At the dawn of the Large Hadron Collider era, the Brout-Englert-Higgs mechanism remains the most appealing theoretical explanation of the electroweak symmetry breaking, despite the fact that the associated fundamental scalar boson has escaped any direct detection attempt. In this thesis, we consider a particular extension of the minimal Brout-Englert-Higgs scalar sector implemented in the Standard Model of strong and electroweak interactions. This extension, which is a specific, "twisted", realisation of the generic two-Higgs-doublet model, is motivated by a relative phase in the definition of the phenomenologically successful CP and custodial symmetries. Considering extensively various theoretical, indirect and direct constraints, this model appears as a viable alternative to more conventional scenarios like supersymmetric models, and gives grounds to largely unexplored possibilities of exotic scalar signatures at present and future collider experiments.

At the dawn of the Large Hadron Collider era, the Brout-Englert-Higgs mechanism remains the most appealing theoretical explanation of the electroweak symmetry breaking, despite the fact that the associated fundamental scalar boson has escaped any direct detection attempt. In this thesis, we consider a particular extension of the minimal Brout-Englert-Higgs scalar sector implemented in the Standard Model of strong and electroweak interactions. This extension, which is a specific, "twisted", realisation of the generic two-Higgs-doublet model, is motivated by a relative phase in the definition of the phenomenologically successful CP and custodial symmetries. Considering extensively various theoretical, indirect and direct constraints, this model appears as a viable alternative to more conventional scenarios like supersymmetric models, and gives grounds to largely unexplored possibilities of exotic scalar signatures at present and future collider experiments.

We review the important features of Little Higgs models, illuminating the mechanisms that generate a naturally light Higgs field while avoiding excessive fine-tuning. The full spectrum of the Littlest Higgs model is analyzed in light of the recent discovery of the Higgs boson mass. We find viable regions in parameter space that are consistent with the standard model. / text

The Standard Model scalar potential contains a minimum at the Electroweak scale, responsible for the masses of the weak gauge bosons through the Higgs mechanism. However, if the Electroweak minimum is only a local minimum, and there exists a global minimum at a higher energy in the Higgs potential, then in a su ciently old universe we would expect the vacuum expectation value to be at the global minimum. The absence of a global minimum at higher energy is related to the condition that the Higgs self coupling is greater than or equal to zero for all energies. For any model that fails this, we expect new physics to enter before the energy at which the coupling becomes negative. We developed tools to automate the derivation of beta functions for renormalisable gauge theories, and used these to carry out evolution of the renormalisation group equations for the Standard Model and three extensions to the Standard Model | the Standard Model with a fourth generation, the Standard Model with right-handed neutrinos and a Left-Right Symmetric Model. We conclude that of these four models, the Standard Model is the only one in which all the couplings remain perturbative, and in which the Electroweak minimum is a global minimum.

In this paper the higgs mechanism for the standard model is constructed in steps. First by considering spontaneous breaking of discrete and continuous global gauge invariance. Then spontaneous breaking of local gauge invariance. These results are then used to construct the electroweak part of the standard model through application of the higgs mechanism. Finally, the LQT-upper bound of 1 TeV for the higgs mass is calculated through unitarity constraints. / I denna artikel konstrueras higgsmekanismen i standardmodellen stegvis. Först genom att beakta spontant symmetribrott av diskreta samt kontinuerliga globala gaugeinvarianser. Därefter spontant symmetribrott av lokala gaugeinvarianser. Dessa resultat används sedan för att konstruera den elektrosvaga delen av standardmodellen genom tillämpning av higgsmekanismen. Slutligen beräknas en övre gräns för higgsmassan, den så kallade LQT-gränsen, via unitaritetsbegränsingar.

