L’expérience ATLAS (A Toroidal LHC Apparatus), collaboration de plus de 3000 scientifiques dans le monde, mène des recherches auprès du grand collisionneur de hadron (LHC). Celui-ci a produit des collisions de faisceaux de protons à une énergie dans le centre de masse de 7 TeV en 2011 et 8 TeV en 2012, période appelée Run1 et représentant 20.3 fb−1 de luminosité intégrée, puis en 2015 et 2016 à 13 TeV pour le Run 2 avec 36.1 fb−1. Le détecteur généraliste ATLAS a un riche potentiel de mesures précises du Modèle Standard (SM) et de recherche de phénomènes de nouvel physique. Deux analyses de physiques sont ici présentées. La première consiste à rechercher dans les données Run 1 la production de tri-bosons, W±W±W∓, se désintégrant totalement ou partiellement en leptons et à étudier le couplage de jauge quartique anormal (aQGC). Le nombre d’événements observés est en accord avec les prédictions du SM. La limite supérieure observée à 95% CL sur la section efficace W±W±W∓ SM est de 730 fb avec une limite attendue de 560 fb en l’absence de production W±W±W∓. La deuxième analyse présentée est la recherche du boson de Higgs doublement chargé effectuée sur les données du Run 2. Ce boson est prédit par un modèle qui prolonge le SM pour permettre des neutrinos massif. Ce modèle introduit plusieurs bosons de Higgs dont l'un est le H±±. Le cas où ce dernier se désintègre en bosons W avec un état final à deux leptons de même charge est plus particulièrement étudié. Le bruit de fond total estimé est en accord avec les données et aucun excès significatif n’est observé. Des limites supérieures sont déduites et le modèle considéré est exclus à 95% CL pour MH±± < 220 GeV. / The ATLAS (A Toroidal LHC Apparatus) experiment, a collaboration of more than 3,000 scientists worldwide, is conducting research at the Large Hadron Collider (LHC). It produced proton beam collisions at an energy in the center of mass of 7 TeV in 2011 and 8 TeV in 2012, period called Run1 and representing 20.3 fb-1 of integrated luminosity, then in 2015 and 2016 at 13 TeV for Run 2 with 36.1 fb-1. The ATLAS general purpose detector has a rich potential for accurate Standard Model (SM) measurements and the search for new physics phenomena. Two physics analyzes are presented here. The first is to search the Run 1 data for tri-bosons production, W ± W ± W∓, totally or partially decayed into leptons and to study abnormal quartic gauge coupling (aQGC). The number of observed events is consistent with the SM predictions. The observed upper limit at 95% CL on the W ± W ± W∓ SM cross section is 730 fb with an expected limit of 560 fb in the absence of W ± W ± W∓ production. The second analysis presented is the search for the doubly charged Higgs boson carried out on the Run 2 data. This boson is predicted by a model that extends the SM to allow massive neutrinos. This model introduces several Higgs bosons, one of which is the H ±±. The case where this latter decays in bosons W with a final state with two leptons of the same charge is more particularly studied. The estimated total background is in agreement with the data and no significant excess is observed. Upper limits are deduced and the model considered is excluded at 95% CL for MH ±± <220 GeV.

This thesis describes searches for a heavy charged Higgs boson decaying into a top and bottom quark pair, and the development of a hardware track trigger with theATLAS experiment. The data for the two searches was collected with the ATLAS detector at the Large Hadron Collider(LHC) with pp collision energies of √s = 8 and 13 TeV, and corresponds to an integrated luminosity of 20.3 and 13.2 fb-1 respectively. The main background for this signal is the production of tt̄ pairs with additional heavy flavor radiation. The searches with a single lepton in the final state found no evidence of a charged Higgs boson, and set 95% CLS upper limits on the production times branching ratio for masses ranging between 200-1000 GeV. The preparation of using the final state with two leptons in future searches is discussed. The design of a hardware track trigger based on pattern matching and linear track fitting was studied for the purpose of reducing the high event rates of the High-Luminosity LHC, which is expected to provide pp collisions with a luminosity about five times the nominal value, in the second half of the 2020’s. A simulation framework was developed to emulate the pattern matching and was used to test its ability to filter hits in high pile-up environments. The results of this simulation, together with simulations of the track fitting and latency, show that such a track trigger is a viable option for the ATLAS experiment in the High Luminosity-LHC era.

