Cette thèse présente une étude de la production de boson de Higgs dans les collisions proton-proton à sqrt(s) = 13 TeV enregistrées avec le détecteur CMS au Grand collisionneur de hadrons (LHC) du CERN, exploitant la voie de désintégration en une paire de bosons Z qui se désintègrent à leur tour en paires d’électrons ou de muons (H->ZZ->4l, l = e,mu).Ce travail s’inscrit dans le contexte du début du Run II du LHC, une nouvelle période de prise de données qui a commencé en 2015 après une interruption de deux ans. Ce redémarrage est marqué par une augmentation de l’énergie dans le centre de masse de 8 TeV à 13 TeV et un resserrement de l’espacement entre paquets de protons de 50 ns à 25 ns. Ces nouveaux paramètres augmentent à la fois la luminosité et placent des contraintes inédites sur le déclenchement, la reconstruction et l’analyse des données de collisions pp. Un effort important est donc consacré à l’amélioration et la réoptimisation du système de déclenchement de CMS pour le Run II, en mettant l’accent sur la reconstruction et la sélection des électrons et sur la préparation de chemins de déclenchement multi-leptons préservant une efficacité maximale pour le canal H->ZZ->4l.Dans un second temps, les algorithmes de sélection hors-ligne des électrons et des muons sont optimisés et leurs efficacités sont mesurées dans les données, tandis que la logique de sélection des candidats à quatre leptons est améliorée. Afin d’extraire des modes de production rares du boson de Higgs tels que la fusion de bosons vecteurs, la production par « Higgsstrahlung » VH et la production associée ttH, une nouvelle répartition des événements sélectionnés en catégories exclusives est introduite, fondée sur des discriminants utilisant le calcul d’éléments de matrice et l’étiquetage de saveur des jets.Les résultats de l’analyse des premières données à 13 TeV sont présentés pour des lots de données enregistrés en 2015 et au début de 2016, qui correspondent à des luminosités intégrées respectives de 2.8 fb-1 and 12.9 fb-1. Le boson de Higgs est redécouvert de façon indépendante à la nouvelle énergie. L’intensité du signal relative à la prédiction du modèle standard, la masse et la largeur de désintégration du boson sont mesurées, ainsi qu’un jeu de paramètres contrôlant les contributions des principaux modes de production attendus. Tous les résultats sont en bon accord avec les prévisions du modèle standard pour un boson de Higgs à 125 GeV, aux incertitudes de mesure près, ces dernières étant dominées par la composante statistique avec l’échantillon de données actuel. Enfin, une autre résonance se désintégrant en quatre leptons est recherchée à haute masse, et aucun excès significatif n’est observé. / This thesis reports a study of Higgs boson production in proton-proton collisions at sqrt(s) = 13 TeV recorded with the CMS detector at the CERN Large Hadron Collider (LHC), exploiting the decay channel into a pair of Z bosons that in turn decay into pairs of electrons or muons (H->ZZ->4l, l = e,mu).This work is carried out in the context of the beginning of Run II of the LHC, a new data-taking period that started in 2015, following a two-year-long shutdown. This restart is marked by an increase of the centre-of-mass energy from 8 TeV to 13 TeV, and a narrowing of the spacing of proton bunches from 50 ns to 25 ns. These new parameters both increase the luminosity and set new constraints on the triggering, reconstruction and analysis of pp collision events. Therefore, considerable effort is devoted to the improvement and reoptimization of the CMS trigger system for Run II, focusing on the reconstruction and selection of electrons and on the preparation of multilepton trigger paths that preserve a maximal efficiency for the H->ZZ->4l channel.Secondly, the offline algorithms for electron and muon selection are optimized and their efficiencies are measured in data, while the selection logic of four-lepton candidates is improved. In order to extract rare production modes of the Higgs boson such as vector boson fusion, VH associated production and ttH associated production, a new classification of selected events into exclusive categories is introduced, using discriminants based on matrix-element calculations and jet flavour tagging.Results of the analysis of first 13 TeV data are presented for two data sets recorded in 2015 and early 2016, corresponding to integrated luminosities of 2.8 fb-1 and 12.9 fb-1, respectively. A standalone rediscovery of the Higgs boson in the four-lepton channel is achieved at the new energy. The signal strength relative to the standard model prediction, the mass and decay width of the boson, and a set of parameters describing the contributions of its main predicted production modes are measured. All results are in good agreement with standard model expectations for a 125 GeV Higgs boson within the incertainties, which are dominated by their statistical component with the current data set. Finally, a search for an additional high-mass resonance decaying to four leptons is performed, and no significant excess is observed.