The Standard Model of particle physics is the best existing theory for describing the interactions between elementary particles. Even though the Standard Model has been confirmed in many experiments, there remain unanswered questions. One of the main questions is how fermions and most gauge bosons get masses; the Standard Model begins with them as massless. The Higgs effect is a mechanism to explain how fermions and several gauge bosons do get masses in the Standard Model. The corresponding Higgs boson is the only particle that has not yet been detected. This Thesis gives a complete review of the Higgs effect and Higgs related topics. It starts with theoretical basics and develops the theory of the Higgs effect within the electroweak section of the Standard Model. The discussion then considers the topics of radiative corrections and the effect of the Higgs boson as a virtual particle, concentrating on the example of the rho-parameter. In addition, experimental and theoretical constraints for the Higgs mass M_H will be given with special emphasis on the Hierarchy Problem which leads to a physically unacceptable Higgs mass when using high energies (of the Grand Unification scale) as a cutoff for the radiative corrections. Furthermore experimental attempts to detect the Higgs boson at LEP2, TEVATRON and LHC will be described and the different decay channels discussed. Finally, alternative theoretical models beyond the Standard Model are motivated and presented, such as supersymmetry, a vectorlike Standard Model and a possible relation between the Higgs and the Inflaton of Cosmology.

After a long history of searches, a Higgs boson H was discovered by the ATLAS and the CMS experiments at the Large Hadron Collider (LHC) in 2012. Its properties fit well the ones predicted by the Standard Model (SM) of particle physics. However, the SM can not explain other established properties of Nature, such as the existence of Dark Matter. For this reason, models beyond the SM should be considered. Such models often predict the existence of several Higgs bosons and this thesis explores some of those models. In particular, the possibility to discover a charged Higgs boson, which would be a clear sign of physics beyond the SM, is studied. A commonly studied extension of the SM is the framework of two-Higgs-doublet models (2HDMs), where there are five Higgs bosons. By confronting the parameter spaces of some 2HDMs with publically available data from the LHC, the prospects for finding the 2HDM Higgs bosons is presented through the calculation of production cross sections and decay branching ratios in various channels. A new kind of 2HDM, called the Stealth Doublet Model is presented and the properties of the Higgs bosons are studied. In this model, it is shown that in particular the properties of the charged Higgs boson H± have new features not exhibited in earlier studied models. Within the parameter space compatible with the LHC results, the production cross section for H± can be sizeable enough to be experimentally observed. Finally, the discovery prospects at the LHC, for a H± in the pp → tH± process, with the decays H± → HW± and H → bb, is studied in various models beyond the Standard Model. It is shown that for the supersymmetric models, this channel is beyond the discovery reach of the LHC. In some of the other studied models, in particular the Aligned 2HDM, the situation is improved and the channel is feasible.

The minimal renormalizable SO(10) Higgs model with the unified gauge sym- metry broken down by the adjoint representation 45S is known to suffer from tachyonic instabilities along all potentially realistic symmetry breaking chains. Few years ago, this issue has been identified as a mere relic of the tree-level cal- culations and the radiative corrections to the masses of the pair of the "most dangerous" pseudo-Goldstone scalars transforming as (8, 1, 0) and (1, 3, 0) with respect to the SU(3)c × SU(2)L × U(1)Y Standard model gauge group were com- puted. Remarkably enough, it turns out that in the minimal potentially realistic renormalizable realization of the model - consisting of 45S ⊕ 126S scalar and 45G gauge fields - there is third pseudo-Goldstone scalar, a full singlet with respect to the Standard model gauge group that, until recently, happened to escape the community's attention. In this thesis we computed the one-loop corrections to its mass employing two different methods within the effective potential approach. In both cases we reduced the complexity of our calculations by decomposition into the Standard model irreducible representations. In the end, we cross-checked the resulting formulae in four distinct symmetry breaking limits in which the pseudo- Goldstone mass was degenerated with those of...

The aim of this project is to understand the structure of the Standard Model of the particle physics. Therefore quantum field theories (QFT) are studied in the both cases of abelian and non-abelian gauge theories i.e. quantum electrodynamics (QED), quantum chromodynamics (QCD) and electroweak interaction are reviewed. The solution to the mass problem arising in these theories i.e. spontaneous symmetry breaking is also studied. / Syftet med detta projekt är att förstå strukturen för partikelfysikens standardmodell. Därför studeras kvantfältsteorier (QFT) i båda fallen av abelska och icke-abelska gaugeteorier, dvs kvantelektrodynamik (QED), kvantkromodynamik (QCD) och elektrosvag växelverkan granskas. Lösningen på massproblemet som uppstår i dessa teorier, dvs. spontant symmetribrott studeras också.