his thesis implements a multivariate analysis in the current cut- based search for the charged Higgs bosons, which are new scalar particles predicted by several extensions to the Standard Model. Heavy charged Higgs bosons (mH± mtop) produced in association with a top quark de- caying via H± → τν are considered. The final state contains a hadronic τ decay, missing transverse energy and a hadronically decaying top quark. This study is based on Monte Carlo samples simulated at CM-energy √ s = 13 TeV for signal and backgrounds. The figure of merit to measure the improvement of the new method with respect to the old analysis is the separation between the signal and background distributions. Four mass points (mH± = 200, 400, 600, 1000 GeV) are considered, and an increase of the separation ranging from 2.6% (1000 GeV) to 29.2% (200 GeV) com- pared to the current cut-based analysis is found. / Denna studie implementerar en flervariabel-analys till den befintliga snitt-baserade analysen av laddade Higgs-bosoner, nya skal ̈arpartiklar fo ̈rutsagda av flertalet fo ̈rl ̈angningar av Standardmodellen. Studien antar tunga lad- dade Higgs-bosoner (mH± mtop) producerade tillsammans med en top- kvark som fo ̈rfaller via H± → τν. Sluttillst ̊andet best ̊ar av ett hadroniskt τ-so ̈nderfall, f ̈orlorad transversell energi och en hadroniskt so ̈nderfallande √ toppkvark. Studien a ̈r baserad p ̊a data f ̈or signal och bakgrund. Fo ̈r att ma ̈ta fo ̈rba ̈ttringen av analysens ka ̈nslighet anva ̈nds avst ̊and mellan bakgrundens och signalens distribu- tioner som godhetstal. Fyra masspunkter (mH± = 200, 400, 600, 1000 GeV) anva ̈nds, och en o ̈kning av avst ̊and fr ̊an 2.6% (1000 GeV) till 29.2% (200 GeV) hittades.

Le travail scientifique présenté dans cette thèse s’appuie sur des collisions proton proton, à 13 TeV dans le centre de masse, produite par le grand collisionneur de Hadron (LHC) et enregistrées entre 2015 et 2017 par le détecteur ATLAS. Après une brève introduction sur le programme de physique du LHC, le premier chapitre donne un aperçu complet du contexte et des motivations théorique. Ensuite une description complète du détecteur ATLAS et plus généralement de l’ensemble expérimental est donnée. Le troisième chapitre traite d’une des mesures des performances du calorimètre électromagnétique ATLAS, à savoir l’étude des efficacités identification électronique à l’aide de W se désintégrant en électrons et neutrinos.Dans les chapitres suivants, le coeur de cette thèse, la recherche et l’étude de la production associée de boson de Higgs avec une paire de quark top, appelée production ttH, est traitée dans le schéma du modèle Standard et au-delà. L’observation de la production ttH avec le détecteur ATLAS a été récemment annoncée en 2018 et représente une étape importante pour le domaine de la physique des hautes énergies. Afin d’élargir le spectre de cette analyse tth au-delà du SM, cet état final du canal 4L a aussi été étudié dans le cadre de la recherche de bosons de Higgs doublement chargés se désintégrant en W$^{\pm}$W$^{\pm}$, H$^{\pm \pm} \rightarrow $ W$^{\pm}$W$^{\pm}$. En conclusions, les perspectives sur les futurs résultats scientifiques dans le modèle Standard (SM) ainsi que au-delà de SM (BSM) qui pourraient être atteint en utilisant les statistiques complètes des Run1 et Run2 combinés sont donnés. / The scientific work presented in this thesis is based on proton proton collision produced, at 13 TeV in the center of mass, created by the Large Hadron Collider (LHC) and recorded between 2015 and 2017 by the ATLAS detector.After a short introduction on the physics program at LHC, the first chapter gives a comprehensive overview of the theoretical context and motivation. Then a complete description of the ATLAS detector and more generally the experimental setup is given with a highlight on the different physics objects used in this research.The third chapter is dealing with a measure of the performances of the ATLAS electromagnetic calorimeter, namely the study of the electron identification efficiencies using W decaying into electrons and neutrinos events.In the following chapters, the core of this thesis work, the search and study of the associated Higgs boson production with top quark pair, so-called ttH production, within the Standard Model schema and beyond is treated. The observation for ttH production with the ATLAS detector recently announced in 2018 represents a significant milestone for the High-Energy Physics field. In order to broaden the spectrum of this ttH analysis beyond the SM, this 4L channel final states was also studied in the context of the search for doubly charged Higgs boson decaying into W±W±, H±±→W±W±.In conclusions, future prospects in Standard Model (SM) as well as Beyond SM (BSM) scientific results that could be reached using the full Run1 and Run2 combined available statistics are given.