The Standard Model of particle physics is a successful theory, yet it is incomplete. Supersymmetry is one of the favoured extensions of the Standard Model, elegantly addressing several unresolved issues. This thesis presents a search for the pair production of supersymmetric particles chargino one and neutralino two, where the neutralino two decays to the lightest neutralino and the 125 GeV Higgs boson. The final states considered for the search have large missing transverse momentum, an isolated lepton and two jets identified as originating from bottom quarks. The analysis is based on 20.3 inverse femtobarns of 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. No excess over Standard Model predictions is observed. The analysis has been combined with three independent searches that probe other decay modes of the Standard Model Higgs boson. Limits are set at 95% confidence level in the context of a simplified supersymmetric model. Common masses of chargino one and neutralino two are excluded up to 250 GeV for a massless neutralino one. The analysis of this dissertation has been reinterpreted in the context of a large scan of the phenomenological Minimal Supersymmetric Standard Model, along with 22 other ATLAS Run 1 searches. The resulting summary paper represents the most comprehensive assessment of the ATLAS constraints on Supersymmetry models to date. / Graduate / 0798 / cdavid@uvic.ca

Deux analyses de physique des particules à hautes énergies sont présentées dans ce manuscrit de thèse, utilisant des événements avec deux leptons de charges opposées et de l'énergie transverse manquante. Ces événements sont sélectionnés parmi 9.7 fb-1 de données totales de collisions pp enregistrées par le détecteur DØ auprès du collisionneur TeVatron du Fermilab à √s=1.96 TeV.La première analyse est la recherche du boson de Higgs se désintégrant en H→WW→μνμν. Aucun excès significatif au-dessus de l'attente pour le bruit de fond n'est observé.Des limites supérieures sur la section efficace de production du boson de Higgs sont donc calculées dans le cadre du modèle standard mais aussi dans l'hypothèse de l'existence d'une quatrième génération de fermions et dans le contexte de couplages fermiophobiques du boson de Higgs.Pour valider la méthodologie de cette recherche, la section efficace de production de la paire de bosons W est mesurée.La seconde analyse est la mesure de l'asymétrie avant-arrière de production de la paire tt. Il s'agit de la première mesure dans le canal dileptonique de l'expérience DØ. Dans ce contexte, une méthode inédite de reconstruction de la cinématique de la paire est utilisée (méthode des éléments de matrice) pour donner une mesure brute de l'asymétrie avant-arrière. A l'aide d'une méthode de calibration dédiée, nous donnons une mesure finale de AFB=18.0 ± 6.0 (stat) ± 3.3 (syst). / Two high energy particle physics analyses are presented in this PhD report using events with two leptons oppositely charged and with missing transverse energy. These events are selected using 9.7 fb-1 of total pp collisions data collected with the DØ detector at the TeVatron at √s=1.96 TeV.The first analysis is the research of the Higgs boson decaying in the H→WW→μνμν channel. No significant excess above the background prediction is observed.Upper limits on Higgs boson production cross-section are computed in the standard model framework but also in the 4th generation of fermions and in the fermiophobic coupling to Higgs boson hypotheses. In order to validate the research methodology, the W boson pair production cross-section is measured.The second analysis is the measurement of the forward-backward asymmetry of the tt pair production. This is the first measurement in the dileptonic channel at DØ experiment. In this context, a new tt pair kinematic reconstruction is used (matrix element method) to give a raw measurement of the forward-backward asymmetry. Thanks to a dedicated calibration method, we give a final measurement of AFB=18.0 ± 6.0 (stat) ± 3.3 (syst).

Foi analisada, através de simulação Monte Carlo, a probabilidade de que um gap de rapidez seja observado na produção do boson de Higgs por fusão de bosons vetoriais com massa (W), no LHC (Large Hadron Collider). Foi analisada, também através do uso de Monte Carlo, a possibilidade de que a existência ou não de um gap de rapidez seja usada na distinção de um Higgs produzido por fusão de bosons vetoriais W de outro produzido por fusão de gluons. Os diferentes modelos para tratamento de processos não perturbativos dos códigos Monte Carlo DTUJET e PYTHIA foram comparados. / The gap survival probability for a higgs boson produced by W fusion at the LHC (Large Hadron Collider) is analyzed through Monte Carlo simulation. The possibility of distinguishing between two higgs production mechanisms, W and gluon fusion is studied using Monte Carlo simulation. The two different models for non pertubative processes, used by DTUJET and PYTHIA are compared.

