Colin Gay

This dissertation presents a search for long-lived heavy neutral leptons (HNLs) in proton-proton collisions at the Large Hadron Collider (LHC). The Standard Model (SM) of particle physics is an extremely successful theory and many of its major predictions have been precisely confirmed. However, the existence of neutrinos, with small nonzero masses, suggests that the SM is incomplete. Introducing HNLs into the SM is a natural way to generate the light neutrino masses through a seesaw mechanism. Theories that postulate the existence of such particles can also explain the asymmetry between matter and anti-matter in our universe and models with at least three HNLs provide a dark matter candidate. This experimental search uses ATLAS data collected between 2015 and 2018 at a centre-of-mass energy of 13 TeV. A non-standard technique is used to search for a displaced vertex from particle trajectories produced in the HNL decay to leptons. The dominant background from uncorrelated leptons crossing in the ATLAS detector is estimated using an object shuffling method. The reconstructed HNL mass is used to discriminate between signal and background. No excess of events is observed and constraints on the strength of the interactions between HNLs and neutrinos are imposed in various scenarios.This dissertation also presents new methods to study the readout system and performance of a silicon strip tracking detector. The LHC is currently undergoing upgrades that will enable it to produce more than ten times the data that has already been collected. To meet the requirements of this challenging new environment, an all-silicon particle tracking system will be installed in ATLAS.

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Many theories beyond the Standard Model suggest new particles at the TeV energy scale, that could be produced in proton-proton collisions delivered by the CERN Large Hadron Collider and collected by the ATLAS experiment. Since 2016, the accelerator has surpassed its expected value of luminosity by up to a factor of two. The data acquisition system of the ATLAS Transition Radiation Tracker had to be upgraded to meet the demand from the challenging beam conditions and the higher trigger rate in Run II beyond the original design of the tracking detector. The improvements developed in the hardware and firmware of the DAQ system are documented in this dissertation, with a study to evaluate the performance of the system. This dissertation also presents two searches for new massive bosons at the TeV scale. The first analysis searches for heavy resonances decaying into a hadronic Z/W/Higgs boson and a photon. The boosted Z/W/Higgs boson is identified using large-radius jet mass and substructure informations. The analysis is based on 36 inverse femtobarns of √s = 13 TeV proton–proton collision data, collected with the ATLAS detector in Run II of the Large Hadron Collider. No significant deviations from the Standard Model prediction is observed. Upper limits are set on the signal cross section multiplied by the branching fraction of resonance for the three different diboson final states at 95% confidence level, excluding those productions at 10–0.2 femtobarns in the resonance mass range of 1–6.8 TeV. The second analysis searches for scalar leptoquark pair productions, where each leptoquark decays into a top quark and an electron or a muon. The search sensitivity is optimized for high leptoquark masses, at which the hadronic decay products of each top quark are contained within a large-radius jet. The analysis exploits the full Run II dataset that corresponds to an integrated luminosity of 139 inverse femtobarns. No significant excess of events is found. Lower mass limits on leptoquarks decaying into electron–top-quark or muon–top-quark pair are set to 1.48 TeV and 1.47 TeV at 95% confidence level.

