La première prise des données du LHC (2010-2012) a été marquée par la découverte du boson scalaire, dit boson de Higgs. Sa masse a été mesurée avec une précision de 0.2% en utilisant ses désintégrations en deux photons et celles en deux bosons Z donnant quatre leptons dans l’état final. Les couplages ont été estimés en combinant plusieurs états finaux, tandis que la précision sur leur mesure pourra bénéficier énormément de la grande statistique qui sera accumulée pendant les prochaines périodes de prise des données au LHC (Run 2, Phase 2).Le canal H→ZZ*→4 leptons, a un rapport d'embranchement réduit mais présente un faible bruit de fond, ce qui le rend attractif pour la détermination des propriétés du nouveau boson. Dans cette thèse, l’analyse conduite pour la mise en évidence de ce mode dans l’expérience ATLAS est détaillée, avec un poids particulier porté à la mesure et au contrôle du bruit de fond réductible en présence d’électrons.Dans le cadre de la préparation de futures prises de données à très haute luminosité, prévues à partir de 2025, deux études sont menées:La première concerne l’observabilité du mode de production du boson de Higgs en association avec des quarks b. Une analyse multivariée, basée sur des données simulées, confirme un très faible signal dans le canal H→2 photons.La seconde concerne la conception et le développement d’un détecteur interne en silicium, adapté à l’environnement hostile, de haute irradiation et de taux d’occupation élevée, attendues pendant la Phase 2 du LHC. Des études concernant l’optimisation de la géométrie, l’amélioration de l’efficacité ainsi que la résistance à l’irradiation ont été menées. A travers des mesures SiMS et des simulations des procédés de fabrication, les profiles de dopage et les caractéristiques électriques attendues pour des technologies innovantes sont explorés. Des prototypes ont été testés sous faisceau et soumis à des irradiations, afin d’évaluer les performances du détecteur et celles de son électronique associée. / The discovery of a scalar boson, known as the Higgs boson, marked the first LHC data period (2010-2012). Using mainly di-photon and di-Z decays, with the latest leading to a four leptons final state, the mass of the boson was measured with a precision of <0.2%. Relevant couplings were estimated by combining several final states, while corresponding uncertainties would largely benefit from the increased statistics expected during the next LHC data periods (Run 2, Phase 2).The H→ZZ*→4l channel, in spite of its suppressed brunching ratio, benefits from a weak background, making it a prime choice for the investigation of the new boson’s properties. In this thesis, the analysis aimed to the observation of this mode with the ALTAS detector is presented, with a focus on the measurement and control of the reducible electron background.In the context of preparation for future high luminosity data periods, foreseen from 2025 onwards, two distinct studies are conducted:The first concerns the observability potential of the Higgs associated production mode in conjunction with two b-quarks. A multivariate analysis based on simulated data confirms a very weak expected signal in the H→di-photon channel.The second revolves around the conception and development of an inner silicon detector capable of operating in the hostile environment of high radiation and increased occupancy, expected during LHC Phase 2. Main studies were concentrated on improving radiation hardness, geometrical and detection efficiency. Through fabrication process simulation and SiMS measurements, doping profiles and electrical characteristics, expected for innovative technologies, are explored. Prototypes were designed and evaluated in test beams and irradiation experiments in order to asses their performances and that of associated read-out electronics.

There are experimental and theoretical indications that the Standard Model of particle physics, although tremendously successful, is not sufficient to describe the universe, even at energies well below the Planck scale. One of the most promising new theories to resolve major open questions, the Minimal Supersymmetric Standard Model, predicts additional neutral and charged Higgs bosons, among other new particles. For the search of the new heavy neutral bosons, the decay into two hadronically decaying tau leptons is especially interesting, as in large parts of the search parameter space it has the second largest branching ratio while allowing for a considerably better background rejection than the leading decay into b-quark pairs. This search, based on proton-proton collisions recorded at sqrt(s) = 13 TeV in 2015 and early 2016 by the ATLAS experiment at the Large Hadron Collider at CERN, is presented in this thesis. No significant deviation from the Standard Model expectation is observed and CLs exclusion limits are determined, both model-independent and in various MSSM benchmark scenarios. The MSSM exclusion limits are significantly stronger compared to previous searches, due to the increased collision energy and improvements of the event selection and background estimation techniques. The upper limit on tan beta at 95% confidence level in the mhmod+ MSSM benchmark scenario ranges from 10 at mA = 300 GeV to 48 at mA = 1.2 TeV.

