The ATLAS Experiment at the CERN Large Hadron Collider is a particle physics experiment to study fundamental particles and their interactions at very high energies. Supersymmetry is a theory of new physics beyond the Standard Model of particle physics. A search for directly produced pairs of the supersymmetric partner of the top quark was performed using 20 fb^-1 of proton--proton collision data at a centre of mass energy of 8 TeV taken in 2012. The search targeted a model where the supersymmetric partner of the top quark (``scalar top'') decays via the supersymmetric partner of the tau lepton (``scalar tau'') into the supersymmetric partner of the graviton (``gravitino''). Scalar top candidates were searched for in pp collision events with either two hadronically decaying taus, two light leptons (electrons or muons), or one hadronically decaying tau and one light lepton. The numbers of events passing the analysis selection criteria agree with the Standard Model expectations. Exclusion limits at the 95% confidence level were set as a function of the scalar top and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV limit set by the LEP experiments to a few GeV below the scalar top mass, lower limits between 490 GeV and 640 GeV were placed on the scalar top mass within the model considered. / Graduate

The first measurement of the spin correlation strength in pairs of top quarks at √s=8 TeV is presented, using data collected using the ATLAS detector, corresponding to an integrated luminosity of 20.3 fb⁻¹. Dileptonically decaying tt^- events are selected with two high-pT leptons and at least two jets, one of which is required to be b-tagged. The azimuthal angle between the two charged leptons, Δφ, is used to measure the degree of spin correlation of top quark pairs. The extracted value of fSM, which is a measure of the spin-correlation strength, is fSM=1.20 ± 0.14. This is the most precise spin-correlation measurement to date. Using the Δφ distribution, a search for Supersymmetric top squarks is performed, with the assumption that the top squarks decay to a top quark and a neutralino. No excess of events is observed and top squarks with masses between the top quark mass and 191 GeV are excluded at 95% confidence level, extending previous limits.

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In its first three years of operation, the Large Hadron Collider at CERN has already proved its worth as a discovery machine and reinvigorated the prospects for discovering new dynamics beyond the Standard Model. This thesis presents a range of experimental methods devised for discovery of supersymmetry, one of the most important extensions of the Standard Model being tested at the LHC. The first chapter discusses the concepts behind a set of transverse mass-bound variables, and explores connections between these variables and others in the literature. Not only are these variables important tools for mass measurement, should sparticles be seen at the LHC, they are also critical components of experimental searches. Accordingly, their performance has been compared in both mass measurement and search contexts, and the results are reported here. This is followed by the details of a search for hadronically-decaying squarks and gluinos, carried out on 4.7 fb−1 of data collected at 7 TeV in 2011 by the ATLAS detector. The search strategy is described, focusing on the implementation of a background estimation method using photon events to determine the background contamination due to invisible Z boson decays. The results of the search are then presented. Stringent limits are set on the production of squarks and gluinos in a variety of model scenarios.

This thesis presents measurements of the the spin correlation strength in top anti-top quark pair production at the LHC using the ATLAS detector. The data used corresponds to 4.6 fb−1 of integrated luminosity taken during 2011 at the LHC at a center of mass energy of 7 TeV. The spin correlation is studied utilising different observables with different sensitivities to the production mechanism, in particular to gluon-gluon fusion in the like or unlike helicity state, quark anti- quark annihilation in the unlike helicity state, or a combination of the three. In addition cuts are made on the invariant mass of the ttbar system to enhance or suppress contributions from different initial state production mechanisms. The analysis presented is a precision test of both ttbar production and decay in the SM. These measurements are compared to the most current theoretical predictions. No deviation from the SM expectation was observed. In a subset of the data, corresponding to an integrated luminosity of 2.1 fb−1, the hypothesis of zero spin correlation is excluded at 5.1 standard deviations.

A search for heavy exotic diboson resonances decaying to llqq final states is presented using pp collision data collected with the ATLAS detector at the Large Hadron Collider. The analysis uses a data sample corresponding to an integrated luminosity of 20.3 fb^-1 at sqrt(s) = 8 TeV collected between April and December 2012. No significant excess of data events over the predicted Standard Model background is observed and 95% confidence level upper limits are set on the product of the production cross-section and the branching ratio for spin-2 Kaluza-Klein gravitons predicted by the bulk Randall-Sundrum model and for Extended Gauge Model W’ bosons. These results are subsequently combined with limits obtained from searches using the lvll, lvqq, and qqqq final states, and new mass limits are set on both signal models.

