2015 and 2018, CERN’s Large Hadron Collider (LHC) collided protons at an unprecedented centre-of-mass energy of √s = 13 TeV, allowing for the production of massive particles in great abundance. Over this period, the ATLAS Experiment collected 139 fb−1 of data suitable for physics analysis; the largest pp dataset to date. Using this dataset, this thesis presents a measurement of the internal structure of jets arising from the production of the most massive of the Standard Model particles, the top quark, in t ¯t pairs, in the case that these jets are produced with high momentum. These results are presented as differential cross-section measurements, unfolded to remove detector effects, and compared to state-of-the-art Monte Carlo simulations. It is found that some observables are modelled poorly by current predictions, and that these substructure observables are sensitive to the choice of parton shower modelling and modelling of the final state radiation. Also presented are testbeam studies which investigate the viability of various silicon pixel sensor designs to be used in the upcoming upgrade of the ATLAS tracking system. Here, it is found that all proposed designs meet the required specification for device efficiency after irradiation and are therefore suitable for use in the High-Luminosity LHC

    Measurement of the associated production of a top quark pair and a Higgs boson (t¯tH) with boosted topologies

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    This thesis presents three studies focusing on boosted topologies that utilise machine learning techniques for boosted H → b¯b reconstruction using the ATLAS detector. The measurement of the t¯tH cross-section is a direct way of accessing the Higgs top Yukawa coupling (yt). Firstly, an all-hadronic feasibility study is shown, aimed at assessing boosted topologies in the all-hadronic t¯tH decay channel. It was found to have low statistical significance, with considerable efforts and data driven techniques required to reduce the QCD-multijet background. Secondly, the boosted contribution to the recent t¯tH, H → b¯b measurement using the full Run-2 ATLAS data set, 139f b−1 at √s = 13 TeV, is analysed. There is a considerable contribution from the boosted region to this result, particularly to the differential cross-section measurement of the Simplified Template Cross-Section (STXS) bins [300, 450) and [450, ∞) GeV. The result of the inclusive profile-likelihood fit is μ = 0.35+0.36−0.34, which corresponds to σ = 1.0(2.7) observed(expected) significance compared to the background-only hypothesis. Thirdly work on retraining the boosted H → b¯b reconstruction deep neural network (DNN) is shown for the Run-2 Legacy re-analysis. The bespoke DNN trained for the analysis showed some improvements over the previous round due to the updated analysis algorithms. It also outperformed the general purpose H → b¯b Xbb tagger. The final motivation for use of the bespoke DNN is that it allows the choice of boosted jet collection (RC-jets vs LR-jets). RC-jets re cluster “small” (∆R = 0.4) jets with ∆R = 1.0 while LR-jets directly cluster the calorimeter clusters with ∆R = 1.0, both using the anti-kt algorithm. The RC-jets jets are found to be advantageous. This is due to the ease of propagating systematics for combining with resolved regions and the good modelling observed using samples made with the Atlfast-2 detector simulation

    Search for heavy resonances and quantum black holes in eμ, eτ, and μτ final states in proton-proton collisions at √s = 13 TeV

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    A search is reported for heavy resonances and quantum black holes decaying into eμ, eτ, and μτ final states in proton-proton collision data recorded by the CMS experiment at the CERN LHC during 2016–2018 at √s = 13TeV, corresponding to an integrated luminosity of 138 fb−1. The eμ, eτ, and μτ invariant mass spectra are reconstructed, and no evidence is found for physics beyond the standard model. Upper limits are set at 95% confidence level on the product of the cross section and branching fraction for lepton flavor violating signals. Three benchmark signals are studied: resonant τ sneutrino production in R parity violating supersymmetric models, heavy Z\u27 gauge bosons with lepton flavor violating decays, and nonresonant quantum black hole production in models with extra spatial dimensions. Resonant τ sneutrinos are excluded for masses up to 4.2TeV in the eμ channel, 3.7TeV in the eτ channel, and 3.6TeV in the μτ channel. A Z\u27 boson with lepton flavor violating couplings is excluded up to a mass of 5.0TeV in the eμ channel, up to 4.3Te V in the eτ channel, and up to 4.1TeV in the μτ channel. Quantum black holes in the benchmark model are excluded up to the threshold mass of 5.6TeV in the eμ channel, 5.2Te V in the eτ channel, and 5.0TeV in the μτ channel. In addition, model-independent limits are extracted to allow comparisons with other models for the same final states and similar event selection requirements. The results of these searches provide the most stringent limits available from collider experiments for heavy particles that undergo lepton flavor violating decays

    Constraining the top quark Yukawa coupling in in single-top Higgs production

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    The work presented in this thesis consists of the first search for the associated production of a top quark and a Higgs boson (tH) with the ATLAS detector at the LHC. It also establishes an important contribution to the fast simulation of the ATLAS detector. A novel analysis of the tH process has been developed using the di-photon (H → γγ) decay channel. The analysis is designed to simultaneously optimise sensitivity to the Standard Model (SM) tH production, and to the scenario in which the top quark Yukawa coupling to the Higgs boson has a negative sign. The ratio of the measured and predicted cross-sections is reported as: σtH/σSM tH = 2.6 +4.2 −3.2 = 2.6 +3.7 −2.9 (stat.) +1.6 −1.1 (syst.). (1) An upper limit on the tH cross-section is set at 10 times the SM expectation, and the negative values of the top quark Yukawa coupling are excluded at 2.2σ. At the time of submission, these results apply the most stringent constraints on the tH cross-section and the top quark Yukawa coupling. The muon punch-through component of the ATLAS fast simulation tool, AtlFastIII (AF3), has been developed. AF3 improves the speed of the ATLAS detector simulation by a factor of 5, facilitating the production of large sets of simulated data that aid in future ATLAS physics results. Muon punch-through occurs when particles created in electromagnetic showers in the ATLAS calorimeters are not fully contained. This can result in signatures that may be mis-reconstructed as muons. Accurate modelling of this effect is essential for understanding muon fakes and for hadronic jet calibration
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