Dans cette thèse la mesure du rapport des sections efficaces de production de bosons W et Z en association avec un jet au LHC est effectuée.Le rapport des sections efficaces est mesuré à l'aide des données issues de collisions proton-proton au LHC à une énergie dans le centre de masse de 7 TeV, recueillies par l'expérience ATLAS en 2010, correspondant à une luminosité intégrée d'environ 40 pb-1. L'analyse est effectuée à l’aide de bosons W et Z se désintégrant dans le canal muon.Il s'agit de la première mesure du rapport des taux de production de bosons W et Z associés à un jet réalisée à une si haute énergie dans le centre de masse, la mesure est la également la première effectuée au LHC. Cette mesure constitue un test très important des calculs théoriques perturbatifs et tire avantage des annulations partielles ou totales de diverses incertitudes expérimentales.La mesure sert aussi pour les recherches de nouveaux phénomènes de physique au LHC à contraindre les bruits de fonds ayant les mêmes signatures topologiques.La valeur du rapport de mesuré est présentée en fonction de différents seuils d’impulsion transverse de jet cumulatifs et est trouvé, a l’intérieur de la précision statistique, en bon accord avec prédictions pertubatives de QCD aux LO et NLO. / In this thesis an investigation of the ratio of the production cross section of W and Z bosons in association with one jet at LHC is performed.The cross-section ratio is measured with LHC proton-proton collision data at a center-of-mass energy of 7 TeV, collected by the ATLAS experiment in 2010, corresponding to an integrated luminosity of approximately 40 pb-1. The analysis is performed selecting W and Z bosons decaying in the muon channel.It is the first time the ratio of W and Z bosons production associated with one jet is measured at such a high center-of-mass energy, the measurement is the first one at the LHC. This measurement constitutes a very important test of perturbative theoretical calculations based on the advantage of the partial and full cancellations of various uncertainties.It also serves as constraining backgrounds with similar topological signatures in the search for new physics phenomena at the LHC. The measured ratio value is presented as a function of cumulative jet transverse momentum thresholds and found, within statistical precision, to agree well with perturbative LO and NLO QCD predictions.

This thesis considers the search of charged Higgs bosons, which are predicted by several extensions of the Standard Model of particle physics. Light charged Higgs bosons (below the top quark mass) can appear in top quark decays and are assumed to decay exclusively to tauons. Two discriminating variables are presented that are sensitive to such a process taking place in top-antitop events with two final state leptons. Distributions of these variables are computed for Monte Carlo simulations and for 35/pb of data from 7 TeV proton-proton collisions recorded in 2010 by the ATLAS experiment at the LHC. Monte Carlo simulations are found to agree well with data, validating the use of these discriminating variables, but no conclusions about the existence of charged Higgs bosons can be made at this point.

A search for the Higgs boson decaying to tau tau using 7.8 fb^-1 of pp collisions at 1.96 TeV collected with CDF II detector is presented. The search is sensitive to four production mechanisms of the Higgs boson: ggH, WH, ZH and VBF. Modes where one tau decay leptonically, and another decay, hadronically, are considered. Two novel techniques are developed and used in the search. A Probabilistic Particle Flow Algorithm is used for energy measurements of the hadronic tau candidates. The signal is discriminated from backgrounds by the Missing Mass Calculator, which allows for full invariant mass reconstruction of tau tau pair. The data are found to be consistent with the background only hypothesis. Therefore a 95% confidence level upper limit on the Standard Model Higgs boson cross section was set. At M_H=120 GeV/c^2 observed limit is 14.9 x sigma_SM x Br(H -> tau tau).

A search for a light charged Higgs boson (H^±) decaying into two jets (c¯s) using pp collisions at s = sqrt(7 TeV) is presented. A data sample corresponding to an integrated luminosity of 4.7 fb^−1 recorded by the ATLAS detector in 2011 has been analysed. The search is performed in the semi-leptonic t¯t channel, where one of the top quarks decays via t --> bH^±. The other top quark decays to bW, where the W boson decays to a lepton (e/μ) and a neutrino. The search is based on the invariant mass distribution of the two light jets in the final state as the discriminating variable. With no observation of an H^± signal, 95% confidence level(CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons. These limits are between 5% and 1% for charged Higgs boson masses between 90 and 150 GeV, and can be considered as model-independent limits on the decay branching ratio of top quarks to any charged boson beyond the Standard Model.