For more than 30 years the Standard Model has been the theoretical foundation for particle physics. The theory has been verified successfully by experimental tests. Its biggest shortcoming is the non-discovery of the Higgs boson,responsible for giving the other particles masses. Despite its success there are hints that the Standard Model is not the complete theory and many extensions of it, such as supersymmetry, have been proposed. Extended theories often predict the existence of a charged Higgs boson and its detection will be a clear sign of physics beyond the Standard Model. The main focus in this thesis is on various phenomenological aspects of the charged Higgs boson. For favorable mass and couplings direct detection is shown to be possible at the Large Hadron Collider in production with an associated W boson. It is also shown how a light charged Higgs can have measurable effects on spin correlations in decays of pair-produced top quarks. The charged Higgs boson can also be seen indirectly, in for example B-meson decays, which can be used to put constraints on its mass and fermion couplings. Exclusion limits in two supersymmetric models are given together with a comparison with the discovery potentials for the LHC experiments. A tool for calculating properties, such as masses and decays, of both charged and neutral Higgs bosons in the Two-Higgs-Doublet Model is also presented. B-meson decays can also be used to test aspects of the strong interaction. Part of this thesis deals with improving and applying phenomenological models to B-meson decays. Although these models are not derived from first principles, their success shows that they capture important features of non-perturbative strong interactions.

In dieser Arbeit wird die Suche nach geladenen Higgs Bosonen (H±) vorgestellt. Dafür wurden Proton–Proton Kollisionen, die bei einer Schwerpunktsenergie von 13 TeV in den Jahren 2015 und 2016 mit einer integrierten Luminosität von 36.1 fb−1 mit dem ATLAS-Experiment produziert wurden, untersucht. Die Existenz solcher geladener Higgs Bosonen wird in verschiedenen Modellen jenseits des Standardmodells vorhergesagt und ist auch dadurch motiviert, das dass Standardmodell nicht immer eine Erklärung für verschiedenste beobachtete Phänomene liefern kann. Diese Arbeit konzentriert sich auf geladene Higgs Bosonen, die eine höhere Masse als das top Quark besitzen und über H± → tb zerfallen. Die H± Produktion, in Verbindung mit einem top und einem bottom Quark pp → tbH±, wird im Massenbereich von 200 bis 2000 GeV untersucht. Die Suche nach H± in dieser Arbeit beschränkt sich auf Endzustände mit einem geladenen Lepton und mehreren Jets. Mit Hilfe eines boosted decision trees werden verschiedenste kinematische Variablen miteinander kombiniert, um dadurch das H± Signal besser von dem Standardmodell Untergrund unterscheiden zu können. Es wurde kein signifikanter Unterschied zum vorhergesagten Standardmodell-Untergrund gemessen und deshalb Ausschlussgrenzen für den Produktionswirkungsquerschnitt mal Verzweigungsverhältnis dieses H± Zerfalls berechnet. Weitere Ausschlussgrenzen wurden für den tanβ Parameter des MSSM Modells für die mhmod− und hMSSM Benchmark-Szenarien bestimmt. Die Ergebnisse dieser Arbeit übertreffen alle bisherigen Ausschlussgrenzen - insbesondere werden zum ersten Mal Erkenntnisse über den Massenbereich bis zu 2000 GeV gewonnen. / This thesis presents a search for charged Higgs bosons produced in proton–proton collisions at a centre-of-mass energy of 13 TeV, using 36.1 fb−1 of data collected by the ATLAS detector at the LHC in 2015 and 2016. The existence of charged Higgs bosons is predicted by various theories Beyond the Standard Model and it is motivated by the inadequacy of the Standard Model to explain some observed experimental phenomena. The work focuses on charged Higgs bosons heavier than the top quark and decaying via H± → tb. The production in association with a top and a bottom quark (pp → tbH±) is investigated in the mass range between 200 and 2000 GeV. A final state containing one charged lepton and jets is considered. Multiple kinematic variables are combined using a boosted decision tree (BDT) in order to separate signal and background. The output of the BDT is used to perform a profile likelihood fit of the Monte Carlo predictions to the observed data. No significant excess of events above the expected Standard Model background is observed, therefore upper limits are set for the cross-section of the charged Higgs boson production times the branching fraction of its decay. Limits are also provided for the tanβ parameter of the MSSM, in the mhmod− and hMSSM benchmark scenarios. The work improves the reach of all previous searches, including for the first time masses ranging up to 2000 GeV.

<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.

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. 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. 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. 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. 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. 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.