The search for additional heavy neutral Higgs bosons predicted in Minimal Supersymmetric Extensions of the Standard Model is presented, using the direct decay channel into two tau leptons which themselves decay hadronically. The study is based on proton-proton collisions recorded in 2011 at a center-of-mass energy of 7 TeV with the ATLAS detector at the Large Hadron Collider at CERN. With a sample size corresponding to an integrated luminosity of 4.5 fb−1, no significant excess above the expected Standard Model background prediction is observed and CLs exclusion limits at a 95% confidence level are evaluated for values of the CP-odd Higgs boson mass mA between 140 GeV to 800 GeV within the context of the mhmax and mhmod± benchmark scenarios. The results are combined with searches for neutral Higgs bosons performed using proton-proton collisions at a center-of-mass energy of 8 TeV recorded with the ATLAS detector in 2012, with a corresponding integrated luminosity of 19.5 fb−1. The combination allowed an improvement of the exclusion limit at the order of 1 to 3 units in tan β. Within the context of this study, the structure of additional interactions during a single proton-proton collision (the “underlying event”) in di-jet final states is analyzed using collision data at a center-of-mass energy of 7 TeV recorded with the ATLAS detector in 2010, with a corresponding integrated luminosity of 37 pb−1. The contribution of the underlying event is measured up to an energy scale of 800 GeV and compared to the predictions of various models. For several models, significant deviations compared to the measurements are found and the results are provided for the optimization of simulation algorithms.

Understanding the origin or Electroweak symmetry breaking within the Standard Model was a key motivation for the construction of the Large Hadron Collider (LHC) experiment at CERN. This thesis presents a search for evidence of Higgs boson production in the 4.7 fb−1 of collision data recorded at a centre-of-mass energy of 7 TeV at the ATLAS detector during 2011. This search is focused on signal events in which a Higgs boson is produced in the mass range 100 < mH < 180 GeV/c2 and subsequently decays to a pair of W bosons or a pair of tau leptons to final states with one hadronically decaying tau lepton and one light lepton. After an event selection criteria has been applied, the number of events in this data sample is consistent with the total background estimate and an upper limit is placed on the SM Higgs boson production rate at 95% confidence level. In addition, the prospects for measuring the SM Higgs coupling strength to tau leptons with the associated Higgs production channels and the full LHC dataset are also presented.

The search for a Higgs boson decaying to a b¯b pair is one of the key analyses ongoing at the ATLAS experiment. Despite being the largest branching ratio decay for a Standard Model Higgs boson, a large dataset is necessary to perform this analysis because of the very large backgrounds affecting the measurement. To discriminate the electroweak H → b¯b signal from the large QCD backgrounds, the associated production of the Higgs with a W or a Z boson decaying leptonically is used. Different techniques have been proposed to enhance the signal over background ratio in the VH(b¯b) channel, from dedicated kinematic cuts, to a single large radius jet to identify the two collimated b’s in the Higgs high transverse momentum regime, to multivariate techniques. The high-pT approach, using a large radius jet to identify the b’s coming from the Higgs decay, has been tested against an analysis based on kinematic cuts for a dataset of 4.7 fb−1 luminosity at √s = 7 TeV, and compatible results were found for the same transverse momentum range. Using a kinematic cut based approach the VH(b¯b) signal search has been performed for the full LHC Run 1 dataset: 4.7 fb−1 at √s = 7 TeV and 20.7 fb−1 at √s = 8 TeV. Several backgrounds to this analysis, such as Wb¯b have not been measured in data yet, and an accurate study of the theoretical description has been performed, comparing the predictions of various Monte Carlo generators at different orders. The complexity of the analysis requires a profile likelihood fit with several categories and almost 200 parameters, taking into account all the systematics coming from experimental or modelling limitations, to extract the result. To validate the fit model, a test of the ability to extract the signal is performed on the resonant V Z(b¯b) background. A 4.8σ excess compatible with the Standard Model rate expectation has been measured, with a best fit value μVZ = 0.93+0.22−0.21. The full LHC Run1 dataset result for the VH(b¯b) process is a limit of (1.3)1.4 x SM (expected) observed, with a best fit value of 0.2±0.5(stat)±0.4(sys) for a Higgs boson of 125 GeV mass.