La thèse décrit une étude de l'état final en paire de leptons tau dans le mode de désintégration semi-leptonique en muons et hadrons en utilisant des données proton-proton collisions à une énergie de centre de masse de 7 TeV avec le détecteur CMS au CERN Large Hadron Collider. La performance de l'algorithme de reconstruction et d'identification des leptons tau est étudiée en utilisant un échantillon de données de collisions proton-proton à $ \ sqrt $ = 7 TeV, ce qui correspond à une luminosité intégrée de 36 pb $ ^ {-1} $. Les leptons tau qui se désintègrent en un seul hadron chargé, avec ou sans hadrons neutres sont reconstruits à l'aide de la technique dite du flot de particules pour la reconstruction des objets en utilisant un nouvel algorithme adapté aux leptons tau et appelé le Hadrons Plus Strips (HPS) qui utilise le trajectographe et le calorimètre électromagnétique de CMS. L'efficacité de l'algorithme de reconstruction est mesurée en utilisant les leptons taus produits lors de la désintégration de bosons Z. Le taux de mauvaise identification de leptons tau se désintégrant de manière hadronique pour les jets produits en association avec un boson W est également déterminé. La première mesure inclusive de production Z $ \ rightarrow \ tau \ tau $ dans les collisions pp au le LHC est présentée dans les états finals muon+jets en utilisant un échantillon de données de 36 pb $ ^ {-1} $. La section efficace mesurée est en bon accord avec les prédictions QCD à l'ordre doublement suivant l'ordre dominant (NNLO). Après avoir identifié le boson Z dans le mode de désintégration di-tau, une recherche inclusive de bosons de Higgs neutres du modèle supersymétrique standard minimal (MSSM) dans les collisions pp est réalisée à une énergie dans le centre de masse de 7 TeV. Les résultats sont basés sur un échantillon de données correspondant à une luminosité intégrée de 36 pb $ ^ {-1} $ et de 4,6 $ fb ^ {-1} $ enregistré par l'expérience CMS dans l'année 2010 et 2011 respectivement. La recherche utilise les désintégrations de bosons de Higgs en paire de leptons tau. Aucun excès n'est observée dans le spectre de masse invariante de paires de leptons tau. Les limites supérieures sur le produit de la section efficace de production de boson de Higgs par le rapport de branchement de la désintégration en paire de leptons tau en fonction de la masse du boson de Higgs pseudoscalaire permettent de donner des limites strictes dans l'espace des paramètres du MSSM. / The thesis describes a study of the tau-pair final state in the semi-leptonic decay mode into muon and hadrons using proton-proton collisions data at a center-of-mass energy of 7 TeV using the CMS detector at the CERN Large Hadron Collider. The performance of tau-lepton reconstruction and identification algorithm is studied using a data sample of proton-proton collisions at $\sqrt{s}$ = 7 TeV, corresponding to an integrated luminosity of 36 pb$^{-1}$. The tau leptons that decay into one charged hadron with or without the association of neutral hadrons is reconstructed using Particle-Flow object reconstruction technique with a novel tau identification algorithm called the Hadron Plus Strips (HPS) algorithm in the CMS tracker and electromagnetic calorimeter. The reconstruction efficiency of the algorithm is measured using $\tau$ leptons produced in Z-boson decays. The hadronically decaying tau lepton misidentification rate for jets produced in association with a W boson is also determined. The first measurement of inclusive Z$\rightarrow\tau\tau$ production in pp collisions at the LHC is presented in muon+hadrons final state using a data sample of 36 pb$^{-1}$. The measured cross-section is in good agreement with the next-to-next-to-leading order (NNLO) QCD prediction. After establishing the Z boson in di-tau decay mode, an inclusive search for neutral minimal supersymmetric standard model (MSSM) Higgs bosons in pp collisions is performed at a center-of-mass energy of 7 TeV. The results are based on a data sample corresponding to an integrated luminosity of 36 pb$^{-1}$ and 4.6 fb$^{-1}$ recorded by the CMS experiment in the year 2010 and 2011 respectively. The search uses decays of the Higgs bosons to tau pairs. No excess is observed in the tau-pair invariant-mass spectrum. The resulting upper limits on the Higgs boson production cross-section times the branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, yield stringent bounds in the MSSM parameter space.