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Foi analisada, através de simulação Monte Carlo, a probabilidade de que um gap de rapidez seja observado na produção do boson de Higgs por fusão de bosons vetoriais com massa (W), no LHC (Large Hadron Collider). Foi analisada, também através do uso de Monte Carlo, a possibilidade de que a existência ou não de um gap de rapidez seja usada na distinção de um Higgs produzido por fusão de bosons vetoriais W de outro produzido por fusão de gluons. Os diferentes modelos para tratamento de processos não perturbativos dos códigos Monte Carlo DTUJET e PYTHIA foram comparados. / The gap survival probability for a higgs boson produced by W fusion at the LHC (Large Hadron Collider) is analyzed through Monte Carlo simulation. The possibility of distinguishing between two higgs production mechanisms, W and gluon fusion is studied using Monte Carlo simulation. The two different models for non pertubative processes, used by DTUJET and PYTHIA are compared.

Ce travail de thése concerne la recherche du boson de Higgsa laide de canaux de désinégration contenant des photons dans létat final, baée sur les données enregistrées par le détecteur ATLAS en 2011 et 2012, a une énergie dans le centre de masse des collisions proton-proton center-mass-energy et 8 TeV. La sélection deséévenements, les principaux bruits de fond, les propriéés du signal, la discrimination statistique entre signal et bruits de fond, ainsi que l'interprétation des ésultats en terme de boson de Higgs du moéle standard sont discués. Dans le canal de désinégration H to gamma+gamma, un excés par rapport au niveau de bruit de fond est clairement visible a une masse mH=126,8 +- 0,2(stat) +- 0,7(syst) GeV et avec une significativité locale de7,4 sigma. En revanche, dans le canal de désinégration rareH to Z+gamma, aucun excés n'est obseré dans la feétre de masse allant de 120 a 150 GeV. Ce résultat est interpéé comme une limiteégalea 11 fois la valeur de la section efficace de production pp to H to Z+gamma prédite par le moéle standard pour une masse du boson de Higgs de mH = 125,5 GeV, proche de celle mesurée dans les canaux engamma+gamma et en quatre leptons. Une reconstruction et une identification efficace des photons, ainsi qu'une connaissance précise des performances du systéme de éclenchement et de l'algorithme d'identification, sont des aspects importants de ces mesures. Une grande partie de ce manuscrit est ainsi consacréea la description de l'optimisation de ces performances et des activités de mesures que j'ai réalisées pendant ces trois années. / This thesis focuses on the searches for the Higgs boson in events with photons in the final states, using the full proton-proton collision data collected by ATLAS at center-mass-energy of 7 TeV and 8 TeV in 2011 and 2012.Higgs boson decays to photon pairs or to a photon and a Z boson decaying to di-electrons or di-muons are investigated. The event selection, the main backgrounds, the signal properties, and the statistical discrimination between the signal and background in data and the interpretation of the results in terms of a Standard Model Higgs boson are discussed. In the H decays to diphoton channel a clear excess over the background is seen at a mass of mH= 126.8 +- 0.2(stat) +- 0.7(syst) GeV, with a local significance of 7.4 sigma. In the rare decay channel H decays to Z+gamma no evidence of excess over the background is observed in the mass range 120-150 GeV, and, for a Higgs boson mass near the one obtained from the combined mass measurement in the diphoton and 4-lepton final states, mH=125.5 GeV, an upper limit of 11 times the SM prediction, at 95% confidence level, is set on the production cross section times the H to Z+gamma cross section. One of the most important ingredient for these measurements is the efficient reconstruction and identification of photons, and a precise knowledge of the trigger and identification performance. A significant part of the document is thus devoted to the photon performance optimisation and measurement activities that I carried on in the past three years.