The study of flavour physics allows for the Standard Model (SM) to be tested to higher energies than can be accessed through direct searches. The SM is known not to provide enough of a difference between matter and anti-matter, termed CP violation, to explain the dominance of matter in our universe. One of the main purposes of the LHCb experiment is to search for new sources of CP violation in the decays of B mesons. Flavour changing neutral current (FCNC) interactions are forbidden at tree level in the SM, and can therefore only be accessed through quantum loops. In New Physics scenarios such as Supersymmetry, new particles could appear in those loops introducing new sources of CP violation. The Bos→ØØ decay proceeds via the b → sss FCNC transition. Triple products provide a method of exploiting the angular distributions of P → V V decays to create T-odd observables. Asymmetries of these T-odd observables, averaged over the initial flavour of the Bos meson provide a measure of T violation. Assuming CPT conservation, violation of time reversal infers CP violation. The CP-violating weak phase in the interference between Bos mixing and the decay to two Ø mesons is predicted to be close to zero in the SM. The measurements of the triple product asymmetries and the CP-violating weak phase have been performed using 1.0 fb-1 of LHCb data. Events where kaon pairs originate from a spin-0 or non-resonant state are accounted for with the associated angular distributions. Triple product asymmetries are measured to be AU = -0:055 ± 0:036(stat) ± 0:018(syst) and Av = 0:010 ± 0:036(stat) ± 0:018(syst). The CP-violating phase is found to be in the interval [-2:46,-0:76] rad at 68% confidence level. The p-value for the hypothesis of zero radians is found to be 16 %. These results represent the most accurate measurements of the triple product asymmetries and the first measurement of the CP-violating weak phase.

Un ordinateur IBM 1800 a été installé au CERN pour assister l'opération du synchrotron à protons (PS). Par l'intermédiaire de systèmes d'acquisition cet ordinateur acquiert un certain nombre de données relatives à l'accélérateur et à ses faisceaux. Certains paramètres peuvent erre commandés par l'ordinateur.<br />Après un bref rappel des caractéristiques de l'accélérateur, de l'ordinateur et de son électronique associée, une analyse détaillée des programmes relatifs aux faisceaux éjectés est donnée. Ces programmes concernent un relevé de données, une surveillance de paramètres et les statistiques sur l'utilisation des protons par les systèmes d'éjection. Deux programmes de contrôle optimisent l'un la position du faisceau éjecté sur la cible externe, l'autre le flux de particules secondaires issues de cette cible.<br /><br />L'utilisation opérationnelle et l'aspect conversationnel de ces programmes sont expliqués.

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The Standard Model of Particle Physics describes the fundamental building blocks of matter and phenomena up to the highest particle interaction energies. The theory demands the existence of a scalar particle: the Higgs boson. The Higgs boson was discovered by the ATLAS and CMS collaborations at CERN using bosonic final states and is measured to have a mass of around 125 GeV. This particle is predicted to decay predominantly into pairs of b-quarks at this mass, but suffers from overwhelming backgrounds from the multijet production expected from QCD interactions. Therefore, H→bb- production in association with a leptonically decaying W or Z boson is considered, with Z → vv-, W → lv and Z → ll, where ` denotes electrons and muons. This thesis presents a search for the Higgs boson decaying into bb- pairs in association with a W or Z boson using the ATLAS detector at the Large Hadron Collider (LHC) at CERN. The analysis uses the full dataset recorded during pp collisions at the LHC in Run-1, corresponding to 4.7 fb-1 at √s = 7 TeV and 20.3 fb-1 at √s = 8 TeV. A multivariate technique and a kinematic cut-based approach have been used to maximize the signal over background ratio, where a particular emphasis on the latter approach is made in this thesis. Final state radiation and reconstruction effects may decrease the bb- resonance resolution significantly, while comparably decreasing the probability of observing the decay over the background. The b quark pairs from the Higgs boson are reconstructed as topological clusters formed to jets in the ATLAS calorimeter. Thus, the reconstruction and calibration of these jets are crucial for the final Higgs mass resolution and paramount for the search and for future precision measurements of V H, H→bb- production. This thesis presents the development and evaluation of advanced techniques to improve the invariant dijet mass reconstruction of the H→bb- candidate. Sequential jet calibrations, semileptonic corrections and pT corrections to account for the interplay between jet resolution/scale and the underlying signal pT spectrum obtained from Monte Carlo simulations have been studied. A major focus has been made on the development and evaluation of an event-level kinematic likelihood fitting framework to exploit the full kinematic potential of V H topologies within the detector uncertainties of the reconstructed final state signatures in order to improve the measurement of the b-tagged jet kinematics. The jet energy calibrations of the H→bb- signal candidates yield an overall improvement of the dijet invariant mass resolution of up to ~30%, and of the expected statistical significance of ~12%. The analysis procedure is validated using the resonant V Z(bb-) production in the same final states as for the Higgs boson search, and is observed, compatible with the Standard Model expectation, with a significance of 4.9 standard deviations and a signal strength of μ^V Z = 0:74+0:17 -0:16. For a Higgs boson mass of 125.36 GeV, the observed (expected) deviation from the background-only hypothesis is found with a significance of 1.4 (2.6) standard deviations and a signal strength is determined to be μ^V H = 0:52±0:32(stat.)±0:24(syst.).