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.

A measurement of the W+ and W− cross section in the electron channel is presented. The measurement is performed using data collected with the ATLAS experiment in 2011, amounting to an integrated luminosity of 4.58 fb⁻¹. The centre of mass energy is √s = 7 TeV. The measurement is performed differentially in the electron pseudorapidity. The W charge asymmetry is also presented. The results are compared to theoretical predictions calculated at next-to-next-to-leading order in perturbative QCD using various Parton Distribution Functions.

In the Standard Model of particle physics, the non-zero masses of the W and Z bosons and the fermions are generated through interactions with the Higgs field, excitations of which correspond to Higgs bosons. Thus, the experimental discovery of the Higgs boson is of prime importance to physics, and would confirm our understanding of fundamental mass generation. This thesis describes a search for the gg → H → WW → lνlν process of Higgs boson production and decay. It uses the LHC Run I dataset of pp collisions recorded by the ATLAS detector, which corresponds to an integrated luminosity of 4.5 fb⁻¹ at √s = 7 TeV and 20.3 fb⁻¹ at √s = 8 TeV. An excess of events is observed with a significance of 4.8 standard deviations, which is consistent with Higgs boson production. The significance is extended to 6.1 standard deviations when the vector boson fusion production process is included. The measured signal strength is 1.11^+0.23<sub style='position: relative; left: -2.1em;'>-0.21</sub> at m_H = 125 GeV. A cross section measurement of WW production, a major background to this search, is also presented using the √s = 7 TeV dataset only.

Phenomenological models have long been employed by nuclear physicists to explain systematic trends in data. The Geometrical Model of Bohr, Mottelson and Rainwater (GM) and the Interacting Boson Model (IBM) are two such models that have been used to study the spectra of even-even nuclei. The IBM differs from previous boson models in that the total number of bosons is conserved and finite. In the GM the bosons of lowest angular momentum have ℓ = 2 and are taken to represent quadrupole shape vibrations, whereas in the IBM the bosons are generally taken to have ℓ = 0, 2 and can be interpreted as correlated pairs of fermions. These models have been extended to handle the neighboring odd-even nuclei by considering the interaction of a fermion with the bosonic space. If the fermionic space consists of the single-particle angular momenta j₁, j₂, ..., then the largest group describing this mixed system of bosons and fermions is the product group Uᴮ(5) x Uᶠ(m(j)) (GM) or Uᴮ(6) x Uᶠ(m(j)) (IBM), where m(j) = Σ(2jᵢ + 1). If one of the subgroups of Uᴮ(5) or Uᴮ(6) is isomorphic to one of the subgroups of Uᶠ(m(j)), then we can combine the two group chains into a common bose-fermi group chain. These combined bose-fermi groups have been used extensively in the Interacting Boson-Fermion Model (IBFM) to study odd-even nuclei and have been claimed as evidence for the existence of supersymmetries; however, the superalgebras associated with these supersymmetries were never identified. We have identified, for the first time, the superalgebras that are associated with some of these combined bose-fermi symmetries. This superalgebra, the non-compact orthosymplectic superalgebra Osp(4s+2/2,R), is fundamentally different than those previously used in the IBFM, where the product algebra was simply embedded into the superalgebra U(6/m(j)). The U(6/m(j)) superalgebras do not imply any particular coupling scheme, and hence cannot be associated with any particular one of the combined bose-fermi algebras. The last few chapters are devoted to a study of coherent states for the non-compact orthosymplectic supergroups Osp(1/2N,R) and Osp(2/2N,R), although the results generalize rather easily to the compact versions of these supergroups. These coherent states, besides being of mathematical interest, form the basis for a study of Osp(M/2N,R) coherent states. (Abstract shortened with permission of author.)

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