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Data collected at the LHC are analyzed by the ATLAS collaboration for evidence of dark matter. In this thesis, a fast simulation of the ATLAS detector response using the Delphes software is assessed for dark matter models with a leptonically decaying Z boson and a pair of dark matter particles in the final state. Limits for the Two Higgs Doublet plus pseudoscalar dark matter model are obtained using simplified systematics, and found to be nearly indistinguishable to limits obtained using the more complex standard ATLAS analysis. / Graduate

<p>The high luminosity phase of the LHC, poised to start taking data in 2027, aims to increase the instantaneous luminosity of the machine to 7.5 x 10³⁴ cm^-2 s^-1. This will make it possible for experiments at CERN to make higher precision measurements on known physics phenomenon as well as to search for “new physics”. However, this motivates the need for hardware upgrades at the various experiments in order to ensure compatibility with the HL-LHC. This thesis describes some of the efforts to upgrade the inner-most layers of the Compact Muon Solenoid, namely the CMS silicon pixel tracking detector. </p> <p>Silicon sensors used to track particles are installed in the detector as part of a pixel sensor module. Modules consist of a silicon sensor-readout chip assembly that is wire-bonded to an HDI, or High Density Interconnects to provide power and signals. </p> <p>As part of the upgrade, 2,541 modules need to be assembled delicately and identically with alignment error margins as low as 10 microns. Assembly will be across three production sites in clean rooms to avoid dust and humidity contamination.</p> <p>In addition, the modules need to survive high magnetic fields and extended close-range radiation as part of the HL-LHC.</p> <p>In line with this effort, new materials and assembly procedures able to sustain such damage are investigated. Techniques to assemble modules are explored, specifically precision placing of parts with a robotic gantry and techniques to protect wirebonds. This is followed by a discussion of the accuracy and repeatability.</p>

The Gas Electron Multiplier (GEM) upgrade project aims at improving the performance of the muon spectrometer of the Compact Muon Solenoid (CMS) experiment which will suffer from the increase in luminosity of the Large Hadron Collider (LHC). After a long technical stop in 2019-2020, the LHC will restart and run at a luminosity of 2 10^34 cm^-2 s^-1, twice its nominal value. This will in turn increase the rate of particles to which detectors in CMS will be exposed and affect their performance. The muon spectrometer in particular will suffer from a degraded detection efficiency due to the lack of redundancy in its most forward region. To solve this issue, the GEM collaboration proposes to instrument the first muon station with Triple-GEM detectors, a technology which has proven to be resistant to high fluxes of particles. Within the GEM collaboration, the Data Acquisition (DAQ) subgroup is in charge of the development of the electronics and software of the DAQ system of the detectors.This thesis presents the work of the author as lead developer of the firmware for the front-end and back-end electronics. These developments have been performed from the ground up and designed to transfer data from the analog front-end to the off-detector electronics while offering extensive control and monitoring capabilities. The developed DAQ chain has been tested extensively during two test beam campaigns which provided results on both the stability of the system and the efficiency of the detectors. The results of these campaigns are described in the present thesis. Further characterization of the electronics is also described along with the procedures and tools built to qualify the components for their installation in CMS. Finally, the results of the irradiation tests performed on the on-detector electronics are presented. These allowed us to understand the effects of radiation on the electronic boards used for the GEM upgrade project and the impact that it has on the design of the firmware for CMS.Additionally, the work of the author on a new architecture for DAQ systems is described. The latter uses modified network topologies and novel web technologies to increase the available bandwidth on the network and yield an event-driven system. / Cette thèse de doctorat s'inscrit dans le projet de mise à niveau du spectromètre à muons du Compact Muon Solénoid (CMS) auprès du Large Hadron Collider (LHC). Après un arrêt pour maintenance prévu en 2019-2020, le LHC reprendra son programme de recherche à une luminosité de 2 $ imes$ 10^34 cm^-2 s^-1, soit deux fois sa valeur nominale. Ceci aura pour conséquence d'accroître le taux de particules auquel seront soumis les détecteurs de CMS et d'entraver l'efficacité de détection de ces derniers. Le spectromètre à muons de CMS sera tout particulièrement touché à cause du manque de redondance dans sa partie avant. Afin de palier à ce problème, il est proposé d'installer des détecteurs Gas Electron Multipliers (GEM) dans la première station à muons. La technologie GEM répond aux besoins de CMS en offrant une excellente efficacité de détection à de hauts flux de particules. Au sein de la collaboration GEM, le sous-groupe en charge du système d'acquisition de données (DAQ) doit développer l'électronique et les logiciels de gestion de la chaîne de lecture des détecteurs.Nous présentons ici le travail de l'auteur réalisé en tant que principal développeur du firmware pour l'électronique du système DAQ. Ces développements visent à créer une architecture qui achemine les données depuis l'électronique de lecture du détecteur jusqu'aux systèmes situés dans la zone de comptage, tout en offrant la possibilité de contrôler et surveiller l'ensemble des composants du DAQ. Le système mis en place a été largement testé durant deux campagnes de tests en faisceaux qui ont fourni des informations concernant la stabilité du DAQ ainsi que des mesures de l'efficacité des détecteurs. Nous décrivons également l'ensemble des travaux réalisés afin de caractériser les composants électroniques avant leur installation dans CMS ainsi que les résultats des tests d'irradiation effectués sur l'électronique du détecteur. Ces derniers permettent de mieux comprendre les conséquences des radiations sur le système et de développer des méthodes de mitigation.De plus, le travail de l'auteur sur la création d'une nouvelle architecture de système DAQ est décrit. Cette dernière combine les avancées récentes en terme de technologies web à une topologie de réseaux non-classique afin d'accroître la bande passante disponible et de créer un système réactif. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished