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No description available.

The Large Hadron Collider (LHC) and the four major experiments set up along its 27 kilometers of circumference (ATLAS, CMS, ALICE and LHCb), have recently started to explore the high–energy frontier at √s = 8 TeV, and will move to even higher energy in just about 2 years. The aim of physics searches at LHC experiments was to complete the picture of the Standard Model (SM) of elementary particles with the discovery of the Higgs boson and to look for specific signatures of models extending the current understanding of particle interactions, at zero and non–zero temperature. In 2012, the official discovery of the Higgs boson, the only missing particle of the StandardModel, was announced by ATLAS and CMS. Other important results include the measurement of rare decay modes in heavy quarks systems, and indications of CP violation in charm decays by LHCb. Signatures of beyond the Standard Model (BSM) physics are currently being looked for in the experimental data, and this often requires the knowledge of quantities that can be computed only with non–perturbative methods. This thesis focuses on some possible extensions of the SM and the analysis of interesting physical observables, like masses or decay rates, calculated using non– perturbative lattice methods. The approach followed for the main part of this work is to model BSM theories as effective field theories defined on a lattice. This lattice approach has a twofold advantage: it allows us to explore non– renormalizable gauge theories by imposing an explicit gauge–invariant cutoff and it allows us to go beyond perturbative results in the study of strongly interacting systems. Some of the issues of the SM that we will try to address include, for example, the hierarchy problem and the origin of dynamical electroweak symmetry breaking (DEWSB). We investigate non–perturbatively the possibility that the lightness of the mass for an elementary scalar field in a four–dimensional quantum field theory might be due to a higher–dimensional gauge symmetry principle. This idea fits in the Gauge–Higgs unification approach to the hierarchy problem and the results we present extend what is known from perturbative expectations. Extra dimensional models are also often used to approach DEWSB. Another approach to DEWSB implies a new strongly interacting gauge sector that extends the SM at high energies and it is usually referred to as Technicolor. The phenomenological consequences of Technicolor can only be studied by non– perturbative methods at low energy since the theory is strongly coupled at large distances. We perform a comprehensive lattice study of fermionic and gluonic scalar bound states in one of the candidate theories for Technicolor BSM physics. We relate our findings to the nature of the newly discovered Higgs boson. New physics is also commonly believed to be hidden in the flavour sector of the SM. In this sector, lattice calculations of non–perturbative input parameters are needed in order to make precise predictions and extract signals of possible new physics. In particular, heavy quark physics on the lattice is still in development and it is important to understand the relevant discretisation errors. We describe a preliminary study of the mixing parameter of heavy–light mesons oscillations in a partially–quenched scenario, using staggered dynamical fermions and domain wall valence fermions.

The Higgs sector is the collection of fields and particles responsible for the spontaneous symmetry breaking of the electroweak symmetry. It is the keystone of the Standard Model of particle physics. While the Standard Model Higgs sector is in agreement with current experiments alternative models often arise to explain experimental anomalies, or to answer puzzles about the Higgs model itself. In this dissertation I explore two such alternative models of the Higgs sector. The first is a model of a composite Higgs boson that is designed to be "minimally fine-tuned." I demonstrate how it generates a light Higgs boson with one fine-tuned parameter. The most accessible expected phenomenological signatures of such a model are heavy resonances decaying into weak vector bosons. I compare the predicted behavior of these resonances to recent experiments at the Large Hadron Collider. The second is alternative model attempts to use multiple Higgs to explain a possible 30 GeV resonant excess in dimuon production arising from Z boson decays. I show that the simplest such model cannot explain the excess, and then argue that all such multiple-doublet models also fail.