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The Standard Model (SM) of particle physics has provided a remarkable description of experimental results across a wide energy range, but nevertheless is regarded as a low energy effective theory. Supersymmetry (SUSY) is an extension of the Poincare symmetry group that predicts partner particles for each of the SM particles. This dissertation focuses on searches for SUSY particles in proton-proton collisions delivered by the Large Hadron Collider and collected by the ATLAS detector.The lightest chargino (C1) and next-to-lightest neutralino (N2) are searched for in proton collisions using 20.3 inverse femtobarns at √s = 8 TeV. Building on the discovery of the 125 GeV Higgs boson in 2012, the targeted decay modes of the chargino and neutralino are N2→hN1 and C1→WN1. A final state of one-lepton, two b-jets from the Higgs boson decay, and missing transverse momentum is explored. No excesses above the SM backgrounds are observed, and the results are statistically combined with analyses targeting other Higgs decay channels. In addition, the direct pair production of squarks or gluinos is searched for using 36.5 inverse femtobarns of data collected at √s = 13 TeV. A final state of one-lepton, 2-6 jets, and missing transverse momentum is explored. Four signal regions are devised to target the wide range of kinematics expected from the decay of the squarks or gluinos. The data is found to be consistent with SM only expectations. The results from both searches are interpreted in a combination of simplified and phenomenological models.The first layer of the Muon Spectrometer end-cap will be replaced with the New Small Wheels (NSW) in 2019 to improve triggering and tracking capabilities in the forward regions of the ATLAS detector. Small-strip Thin Gap Chambers (sTGC) are one of the two detector technologies that will be used in the NSW, and are studied under prolonged radiation in preparation for their installation. A prototype sTGC detector is irradiated with a Strontium-90 source of intensity 10mCi. No degradation of the signal characteristics is observed up to 11.8 C/cm of accumulated charge per cm of wire.

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This work presents the results of two analyses searching for massive resonances decaying to top-antitop quark pairs in sqrt(s) = 7 TeV proton-proton collisions measured with the ATLAS detector at the CERN Large Hadron Collider. The peculiar position of the top-quark within the Standard Model, as the most massive particle, motivates many theories predicting the existence of new massive particles that couple strongly to top-quarks. These particles may manifest themselves at the LHC as new massive resonances decaying to top-antitop quark pairs. An analysis conducted on the first 2.04/fb^-¹ of data collected in 2011 searched for resonances in the subset of top-pair events where the W-bosons from each top decay decayed leptonically to a final state electron or muon. A second analysis conducted on the full 4.7/fb^-¹ of 2011 data searched for resonances in the subset of top-quark pair events where W-bosons from each top decay decayed hadronically to a diquark pair. The analysis focused on the case were each top-quark is sufficiently boosted such that the resultant fully hadronic system is highly collimated, and reconstructable as a single hadronic jet with large angular separation. In each analysis, the leptophobic Z' from topcolor assisted technicolor models, and the KK-gluon from Randall-Sundrum warped extra-dimension models were chosen as benchmarks to test for the presence of narrow and wide resonances respectively. No significant deviation from the Standard Model was observed in either analysis. The results were used to set 95% C.L. upper limits on the cross-section times branching ratio to top-quark pairs as a function of resonance mass for each benchmark model. The existence of a Randall-Sundrum KK-gluon was excluded at the 95% C.L. over the mass range 500 to 1620 GeV.

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This dissertation presents a search for direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum in 20.7 inverse femtobarns of 8 TeV (center of mass) proton-proton collisions delivered by the Large Hadron Collider and collected with the ATLAS detector. The Standard Model augmented with the minimal formulation of universal extra dimensions (MUED) is phenomenologically similar to supersymmetry. This search is also sensitive to MUED cascades characterized by the presence of n=1 Kaluza-Klein excitations of Standard Model W and Z bosons. No significant excess over Standard Model predictions is observed. The results are interpreted in the context of simplified supersymmetric models and MUED. New limits are set at the 95% confidence level on the parameter spaces of these models.

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This work presents the result of a search for a new long-lived neutral particle decaying into a muon and charged hadrons in proton-proton collisions at a centre-of-mass energy of 7 TeV with a total integrated luminosity of 4.4 inverse femtobarn, using the ATLAS detector located at the Large Hadron Collider (LHC). Many extensions to the current Standard Model of particle physics predict the existence of such new particles, including the neutralino in an R-parity violating supersymmetry scenario. In this search, a set of selection criteria has been established to be sensitive to this kind of signal, in addition to evaluating the background in a data-driven manner. No excess of events above the expected background is observed with the collected data and limits are set on the squark pair production cross section, multiplied by the branching ratio for a squark to decay, via a long-lived neutralino, to a muon and charged hadrons, as a function of the neutralino lifetime.

