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This thesis presents measurements of the branching fractions B(Ds -> lvl) and mea- surements of the pseudoscalar decay constant, fDs , using all the data acquired at the BaBar detector at SLAC National Accelerator Laboratory, which were collected from September 1999 to April 2007, and comprised 531 fb-1. The following mea- surements are made: B(Ds -> eve) = (6:11 +/-0:38 +/- 0:33) * 10-3, B(Ds -> tvt) = (5:06 +/- 0:34 +/- 0:50) * 10-2 , and a limit B(Ds -> eve) < 1:46 * 10-4 is obtained. Using these measurements a value of fDs = 252 +/- 6 +/- 7 +/- 1MeV is obtained, where the first uncertainties account for the statistical limitations of the data, the second uncertainties account for the systematic uncertainties, and the third uncertainties account for uncertainties associated with other physical constants (dominated by the lifetime of the Ds meson).

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Results of the search for the rare decay $B_s^0 \to \mu^+ \mu^-$ using data collected by the D0 detector at the Fermilab Tevatron collider are presented. This analysis covers the full Run II data set, corresponding to approximately 10.4~fb$^{-1}$ of integrated luminosity in $p\bar{p}$ collisions at a center of mass energy of 1.96 TeV. The analysis used new variables and a multivariate technique to improve the background reduction. After seeing fewer events than expected from background, a new Tevatron best observed limit was set on the branching fraction of the decay at $\cal{B}$($B_s^0 \to \mu^+ \mu^-$) $< 15 \times 10^{-9}$ ($12 \times 10^{-9}$) at the 95\% (90\%) C.L.

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L'objectif principal de cette thèse est de fournir une mesure aussi précise que possible de la masse du quark top dans son canal de désintégration semi-leptonique. Cette mesure expérimentale est réalisée à l'aide du détecteur ATLAS, installé auprès du LHC, un collisionneur proton-proton. Les motivations principales à cette mesure de précision sont les contraintes qu'elle apporte aux modèles de physique. Par ailleurs, la masse du quark top est un paramètre permettant d'estimer l'état de stabilité du vide dans le cadre du Modèle Standard, à l'échelle de Planck. Les jets sont des objets dont l'étalonnage en énergie est cruciale pour la mesure. Une étude détaillée de cette calibration est présentée, et notamment l'impact des conditions réelles de prises de données sur la performance de ces objets. La mesure de masse utilisant les données prises en 2011 à une énergie dans le centre de masse de 7 TeV est détaillée. Cette mesure utilise une méthode d'analyse particulière, appelée méthode des templates tridimensionnels. La masse du quark top mesurée est: mtop = 172.01 ± 0.92 (stat) ± 1.17 (syst) GeV. Les données prises en 2012 à une énergie dans le centre de masse de 8 TeV sont aussi analysées, et une mesure préliminaire de la masse du quark top est fournie: mtop = 172.82 ± 0.39 (stat) ± 1.12 (syst) GeV, la combinaison des deux mesures donnant le résultat le plus précis de cette thèse: mtop = 172.64 ± 0.37 (stat) ± 1.10 (syst) GeV. / The main goal of this thesis is to provide a measurement as accurate as possible of the top quark mass in the semi-leptonic decay channel. This experimental measurement is made thanks to the ATLAS detector near LHC, a proton-proton collider. The main interests for this precison measurement are the physics constraints to the theoretical models of fundamental constituents. Besides, the top quark mass is a parameter allowing to have more information on the vacuum stability at the Planck scale within the Standard Model. Jet energy calibration is crucial to this measurement. The impact of real data taking conditions on this calibration and on jet performance is detailed. The top quark mass measurement using 2011 data collected at an energy in the center-of-mass of 7 TeV is presented. It is using a tridimensional template analysis method. The measured top quark mass is: mtop = 172.01 ± 0.92 (stat) ± 1.17 (syst) GeV. The 2012 data collected at an energy in the center-of-mass of 8 TeV are also analysed, and a preliminary result for the top quark mass is provided: mtop = 172.82 ± 0.39 (stat) ± 1.12 (syst) GeV, the combination of both measurements being the most accurate result of this thesis: mtop = 172.64 ± 0.37 (stat) ± 1.10 (syst) GeV.

This thesis is an investigation of the subject of extra dimensions in particle physics. In recent years, there has been a large interest in this subject. In particular, a number of models have been suggested that provide solutions to some of the problem with the current Standard Model of particle physics. These models typically give rise to experimental signatures around the TeV energy scale, which means that they could be tested in the next generation of high-energy experiments, such as the LHC. Among the most important of these models are the universal extra dimensions model, the large extra dimensions model by Arkani-Hamed, Dimopolous, and Dvali, and models where right-handed neutrinos propagate in the extra dimensions. In the thesis, we study phenomenological aspects of these models, or simple modifications of them. In particular, we focus on Kaluza–Klein dark matter in universal extra dimensions models, different aspects of neutrino physics in higher dimensions, and collider phenomenology of extra dimensions. In addition, we consider consequences of the enhanced renormalization group running of physical parameters in higher-dimensional models. / QC 20120427

Made available in DSpace on 2015-05-14T12:14:04Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 639748 bytes, checksum: 09ac6e93733c265a01578d25967f780c (MD5) Previous issue date: 2009-10-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we develop some phenomenological aspects of a model based on the gauge symmetry SU(3)CNSU(3)LNU(1)N (311νR), that include righthanded neurinos in its matter content. We establish all the necessary structure for the study of the phenomenological aspects of the model. First, we obtain bounds on the angle that mix the gauge bosons Z and Z′, taking into account a requirement of the model, namely, that the number of fermionic families is exactly three. Also we implement the mass for both species of neutrinos (left-hand and right-handed) via type II seesaw mechanism, obtained adding to the model a sextet of scalars which will be responsible by the violation of the lepton number. The main consequence is the rising of a sterile neutrino with mass in the KeV range, satisfying all the cosmological and astrophysical constraints. This neutrino will a viable candidate for warm dark matter in the universe. / Nesta tese, desenvolvemos alguns aspectos relacionados `a fenomenologia de um modelo com simetria de gauge SU(3)CNSU(3)LNU(1)N (331νR), que possui neutrinos de m ao direita no seu conte´udo de mat´eria. Estabelecemos toda a estrutura necess´aria para o estudo dos aspectos fenomenol´ogicos do modelo. Primeiro, vamos obter v´ınculos no angulo que mistura os b´osons de gauge Z e Z′, levando em conta uma exig encia imposta pelo modelo, ou seja, que o n´umero de fam´ılias fermi onicas ´e exatamente tr es. Tamb´em implementamos termos de massa para ambas as esp´ecies de neutrinos (m ao esquerda e m ao dieita) via mecanismo seesaw tipo II, obtido adicionado ao modelo um sexteto de escalares, que ser´a respons ´avel pela viola¸c ao do n´umero lept onico. A principal consequ encia ´e o surgimento de um neutrino est´eril com massa na escala de KeV , satisfazendo todas as restri¸c oes cosmol´ogicas e astrof´ısicas. Esse neutrino ser´a um vi´avel candidato `a mat´eria escura morna do universo.