Since the observation of a narrow mass resonance consistent with the Higgs boson by ATLAS and CMS collaborations in 2012, a number of important studies have been made in order to understand the properties of the newly discovered particle. The most fundamental precision measurements include the Higgs coupling to other particles and itself; properties that have a direct relation with the total decay width. The theoretical total width of the Standard Model (SM) Higgs boson is extremely small (4.2 MeV), making its direct measurement by the LHC experiments non feasible due to the finite experimental detector resolution. Recent publications have shown a novel way to set an indirect limit on the total Higgs boson width by using measurements of both off-shell and on-shell production. This thesis presents a determination of the off-shell Higgs boson coupling, and a further interpretation of the Higgs total width in the H → ZZ → 4l channel (l = e; μ). The results are based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb-1 at a collision energy of √s = 8 TeV. Using the CLs statistical method and assuming the same higher-order QCD corrections applied for both signal and background processes, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is 7.3 (with the yields normalized to the SM expectation). Similarly, the 95% CL upper limit on the Higgs total width is 24.7 MeV. The LHC will undergo its last big upgrade in 2021, in preparation for the high-luminosity LHC Run (HL-LHC), with a luminosity increase of approximately a factor of 5 beyond its nominal design rate. Raising the muon transverse momentum threshold becomes a necessity in order to maintain a low online selection rate with the existing trigger system, at the cost of a reduced efficiency for the electroweak scale physics. An alternative to this approach is a proposed design of a first-level hardware trigger that uses tracking information. Being able to use tracking information at the first level of the ATLAS trigger in the implementation of a muon isolation algorithm offers an extra handle for differentiating between signal and background. The second part of this thesis presents studies on the performance of tracking-based muon isolation designed for a first-level hardware trigger system. These studies demonstrate the improved trigger performance of the muon isolation algorithm when compared to an increase of the transverse momentum threshold of the muon candidates.

No description available.

Le travail de cette thèse a été mené au sein de la collaboration CMS (Compact Muon Solenoid), une des quatre experiences en cours d'installation au LHC (Large Hadron Collider) du CERN. La thèse se focalise sur le calorimètre électromagnétique de CMS: les performances du detecteur sont étudiées en détail en utilisant soit des données de tests réalisés sur faisceau d'électrons soit une simulation complète du détecteur. Une étude du bruit de fond électronique dans la chaîne de lecture a conduit à l'élaboration de deux méthodes de reconstruction de l'amplitude des signaux, l'une en cas de saturation de la chaîne et l'autre en cas de bruit de fond cohérent. La thèse presente aussi une étude des méthodes de reconstruction des électrons dans CMS, qui a conduit à la première définition dans CMS de la qualité de mesure d'un électron, à une procédure pour établir l'échelle d'énergie des électrons et à la combinaison des informations du calorimètre et du trajectomètre pour obtenir une estimation optimale de l'impulsion de l'électron au vertex. Ces resultats sont appliqués à l'analyse du potentiel de découverte de l'Higgs dans le canal de désintegration H->ZZ(*)->4e pour lequel une te! chnique standard de selections séquentielles ainsi qu'une technique optimale utilisant des reseaux de neurones sont proposées.