<p>The Standard Model describes all elementary particles known today, but at larger energies it will have to be complemented with new particles and interactions. To be able to distinguish new physics at proton colliders such as LHC at CERN, it is essential to have an appropriate description of the colliding protons and their interactions. The study of the proton is important also in itself, to get a better understanding of the non-perturbative aspects of the strong interaction.</p><p>In paper I-IV of this thesis, a model for the non-perturbative dynamics of quarks and gluons is developed, based on quantum fluctuations in hadrons. The parton distributions of the proton are given by momentum fluctuations, with sea quark distributions generated by fluctuations into baryon-meson pairs. This model can reproduce proton structure function data, as well as measured asymmetries between up and down valence quark distributions and between the anti-up and anti-down sea. It provides an intrinsic charm quark component as indicated by data. It also predicts an asymmetry in the strange sea of the proton, which can explain the NuTeV anomaly first attributed to new physics beyond the Standard Model.</p><p>Charged Higgs bosons are predicted by several theories for new physics, including Supersymmetry. At proton colliders, the predicted dominant production mechanism is in association with top and bottom quarks. In paper V-VII, different contributions to this production are studied, and an algorithm is developed for combining the two dominant processes gb -> tH^+/- and gg -> tbH^+/-. The algorithm gives a smooth transition from small to large transverse momenta of the b-quark, which is important when the b-quark is observed. It also gives arguments for the choice of factorisation scale in the process.</p>

<p>This thesis treats two different projects, both aiming at experimental tests at hadron colliders of some specific predictions of the Standard Model (SM) of particle physics and of its Minimal Supersymmetric extension (MSSM). The thesis is based on four papers.</p><p>Papers I-III study the discovery potential for a heavy charged Higgs boson of the ATLAS experiment at the Large Hadron Collider (LHC). Charged Higgs bosons are part of the Higgs sector in many extensions to the SM and their detection would be an unambiguous sign of new physics.</p><p>If the charged Higgs boson is heavier than the top quark, its dominant decay mode is into a top and a bottom quark. Searches in this decay channel at ATLAS are difficult mainly due to the large top-quark pair-production background. The possible gain obtained when requiring 4 b-tagged jets for the signal events is studied, but no significant improvement compared to an analysis requiring 3 b-tagged jets is found.</p><p>If the masses of supersymmetric particles are sufficiently small, heavy charged Higgs bosons can decay into a chargino-neutralino pair. We show that in this decay channel charged Higgs bosons can be detected, given a favourable choice of parameters governing the MSSM.</p><p>In a large-mass-splitting MSSM scenario, the charged Higgs decay into a W boson and a neutral Higgs can have a large branching fraction. We conclude, however, that charged Higgs searches in this decay channel are made difficult by a large, irreducible SM background.</p><p>Paper IV describes a measurement of the top-quark pair production cross-section performed with the DØ detector at the Tevatron collider. Signal events in the muon+jets decay channel are selected using topological event characteristics and a preliminary result of σ(ttbar)=3.8 +1.1-1.1(stat) +0.9-0.8(syst) +0.3-0.2(lumi) pb is obtained using an integrated luminosity of 363/pb.</p>

The Standard Model describes all elementary particles known today, but at larger energies it will have to be complemented with new particles and interactions. To be able to distinguish new physics at proton colliders such as LHC at CERN, it is essential to have an appropriate description of the colliding protons and their interactions. The study of the proton is important also in itself, to get a better understanding of the non-perturbative aspects of the strong interaction. In paper I-IV of this thesis, a model for the non-perturbative dynamics of quarks and gluons is developed, based on quantum fluctuations in hadrons. The parton distributions of the proton are given by momentum fluctuations, with sea quark distributions generated by fluctuations into baryon-meson pairs. This model can reproduce proton structure function data, as well as measured asymmetries between up and down valence quark distributions and between the anti-up and anti-down sea. It provides an intrinsic charm quark component as indicated by data. It also predicts an asymmetry in the strange sea of the proton, which can explain the NuTeV anomaly first attributed to new physics beyond the Standard Model. Charged Higgs bosons are predicted by several theories for new physics, including Supersymmetry. At proton colliders, the predicted dominant production mechanism is in association with top and bottom quarks. In paper V-VII, different contributions to this production are studied, and an algorithm is developed for combining the two dominant processes gb -&gt; tH+/- and gg -&gt; tbH+/-. The algorithm gives a smooth transition from small to large transverse momenta of the b-quark, which is important when the b-quark is observed. It also gives arguments for the choice of factorisation scale in the process.