This thesis presents searches for a charged Higgs boson (H±) in proton-proton collisions with center-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. Multiple search channels are used with the common characteristic of at least one charged lepton (electron or muon) that effectively reduces the multi-jet background and is used for efficient triggering. Charged Higgs bosons decaying to a tau lepton and a neutrino are searched for using final states with two charged leptons, or one charged lepton and a hadronically decaying tau. A significant background originates from quark- or gluon-initiated jets that may be misidentified as hadronic tau decays. Methods to estimate this background are presented, including a largely data-driven matrix method. Signal processes with a charged Higgs boson mass below or above that of the top quark are considered. With the dataset collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb-1, upper limits at 95% confidence level are placed on the branching fraction B(t→bH±)×B(H±→τν) in the range 1.1–0.3% for charged Higgs boson masses between 80 GeV and 160 GeV, and on the top-quark associated H± production cross section in the range 0.93 to 0.03 pb for charged Higgs boson masses between 180 GeV and 1 TeV.

This thesis examines the possible creation of (4+n)-dimensional black holes at Large Hadron Collider (LHC) at CERN, consequent decays of such black holes via Hawking radiation and probable formation of Higgs boson among black hole decay products. Firstly, a theoretical background was presented including black hole physics, Hawking radiation, large extra dimensions, brane-bulk models, 4+n black holes and Higgs mechanism. Then, a simulation modeling black hole formation and decay including 130 GeV Higgs as a decay product at LHC interfaced with a detector simulation of Compact Muon Selenoid (CMS) was analysed focusing especially on the Higgs decay channels and properties of Hawking radiation. Both theoretical assumptions and simulation analysis point out that black hole production and the signatures of black hole decay products could carry crucial information on dimensionality and structure of spacetime Furthermore there is a significant possibility to observe 130 GeV Higgs boson especially in the Black Hole -&gt / H -&gt / jj and Black Hole -&gt / H &amp / #8211 / &gt / WW/ZZ -&gt / lnln decay channels.

The highly celebrated discovery of a new particle with a mass of 125 GeV in proton-proton collisions by the ATLAS and CMS experiments at the CERN Large Hadron Collider in 2012 has been shown to be compatible with the Standard Model description of the Higgs boson. However, in order to fully verify the Standard Model nature of the Higgs boson, most of its properties still remain to be measured. Such measurements include differential cross section measurements, which are shown here for the H→WW decay channel and the coupling of the Higgs boson to bottom quarks, for which a study of future prospects is presented. Differential fiducial cross section measurements of the Higgs boson were performed in the H → WW*→lvlv channel at the ATLAS detector with 20 fb−1 of √s = 8 TeV collision data. For Higgs bosons produced by gluon-gluon fusion, the cross section is measured as a function of kinematic variables, including transverse momentum and rapidity of the Higgs boson, as well as the number of jets associated with the Higgs event. The obtained distributions are unfolded to a fiducial volume using a two-dimensional iterative Bayesian algorithm. The measured fiducial differential cross sections are compared to predictions from Monte Carlo generators. The total cross section measured in the fiducial volume defined by the charged lepton and neutrino kinematic properties is 36.0 ± 9.7 fb. Additionally the jet-veto efficiency in the fiducial volume is extracted from the differential cross sections. An analysis is presented of Higgs boson production and decay into bottom quarks in association with a vector boson at the ATLAS detector for the future high-luminosity LHC with proton-proton collisions at √s = 14 TeV. The vector bosons are reconstructed from Z→l+l− or W→lv final states, where l is an electron or muon. The analysis uses generator-level Monte Carlo samples to which efficiency and resolution smearing functions are applied. These reproduce the expected resolution of the upgraded ATLAS detector for the foreseen amount of pile-up due to multiple overlapping proton-proton collisions. The analysis of the ZH(→ l+l−b¯b) channel is presented and results are combined with the WH(→lvb¯b) channel from a corresponding study. For an integrated luminosity of 300 fb−1 using an average pile-up of 60, the expected significance is 3.9 σ with an expected error on the signal strength of 25%. Likewise, for 3000 fb−1 using an average pileup of 140 the expected significance is 8.8 σ , and the error on the signal strength is expected to be about 15%.

