Les premières années d’opération du LHC à 7 et 8 TeV ont permis de tester de nombreux aspects du modèle standard (MS) de la physique des particules avec comme point d’orgue la découverte d’un boson scalaire compatible avec celui du MS. Cependant, malgré son succès, le MS n’explique pas tout. De nombreux modèles essayent donc de combler les manques du MS et certains requièrent un secteur scalaire étendu. Cette thèse étudie ce secteur scalaire et présente en particulier la recherche d’un nouveau boson scalaire massif en utilisant les 35,9 fb-1 de données récoltées au LHC à 13 TeV en 2016. Cette recherche se concentre sur le processus H→ZZ→2l2ν où l est un electron ou un muon, et ν un neutrino. Une analyse statistique des distributions de masse transverse du boson (Z→l^+ l^-) reconstruit et de l’énergie transverse manquante est réalisée en catégories de jets et de saveurs de lepton après une sélection des événements dans le but de faire sortir un signal potentiel par rapport aux bruits de fond. Divers scenarios de masse, largeur et de mode de productions sont testés pour un boson scalaire H et aucun excès significatif n’est observé dans les données pour une gamme de masses allant de 300 GeV à 3 TeV. Des limites supérieures à 95% de niveau de confiance sur la section efficace σ(pp→H→ZZ) sont établies. Ces résultats sont combinés avec deux autres canaux étudiés par la collaboration CMS :H→ZZ→4l et H→ZZ→2l2q, où q représente un quark. Cette combinaison est publiée (CMS Collaboration, « Search for a new scalar resonance decaying to a pair of Zbosons in proton-proton collisions at √s=13 TeV », JHEP, vol. 06, pp. 127,2018) et aucun excès significatif n’est observé dans les données pour des masses entre 130 GeV et 3 TeV. Un bruit de fond majeur pour cette analyse est le bruit de fond instrumental Z+jets. Son estimation est faite non pas par simulation mais en utilisant les données récoltées par CMS. Un chapitre entier est dédié à l’étude de ce bruit de fond et les incertitudes sur celui-ci sont estimées en détails pour la première fois. En particulier les sources d’incertitudes sont divisées en celles d’origine statistique, systématique et celles inhérentes à la méthode d’estimation de ce bruit de fond. Elles sont respectivement de l’ordre de 50%, 50% et 10%. / The first years of operation of the LHC at 7 and 8TeV (run 1) allowed to probemany aspects of the Standard Model (SM) of particle physics, confirming many predictions up to a high level of precision. An awaited event was the discovery of a scalar boson compatible with the SM in 2012. However, despite its success, the SM cannot explain everything on its own. Models going beyond the SM try to address such issues, and in particular classes of BSM theories require an extended scalar sector.The scope of this thesis falls within the direct search for heavy BSM scalar boson(H) using 35,9 fb-1 of data taken at LHC at 13TeV during 2016 operations. Inparticular, the thesis focuses on H→ZZ→2l2ν where l is an electron or a muon, and ν a neutrino.A statistical analysis of the transversemass distributions of the reconstructed Z boson and the missing transverse energy isperformed in categories of jet and lepton flavour after selecting events in order to enhancethe number of potential signal events in comparison to background events. Variousscenarii of masses, widths and production mechanisms (gluon fusion and vector boson fusion) are tested and no excess is observed in data compared to backgroundestimations for a scalar mass between 300 GeV and 3TeV. Upper exclusion limits onthe cross section of a potential signal are set using the CLs method.The results of this analysis are combined with two other channels studied by theCMS collaboration: H→ZZ→4l and H→ZZ→2l2q, where q is a quark.This combinationhas been published (CMS Collaboration, « Search for a new scalar resonance decaying to a pair of Zbosons in proton-proton collisions at √s=13 TeV », JHEP, vol. 06, pp. 127,2018) and no significanthint for BSM physics is observed for a scalar mass between 130 GeV and 3TeV.A critical background to control for this analysis is the Z+jet background. Its estimation is done using a data-driven method.A full chapter is dedicated to the study of this background. The present thesis is the first time that the uncertainties on the data-driven estimationof the Z+jet background are estimated in detail. Uncertainties are dividedinto statistical uncertainties, systematic uncertainties and uncertainties inherent to themethod, the latter being computed from a closure test of the method and evaluated to10%. The relative statistical and systematic uncertainties are both of the order of 50%. / Option Physique du Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The Higgs boson and the top quark have been a focus in modern elementary particle physics research because of their special roles in the Standard Model (SM) of particle physics. The studies of both particles are crucial for revealing the unsolved puzzles of modern particle physics. The coupling between the Higgs boson and the top quark, the top-Yukawa coupling, is one of the fundamental parameters in the SM that can potentially direct the future development of the theory of elementary particle physics. This thesis presents two analyses on the Higgs boson and the top quark, using proton-proton (pp) collision data collected by the ATLAS detector during 2012 and 2015. A search for the SM Higgs boson produced in association with a top quark pair (tt̄H) was performed using 20.3 fb⁻¹ of pp collision data at a centre-of-mass energy of √s = 8 TeV. The search is designed to be primarily sensitive to the H → bb decay mode. Events with one of two electrons or muons are used in the search. No significant excess of events is observed above the background predicted by the SM. An observed (expected) upper limit on the signal strength of 3.4 (2.2) times the SM prediction is obtained at 95% confidence level. The tt̄H signal strength, represented by the ratio of the measured tt̄H cross-section to the SM prediction, is found to be μ = 1.5 ± 1.1 for a Higgs boson mass of m_H = 125 GeV. A measurement of the top quark pair (tt̄) production cross-section was performed using 3.2 fb⁻¹ of pp collision data at √s = 13 TeV. The measurement uses events with an opposite-charge-sign electron-muon pair and exactly one and two jets originating from b quarks. The inclusive tt̄ production cross-section is measured to be σ_tt̄ = 818 ± 8(stat) ± 27(syst) ± 19(lumi) ± 12(beam) pb, where the uncertainties arise from data statistics, analysis systematic effects, the integrated luminosity and the LHC beam energy. The total relative uncertainty is 4.4%. The result is consistent with the theoretical prediction at the next-to-next-to-leading order accuracy in the strong coupling constant αs of QCD, with the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms. A fiducial cross-section corresponding to the experimental acceptance of leptons is also measured.

