Dans cette thèse, je présente ma contribution à la recherche du boson de Higgs du Modèle Standard dans son canal de désintégration en deux leptons taus dans le sous-canal τlepτhad. J’ai contribué aux différentes étapes de cette analyse. Premièrement, j’ai travaillé sur le développement du framework utilisé pour produire les fichiers d’analyse qui contiennent les informations nécessaires à cette étude. J’ai également participé au développement du framework utilisé pour introduire les prédictions des bruits de fond pour ce canal, tester l’accord entre ces prédictions et les données d’ATLAS, et à la production des fichiers utilisés dans l’étude statistique finale. J’ai également travaillé sur le modèle statistique, qui prend en compte les erreurs statistiques et systématiques, pour en extraire une mesure de la force du signal pour ce canal du couplage du Higgs aux leptons. Ma contribution à l’amélioration de la reconstruction d’un tau hadronique, à l’aide des algorithmes qui identifient les traces de conversion des photons provenant de la désintégration des pions neutres, est également détaillée. / In this thesis, I show my contribution to the search for the Standard Model Higgs Boson decaying into two tau leptons in the tlepthad sub-channel. I contributed to the different stages of this analysis. First, I worked on the development of the framework used to produce analysis files that contain the information necessary for this study. I also participated in the development of the framework used to introduce the predictions of the background for that channel, test the agreement between the predictionsand the ATLAS data, and on the production of files used in the final statistical analysis. I also worked on the statistical model, which takes into account the statistical and systematic errors to extract a measure of the signal strength for the Higgs coupling channel to leptons. My contribution to the improvement of reconstruction of a hadronic tau, using algorithms that identify photon conversion traces from the decay of neutral pions, is also detailed.

Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) du CERN a redémarré au printemps 2015 pour trois ans (Run 2) avec une énergie dans le centre de masse de 13 TeV.Une mesure précise de l’efficacité de reconstruction des électrons avec le détecteur ATLAS au LHC, est présente dans la première partie de cette thèse en utilisant les données récoltées au cours de l’année 2015. Cela a permis d’extraire les rapports d’efficacité de reconstruction des électrons, lesquels sont utilisés par toutes les analyses d’ATLAS impliquant des électrons. Les résultats montrent d'une part une grande efficacité de reconstruction des électrons avec le détecteur ATLAS, et une bonne compréhension de ses performances d’autre part.La seconde partie de ce manuscrit est dédiée à la recherche de la production associée du boson de Higgs avec une paire de quarks top (bar), qui pourrait permettre une première mesure directe du couplage de Yukawa entre le boson de Higgs et le quark top. Une déviation dans la mesure par rapport aux prédictions du Modèle Standard serait une preuve manifeste de Nouvelle Physique. La signature de l’état final avec deux leptons de même charge électrique électrons ou muons) est examinée en utilisant les premiers 10\% du total de données attendues pour le Run 2. Une méthode améliorant l'estimation des événements avec de faux (non-prompt) leptons a développé}e et est discutée en détails dans ce rapport. / The Large Hadron Collider (LHC) at CERN restarted in spring 2015 for three years (Run2) at an unexplored center-of-mass energy of 13 TeV. A precise measurement of electron reconstruction efficiency in ATLAS, one of the two general purpose experiments of the LHC, is presented in the first part of this thesis with $Zee$ data sample recorded in 2015. This allows to extract scale factors between data and simulation that are used by all ATLAS physics analyses involving electrons. The results show the high ability of the ATLAS detector to reconstruct electrons from one hand and the good understanding of its performance on the other hand. The second part of the thesis is dedicated to a search for the Higgs boson production in association with a top quark pair (ttH), which could allow a first direct measurement of the top quark Yukawa couplingand could reveal new physics. The signature with two same-charge light leptons (electron or muon) without a hadronically decaying tau lepton final state is examined using the first 10% of the total expected Run2 dataset. Events with fake (non-prompt) leptons represent the main reducible background of this signature. An improved method to estimate it has been developped and is discussed in details in this thesis. Driving the total error, fake leptons background is found to be 1.5 to 3.6 times higher than in simulation and represent between 32 and 48% of the total background. The best-fit value of the ratio of observed and Standard Model cross sections of ttH production process, combining with other multilepton channels, is 2.5±0.7(stat)+1.1-0.9(syst), and an upper limit on this ratio of 4.9 (2.3 expected) is found at 95% confidence level.

<p dir="ltr">This thesis presents two studies in search for violation of lepton flavor universality as predicted by the Standard Model. The first searches for signs of the violation by studying beyond the Standard Model (BSM) physics models involving same flavor and opposite sign dilepton pair and bottom quarks as final states. This study was done using the dataset collected during years of 2016, 2017 and 2018, with center of mass energy $\sqrt{s} = 13$ TeV and integrated luminosity of 138 $fb^-1$. In the study, scale factors were derived in order to correct deviations between simulation and real life data, specifically for high transverse momentum muons and top\&anti-top quark background. Furthermore, lower limits of energy scale were calculated leading to exclusion of the BSM models with energy scale values lower than that of the calculated value.</p><p dir="ltr">The second study also searches for of the lepton flavor universality violation, but in the specific decay of a tauon into three muons ($\tau \rightarrow 3\mu$). In the study, graph based neural network model (GNN) designed to classify $\tau \rightarrow 3\mu$ events at the CMS detector was converted to high level synthesis (HLS) code, so that the GNN could be coded into a custom hardware such as field programmable gate arrays (FPGA) for deployment. Moreover, techniques such as pruning and quantization were applied in an attempt to make the GNN more light weight, due to strict requirements of FPGA.</p>

The coupling of the Standard Model top quark to the Higgs boson is O(1), which leads to large quantum corrections in the perturbative expansion of the Higgs boson mass. Possible solutions to this so-called naturalness problem include supersymmetric models with gluinos and stop squarks whose masses are at the electroweak scale, O(1 TeV). If supersymmetry is realised in nature at this scale, these particles are expected to be accessible with the Large Hadron Collider at CERN. A search for gluino pair production with decays mediated by stop- and sbottom-squark loops in the initial 14.8 ifb of the ATLAS run 2 dataset is presented in terms of a pair of simplified models, which targets extreme regions of phase space using jet substructure techniques. No excess is observed and limits are set which greatly extend the previous exclusion region of this search, up to 1.9 TeV (1.95 TeV) for gluinos decaying through light stop (sbottom) squarks to the lightest neutralinos. A performance study of top tagging algorithms in the 20.3 ifb 2012 dataset is also presented, which includes the first measurements of substructure-based top tagging efficiencies and fake rates published by ATLAS, as well as a detailed comparison of tagger performance in simulation. A benchmarking study which compares commercially available cloud computing platforms for applications in High Energy Physics, and a summary of ATLAS liquid argon calorimeter data quality work focused on monitoring and characterising the sporadic phenomena of Mini Noise-Bursts in the electromagnetic barrel calorimeter are also included. / Graduate / 0798 / matt.leblanc@cern.ch