No description available.

This thesis presents details of the search for the Standard Model Higgs boson, in the low mass region (100 ≤ MH < 150 GeV), with the ATLAS detector at CERN. In this range, a Higgs boson may be produced in association with a W or Z-boson and decay predominantly to two b-quarks (H → b¯b). Specifically events having large Higgs boson transverse momentum (pT > 200 GeV) and large recoiling W- boson transverse momentum (pT > 200 GeV) are considered as a means to reduce the contribution from background processes. In this high pT (boosted) regime, novel jet-substructure techniques are applied to the reconstructed jets resulting from the Higgs boson decay. In order to use these jet-substructure techniques, b-tagging efficiency scale factors in the jet-substructure regime have also been derived for the first time. Details of their derivation are presented for many b- tagging algorithms, with 14.3 fb¯¹ of ATLAS proton-proton collision data in 2012 at √s = 8 TeV. These, and their associated systematic uncertainties, are then applied to the Higgs boson search. No significant measurement of Higgs boson production was made, based on 20.4 fb¯¹ of ATLAS proton-proton collision data in 2012 at √s = 8 TeV. For a Higgs boson mass of 125 GeV, an exclusion limit of 6.12 x σSM was found at the 95% confidence level, and a signal strength of 0:93 ± 2:63 was measured, consistent with both background-only and signal (Standard Model Higgs boson) plus background hypotheses.

Le mécanisme de Higgs, introduit en 1964, propose une solution à un problème majeur du modèle standard de la physique des particules : l'origine de la masse. Ce mécanisme prédit l'existence d'un boson scalaire, de masse non prédite par la théorie et qui n'a encore jamais été observé expérimentalement (Juin 2012). Le Tevatron, un accélérateur hadronique basé à Fermi National Accelerator Laboratory près de Chicago, a permis la prise de données à l'aide des deux détecteurs CDF et DØ depuis 1983 jusqu'en septembre 2011. Accumulant une statistique de près de 10fb-1 à analyser. La production associée du Higgs et d'un boson vecteur est le canal principal de recherche pour un Higgs standard léger. A l'aide des données collectées par DØ, nous recherchons ce mode de production. La production du boson de Higgs étant très rare, nous avons développé des techniques sophistiquées de manière à améliorer la sensibilité au signal, telles que l'identification des jets de quarks beaux ou encore des méthodes basées sur des discriminants multivariés. Au final, une approche statistique nous permet de poser une limite supérieure sur le taux de production du Higgs observé (resp. attendu) rapporté aux prédictions du modèle standard. Les résultats obtenus dans le canal WH avec la statistique finale de l'expérience DØ au Tevatron sont de 3.15 (resp. 3.97) pour un boson de Higgs de 115 GeV/C2. / Higgs mechanism, introduced in 1964, gives a satisfactory solution to a major problem of the standard model of elementary particles : the origin of the mass. It predicts the existence of the Higgs scalar boson, which mass is not defined by the theory and which has not been discovered experimentally yet (June 2012). The Tevatron, a hadron accelerator based at Fermi National Accelerator Laboratory near Chicago, took data with its two multi-purpose detectors CDF and DØ since 1983 up to september 2011. Leaving about 10 fb-1 of statistics to analyze. Associated production of Higgs and vector gauge boson is the main search channel for a light standard Higgs boson. Using data collected by DØ, we are looking for this production mode taking advantage of sophisticated techniques to improve the signal sensitivity like b-jet identification and multivariate discriminants. In the end, a statistical approach allows us to set an upper limit on the ratio between the observed (resp. expected) Higgs production and its theoretical cross section. The results obtained in the WH channel using 9.7 fb-1 at DØ is 3;15 (resp. 3.96) for a 115 GeV/c2 Higgs boson.

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.

No description available.