In this thesis, the search for the production of the Higgs boson in associationwith two top quarks is presented. The main focus of this work is on the analysis optimised for the decay of the Higgs boson to a b-quark pair. The analysis is performed using 36.1 fb−1 of ppcollision data at a centre of mass energy sqrt(s)=13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015 and 2016. The signal strength of ttH in relation to the Standard Model prediction for a Higgs boson with a mass of 125 GeV is measured to be mu(ttH) = 0.87 +0.64−0.61, with signal strengths greater than 2.0 excluded at the 95% confidence level. The combination of this analysis with searches targeting additional Higgs boson decay modes is subsequently presented. The measured signal strength in relation to the Standard Model prediction is mu(ttH) = 1.2 +/-0.3. This correspondsto an observed (expected) significance for ttH of 4.2sigma (3.8sigma), constituting evidence for the ttH production mode. Finally, a study into the ability to observe and model the colour connection of b-quarks in ttH(bb) and ttbar+jets events is presented. The jet pull angle observable is used to investigate the effect of colour connection on jet substructure. Such an observable is found to be sensitive to the underlying colour structure in events, showing differences between b-quarks which decay from a colour singlet in comparison to a colour octet. However, the effect is found to be small and a larger dataset is required to measure the effect in ttH events.

This thesis presents two measurements made using data collected by the LHCb detector, operating at the Large Hadron Collider accelerator at the CERN particle physics laboratory. The first is a measurement of the production rates of promptly produced D0, D+, Ds+, and D*+ open charm mesons, using data collected in 2015 at a proton-proton centre-of-mass energy of √s = 13 TeV. The second is a search for direct CP violation in two three-body decays of the Lambda_c charm baryon, pKK and ppipi, using data collected in 2011 at √s = 7 TeV and in 2012 at √s = 8 TeV. For each measurement, motivation and context are given from the standpoint of improving the theoretical understanding of the Standard Model and searching for signs of physics that cannot be explained by it, and then the various statistical analysis techniques used to extract physical quantities from the data are explained. The systematic limitations of the method are explored and quantified, and then the results are presented.

Orientador: Eduardo de Moraes Gregores. / Dissertação (mestrado) - Universidade Federal do ABC. Programa de Pós-Graduação em Física, 2012.

Estágio realizado no CERN e orientado Eva Sanchez-Corral Mena / Tese de mestrado integrado. Engenharia Informática e Computação. Faculdade de Engenharia. Universidade do Porto. 2008

Estágio realizado na ATLAS Distributed Computing Group e orientado por Markus Elsing / Tese de mestrado integrado. Engenharia Informátca e Computação. Faculdade de Engenharia. Universidade do Porto. 2008

Current estimates put Dark Matter to 26% of the energy-matter content of the universe, but very little is known about it other than its gravitational interactions. Eorts to learn more about Dark Matter include searching for it at high energy particle colliders. The lack of information about the nature of Dark Matter makes this a complicated task, and many searches are performed in dierent channels, and considering dierent theoretical models. In this thesis, I explore two such analyses, performed in the ATLAS collaboration using data from the ATLAS detector at the Large Hadron Collider at CERN: the tW+MET (missing transverse energy) nal state and the tt+MET nal state. I have made a generation-level study of the overlap between the signal regions used, and come to the conclusion that there is some. I have also compared the models used in these analyses, the 2HDM+a and the simplied spin-0 pseudoscalar model. Given the simplications made in my study, however, more sophisticated approaches should be used before anything conclusive can be said.

ATLAS est une expérience généraliste de physique des particules située sur le collisionneur LHC du CERN. Dans ce document, est résumée mon activité entre 2004 et 2009 au sein de la collaboration ATLAS, qui s'est articulée autour de 2 axes forts : - étude de la mise en route du calorimètre à argon liquide; - estimation du potentiel de découverte de nouveaux bosons de jauge massifs. Les perspectives pour les années 2010-2015, très prometteuses avec les premières collisions attendues sont également détaillées.

L'origine de la masse des particules élémentaires est liée au mécanisme de brisure de la symétrie électrofaible. Son étude sera l'un des enjeux majeurs de l'expérience Atlas auprès du Large Hadron Collider, au Cern à partir de 2008. Dans la plupart des cas, les recherches seront limitées par notre connaissance des performances du détecteur, telles que la précision avec laquelle l'énergie des particules est reconstruite ou l'efficacité avec laquelle elles sont identifiées. Ce mémoire d'habilitation présente un travail portant sur la reconstruction des électrons dans Atlas avec des données simulées et des données prises durant le test en faisceau combiné qui s'est déroulé en 2004. L'analyse des données d'Atlas nécessite l'utilisation de ressources de calcul et de stockage importantes qui a impliqué le développement d'une grille de calcul mondiale dont un des noeuds est développé au laboratoire. Le manuscrit présente aussi l'effort effectué au LPNHE Paris pour la mise en place d'un Tier 2 en r\égion Ile de France.