La notion de symétrie de jauge est au coeur de notre compréhension de l'interaction électrofaible et permet d'expliquer l'ensemble des observations expérimentales actuelles. Pourtant, l'incompatibilité intrinsèque entre l'invariance de jauge et la masse des particules nécessite d'introduire une nouvelle particule, le boson de Higgs, toujours non observée à ce jour. Cette thèse présente l'analyse de 7.3/fb de collisions protons-antiprotons à sqrt{s} = 1.96 TeV enregistrées par le détecteur D0 au Tevatron en vue de la recherche du boson de Higgs dans l'état final mu+tau. Cette analyse vient compléter les canaux principaux dimuons, électron-muon et diélectrons en exploitant également la désintégration H -> WW -> lvlv, majoritaire dans la fenêtre de masse accessible au Tevatron. L'état final contenant un lepton tau, leur identification parmi les jets a été améliorée d'environ 15% grâce au développement de plusieurs idées : l'ajustement des paramètres du réseau de neurones d'identification, la prise en compte de certaines dépendances cinématiques des performances de l'algorithme, l'exploitation du temps de vie du lepton tau et une étude exhaustive visant à inclure la mesure du détecteur de pieds de gerbe dans le processus d'identification. Dans un second temps, la recherche du boson de Higgs dans l'état final mu+tau étant dominée par le bruit de fond W+jets (où un jet est faussement identifié comme un lepton tau), une méthode a été élaborée pour obtenir une modélisation convenable de ce bruit de fond, non fournie par la simulation par défaut. Cette méthode est basée, entre autres, sur l'étude de la corrélation de charge entre le muon et le candidat tau qui permet de mesurer ce bruit de fond dans les données en excluant la région du signal. Ensuite, l'exploitation des cinématiques et/ou topologies différentes du signal et du bruit de fond a permis d'optimiser cette recherche atteignant alors une sensibilité attendue (observée) de 7.8 (6.6) fois le Modèle Standard pour une masse de 165 GeV/c2. Enfin, l'interprétation de l'analyse dans un scénario à quatre familles de fermions a été effectuée. Pour la première fois, cette analyse est incluse dans les combinaisons D0 et Tevatron présentées à Moriond EW et EPS 2011. / The gauge symmetry is the heart of our understanding of the electroweak interaction and describes all the current experimental results. However, the intrinsic incompatibility between the gauge invariance and the mass of particles leads to the introduction of a new particle, the Higgs boson, for which we have no experimental evidence as of today. This thesis describes the Higgs boson search in the mu+tau final state in 7.3/fb of protons-antiprotons collisions at sqrt{s} = 1.96 TeV collected by the D0 detector at the Tevatron. This analysis completes the golden channels (dimuons, electron-muon, dielectrons) exploiting the decay chain H-> WW ->lvlv, which is the main Higgs boson decay mode in the mass window accessible to the Tevatron. Since the final state includes a tau lepton, work was done to improve their identification among jets. An increase of 15% was achieved thanks to the the following : changing tuning parameters for the tau identification neural network, use of the kinematical dependence of the algorithm performances, incorporation of the tau lepton life time information and full study of the additionnal information coming from the central preshower measurements. Then, since the dominant background of the mu+tau Higgs boson search is W+jets (where one jet fakes a tau), a method was developed to obtain good modeling of this background, not provided by the default simulation. This method is based, among other things, on the charge correlation between the muon and the tau candidate which allows for calibration of this background in the data excluding the signal region. Finally, all the kinematic and/or topological differences between the signal and the background were exploited to optimize this search, reaching an (observed) expected sensitivity of 7.8 (6.6) times the Standard Model for mH = 165 GeV/c2. In addition, this result was also interpreted in a fourth fermion generation scenario. For the first time, this analysis is included in the D0 and Tevatron combinations, both presented at Moriond EW and EPS 2011.