En utilisant les outils de Monte Carlo et les données de test en faisceau, la performance d'un détecteur au futur collisionneur linéaire international a été étudiée. La contribution de cette thèse porte sur deux parties; d'une part sur une mesure de précision de la masse du boson Higgs et de la section efficace de la production avec le processus e^+e^- → HZ où le boson Z se désintègre en paire μ^+μ^− et d'autre part sur une analyse des données de test en faisceau de la collaboration CALICE (CAlorimeter for Linear Collider Experiment). Pour un Higgs de 120GeV, nous avons obtenu une précision de 38.4MeV sur la masse de Higgs et de 5% sur la section efficace en choisissant une énergie dans le centre de masse optimale de 230 GeV et avec une luminosité intégrée de 500 fb^-1. Ces résultats sont indépendants d'un modèle de Higgs donné puisque aucune information sur la désintégration du Higgs n'a été utilisée dans l'analyse. Si on suppose que le Higgs est celui du modèle standard ou il se désintègre principalement en particules invisibles, la précision peut être améliorée de façon significative (29MeV pour la masse et 4% pour la section efficace). Pour l'analyse des données de test en faisceau, mon travail concerne deux aspects. Premièrement une vérification sur la qualité des données en temps quasi réel et deuxièmement une mesure précise sur la résolution angulaire d'une gerbe électromagnétique dans le calorimètre prototype utilisé dans le test en faisceau. Le but pour la vérification de la qualité des données est de détecter des problèmes éventuels sur l'ensemble du détecteur y compris l'électronique, le système de haute tension et d'acquisition, et de classer des différentes données pour faciliter les analyses offlines. Pour déterminer la résolution angulaire du calorimètre électromagnétique, nous avons développé un algorithme qui est basée uniquement sur le dépôt d'énergie dans différentes cellules produites par le faisceau d'électrons sans utilisant l'information du détecteur de trace devant le calorimètre. Celle-ci est importante pour pouvoir identifier le composant neutre d'un jet. Nos résultats montrent que la dépendance de la résolution angulaire en énergie du faisceau est similaire à celle de la résolution en énergie et peut être décrite par (74/√(E/GeV)＋ 8.7)mrad.

<p>Conventional searches for the charged Higgs boson using its production in association with Standard Model (SM) quarks is notoriously weak in the mid-tanB range. Hoping to find an alternate channel to fill this gap, the production of the charged Higgs boson in association with supersymmetric squarks is studied. Using Monte Carlo generators the production at the LHC is simulated within the non universal Higgs mass model (NUHM). If the six parameters of the model (m₀, m_1/2, A₀, tanB, u, m_A) induce small masses of the stop, sbottom and charged Higgs, the production cross section can be of the order pb. Through scans of the input parameter the cross section is maximized, with the requirement that the stop decays directly to a neutralino - simplifying detection, in the point (m₀, m_1/2, A₀, tanB, u, m_A) = (190, 187, -1147, 179, 745, 13.2) where the cross section is 559 fb.</p><p>The production is compared to the irreducible backgrounds stop, stop, t, tbar and t, tbar + 2 jets. The former poses no severe constraints and can be easily removed using appropriate cuts. The latter, SM background, has a cross section almost 1000 times larger and strong cuts must be imposed to suppress it. Neglecting hadronization and systematic effects, we show that a 5 sigma discovery is possible at 133 fb^-1. In this range, mH+ = 194 GeV and tanB = 13.2, other channels have little or no prospects of detecting the charged Higgs and the studied process shows good prospects for complementing charged Higgs searches at the LHC in the mid-tanB range.</p>

Conventional searches for the charged Higgs boson using its production in association with Standard Model (SM) quarks is notoriously weak in the mid-tanB range. Hoping to find an alternate channel to fill this gap, the production of the charged Higgs boson in association with supersymmetric squarks is studied. Using Monte Carlo generators the production at the LHC is simulated within the non universal Higgs mass model (NUHM). If the six parameters of the model (m0, m1/2, A0, tanB, u, mA) induce small masses of the stop, sbottom and charged Higgs, the production cross section can be of the order pb. Through scans of the input parameter the cross section is maximized, with the requirement that the stop decays directly to a neutralino - simplifying detection, in the point (m0, m1/2, A0, tanB, u, mA) = (190, 187, -1147, 179, 745, 13.2) where the cross section is 559 fb. The production is compared to the irreducible backgrounds stop, stop, t, tbar and t, tbar + 2 jets. The former poses no severe constraints and can be easily removed using appropriate cuts. The latter, SM background, has a cross section almost 1000 times larger and strong cuts must be imposed to suppress it. Neglecting hadronization and systematic effects, we show that a 5 sigma discovery is possible at 133 fb-1. In this range, mH+ = 194 GeV and tanB = 13.2, other channels have little or no prospects of detecting the charged Higgs and the studied process shows good prospects for complementing charged Higgs searches at the LHC in the mid-tanB range.