This work presents a study of the discovery potential for the neutral supersymmetric Higgs bosons h/A/H decaying to tau pairs with the ATLAS experiment at the LHC. The study is based on Monte Carlo samples which are scaled to state-of-the-art cross sections. The analyses are designed assuming an integrated luminosity of 30 1/fb and a center-of-mass energy of sqrt(s) = 14 TeV. The results are interpreted in the mmax h benchmark scenario. Two final states are analyzed: The dileptonic channel where the two tau leptons decay to electrons or muons and the lepton-hadron channel where one tau decays to an electron or muon and the other tau decays to hadrons. The study of the dilepton channel is based completely on the detailed ATLAS simulation, the analysis of the lepton-hadron channel is based on the fast simulation. The collinear approximation is used to reconstruct the Higgs boson mass and its performance is studied. Cuts are optimized in order to discriminate the signal from background and to maximize the discovery potential given a certain Higgs boson mass hypothesis. In the lepton-hadron channel the selection is split into two analyses depending on the number of identified b-jets. Procedures to estimate the dominant backgrounds from data are studied. The shape and normalization of the Z to tautau background are estimated from Z to leptonlepton control regions. The ttbar contributions to the signal regions are estimated from ttbar control regions. The individual analyses are combined and sensitivity predictions are made depending on the Higgs boson mass mA and the coupling parameter tanβ. The light neutral MSSM Higgs bosons with mA = 150 GeV can be discovered when at least tanbeta = 11 is realized in nature. The heavy neutral MSSM Higgs bosons with mA = 800 GeV can be discovered for tanbeta ≥ 44. However, due to the large width of the reconstructed Higgs boson mass and the mass degeneration, only the sum of at least two of the three Higgs boson signals will be visible.

En Juillet 2012, les expériences ATLAS et CMS annoncent la découverte d'une nouvelle particule de masse 125 GeV, compatible avec le boson de Higgs prédit par le Modèle Standard. Pour établir la nature de ce boson de Higgs et la comparer au Modèle Standard, il est nécessaire de mesurer le complage du boson de Higgs au fermions. En particulier le quark top possède le plus fort couplage de Yukawa avec le boson de Higgs. Ce couplage est accessible par le processus de production d'un boson de Higgs en association avec une paire de quarks tops (ttH). Cette thèse présente la recherche d'évènement ttH où le boson de Higgs se désintègre en deux quark b dans les données du Run 2 recueillies en 2015 et 2016 par le détecteur ATLAS. La composition du bruit de fond ainsi que la mesure du signal ttH dans les données sont obtenues à partir d'un ajustemement statistique des prédictions aux données. Le bruit de fond tt+jets étant la plus grande source d'incertitudes sur le signal, une attention particulière est portée à sa description.La détection des jets issus de quarks b, appelé b-tagging, est primordiale pour l'analyse ttH(H->bb) dont l'état final contient quatre quarks b. Afin d'améliorer la compréhension des performances des algorithmes de b-tagging pour le Run 2, la définition des jets de saveur b dans les simulations Monte Carlo est revisitée. Les algorithmes standards du b-tagging ne permettant pas la différenciation des jets contenant un ou deux quarks b, une methode spécifique à été développée et est présentée dans cette thèse. / In July 2012, the ATLAS and CMS experiments announced the discovery of a new particle, with a mass about 125 GeV, compatible with the Standard Model Higgs boson. In order to assess if the observed particle is the one predicted by the Standard Model, the couplings if this Higgs boson to fermions have to be measured. In particular, the top quark has the strongest Yukawa coupling to the Higgs boson. The associated production of a Higgs boson with a pair of top quarks (ttH) gives a direct access to this coupling. The ttH process is accessible for the first time in the Run 2 of the LHC thanks to an upgrade of the detector and the increase of the center of mass energy to 13 TeV. This thesis presents the search for ttH events with the Higgs boson decaying to a pair of b-quarks using data collected by the ATLAS detector in 2015 and 2016. The description of the background and the extraction of the ttH signal in data are obtained by a statistical matching on predictions to data. In particular the tt+jets background is the main limitation to signal sensitivity and is scrutinized.The identification of jets originating from b-quarks, called b-tagging, is a vital input to the search of ttH(H->bb) events because of the four b-quarks in the final state. For Run 2 the definition of b-flavoured-jets in Monte Carlo simulations is revisited to improve the understanding of b-tagging algorithms and their performance. Standard b-tagging algorithms do not separate jets originating from a single b-quark from those originating from two b-quarks. Thus a specific method has been developed and is reviewed in this thesis.