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The question of physics beyond the Standard Model remains as crucial as it was before the discovery of a Higgs boson at the Large Hadron Collider, as the theoretical and experimental shortcomings of the Standard Model remain unresolved. Indeed, theoretical problems such as the hierarchy of energy scales, the Higgs mass fine-tuning and the large number of postulated parameters need to be addressed, while the experimental observations of dark matter, dark energy and neutrino masses are not explained by the Standard Model. Many hypotheses addressing these issues predict the existence of new neutral high-mass resonances decaying into muon pairs.This dissertation documents a search for this process using 25.5 inverse femtobarns of proton-proton collision data collected by the ATLAS experiment in Run‑I of the Large Hadron Collider. After evaluating the performance of the detector for reconstructing muons at very high momentum, the event yields observed as a function of the invariant mass of muon pairs are compared with expected values from Standard Model processes.The observed yields are found to be in good agreement with Standard Model predictions, and no significant excess of events is found. New gauge bosons with couplings to fermions equal to these of the Standard Model Z boson and with masses lower than 2.53 TeV are therefore excluded at 95% confidence level. A statistical combination with the results of the search for the same particle decaying into electron pairs yields a lower mass limit of 2.90 TeV at 95% confidence level. Limits are also placed in the context of two classes of models inspired by Grand Unification Theories: gauge theories with the E₆ symmetry group, as well as Minimal Z' Models.

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The Standard Model of particle physics has proven itself as a wildly successful description of the microscopic world. Yet, despite all its accomplishments, deep mysteries remain for which the Standard Model offers no explanation. This thesis is dedicated to the phenomenological study, as well as the creation of, a number of well-motivated extensions to the Standard Model, each originally designed to explain a known flaw. The Randall-Sundrum model employs an extra dimension to exponentially suppress all mass scales in the theory, generating the electroweak-Planck hierarchy with no recourse to fine-tuning of parameters. In this way, it can lead to unwanted enhancements of higher dimensional operators inducing strongly constrained processes such as proton decay. We investigate the effects of the Randall-Sundrum model on a similar process, neutron-antineutron oscillation, and find that, in contrast to the example of proton decay, it is naturally suppressed below experimental limits by virtue of the flavor structure of the operators inducing it.The DZero collaboration has reported strong evidence of CP violation beyond the Standard Model in measurements of the like-sign dimuon asymmetry at the Tevatron. The nature of the signal suggests the presence of new physics in the form of new bosonic particles with weak scale masses and new sources of CP violation in the mixing of neutral B mesons. We note that this is exactly what is needed for viable electroweak baryogenesis in extensions of the Standard Model. We explore the potential for simultaneously explaining the dimuon asymmetry and the baryon asymmetry of the Universe using a Two-Higgs-Doublet-Model with an unusual flavor structure.Both the CDF and DZero collaborations measure a strong preference for top quarks, produced in pairs at the Tevatron, to propagate in the direction of the initial proton beam, in direct conflict with Standard Model predictions. A class of new physics models with novel flavor interactions has been proposed to explain this behaviour through a Rutherford enhancement in the forward direction. We show that such interactions can also simultaneously explain a long-standing tension between different measurements of the CKM matrix element Vub through the generation of large loop-induced right-handed charge currents.

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A generic search for new detector-stable high-mass particles at the LHC using the ATLAS detector is conducted, by measuring the velocity of candidate tracks in ATLAS’ Inner Detector. Four novel new velocity estimation techniques are developed, the best of which achieve velocity resolutions of better than 8% in simulation. 2.8 fb⁻¹ of 2011 ATLAS data is analyzed via these methods, yielding no significant signal for new stable massive particles at the sub-TeV scale, and an exclusion limit at the 95% confidence level for the existence of generic stable massive gluinos below a mass of 665 GeV.

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