Composite Higgs models provide a well-motivated solution to the naturalness problem plaguing the Standard Model of particle physics. We study the collider phenomenology of a class of composite Higgs models with an underlying fermionic description. The Higgs appears as a pseudo Nambu-Goldstone boson (pNGB) originating from the spontaneous breaking of the global symmetry when the strongly interacting composite sector condenses. There will be further pNGBs, however, both colour singlets and coloured states. We collect and derive bounds on the masses of the coloured pNGBs for all popular cosets. For the purely electroweakly charged states we determine simplified model bounds on Drell-Yan pair production with decays into vector bosons or third generation quarks, covering all possible processes within this model class. As a complete model we study the SU(5)/SO(5) coset due to its rich particle content including a doubly charged scalar $S^{++}$. We first apply the simplified model bounds and then perform simulations of the full model to determine bounds on several mass scenarios. We show that this model features the process $pp \to S^{++} S^{--} \to W^+ t\bar b W^- \bar tb$. We design a search proposal for this process employing deep learning techniques to train neural networks to separate signal from background events. The best performing network is a combination of a convolutional neural network using jet images and a multilayer perceptron on kinematic variables. We assess the discovery reach and expected exclusion limit for this process at the high-luminosity LHC. The models also contain colour triplet fermionic resonances, required to give mass to the top quark through the mechanism of partial compositeness. We systematically classify the production and decay channels and derive bounds on pair production for a few sample processes. Finally we turn to the spin-1 resonances, focusing on the coloured states. Among them there is a ubiquitous vector octet $\mathcal V_8$ that mixes with the gluon, allowing for single production. For all models, we calculate the relevant Lagrangian, derive bounds on $\mathcal V_8$ production, and explore which signatures can be expected from vector pair production at future colliders. / Composite Higgs Modelle bieten eine gut motivierte Lösung des Natürlichkeitsproblems des Standardmodells der Teilchenphysik. Wir untersuchen die Beschleuniger-Phänomenologie einer Klasse von Composite Higgs Modellen mit zu Grunde liegender fermionischer Beschreibung. Das Higgs ist ein pseudo-Nambu-Goldstone-Boson (pNGB), das der spontanen Brechung der globalen Symmetrie entspringt, wenn der stark wechselwirkende Composite-Sektor kondensiert. Es gibt allerdings noch zusätzliche pNGBs, sowohl QCD-Singuletts als auch farbgeladene Zustände. Wir sammeln und bestimmen Ausschlussgrenzen auf die Massen der farbgeladenen pNGBs für alle Zustände in dieser Modellklasse. Für die nur elektroschwach geladenen Zustände bestimmen wir Beschränkungen auf vereinfachte Modelle für die Drell-Yan-Paarproduktion mit Zerfällen in Vektorbosonen oder Top- und Bottom-Quarks. Dabei decken wir alle in dieser Modellklasse möglichen Prozesse ab. Als konkretes Beispiel betrachten wir das SU(5)/SO(5) Modell, da es besonders viele Teilchen enthält, unter anderem einen doppelt geladenen Skalar $S^{++}$. Zuerst wenden wir die Beschränkungen auf vereinfachte Modelle an, dann führen wir Simulationen des ganzen Modells durch, um Beschränkungen in verschiedenen Massenszenarien zu bestimmen. Wir zeigen, dass dieses Modell auf den Prozess $pp \to S^{++} S^{--} \to W^+ t\bar b W^- \bar tb$ führt. Wir schlagen eine Strategie zur Suche nach diesem Prozess vor. Diese verwendet Deep-Learning-Methoden um neuronale Netze zu trainieren, die die Signalevents von den Hintergründen trennen sollen. Das beste Netzwerk ist eine Kombination aus einem Convolutional-Neural-Network, das Jet-Bilder verwendet, und einem Multilayer-Perceptron basierend auf kinematischen Variablen. Wir arbeiten heraus bis zu welcher Masse des Skalars der Prozess am High-Luminosity-LHC entdeckt oder ausgeschlossen werden kann. Die Modelle enthalten zudem fermionische Farbtriplett-Resonanzen. Diese sind notwendig, um dem Top-Quark mit dem Partial-Compositeness-Mechanismus seine Masse zu geben. Wir klassifizieren systematisch deren Zerfallskanäle und bestimmen Beschränkungen auf die Paarproduktion für einige ausgewählte Prozesse. Schließlich betrachten wir die Spin-1 Resonanzen, wobei wir uns auf die farbgeladenen Zustände konzentrieren. Unter ihnen gibt es in allen Modellen ein Vektor-Oktett $\mathcal V_8$, das mit dem Gluon mischt und daher einzeln produziert werden kann. Wir berechnen für alle Modelle den relevanten Teil der Lagrange-Dichte, bestimmen Beschränkungen auf die $\mathcal V_8$ Produktion und diskutieren welche Signaturen von der Paarproduktion von Vektoren an zukünftigen Beschleunigern zu erwarten sind.

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.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

This thesis describes a measurement of the Standard Model Higgs boson coupling to fermions in decays to two τ-leptons, a search for charged Higgs bosons in decays to a τ-lepton and a neutrino, as well as the reconstruction, identification and triggering of hadronically decaying τ-leptons. The data considered are collected by the ATLAS experiment at the Large Hadron Collider. The reconstruction and identification of hadronically decaying τ-leptons in the ATLAS experiment during Run 2 of the Large Hadron Collider are described, and the performance of the τ trigger is measured in events with top–antitop-quark pairs using data from 13 TeV proton–proton collisions, corresponding to an integrated luminosity of 3.2 fb-1, collected in 2015, and 11.5 fb-1, collected in 2016. Hadronically decaying τ-leptons are of importance to many physical processes involving Higgs bosons. The coupling of the Standard Model Higgs boson to fermions is measured in decays to two τ-leptons using 7 TeV data corresponding to an integrated luminosity of 4.5 fb-1, collected in 2011, and 8 TeV data corresponding to an integrated luminosity of 20.3 fb-1, collected in 2012. The signal strength is measured to be μ = 1.4, corresponding to an excess over the background-only model of 4.5σ. Charged Higgs bosons are searched for in decays to a τ-lepton and a neutrino, where the τ-lepton decays hadronically. 13 TeV data corresponding to an integrated luminosity of 14.7 fb-1, collected in 2015 and 2016, are used. No excess over the Standard Model background is observed, and the 95 % confidence-level exclusion limits on σ(pp → [b]tH+) × (H+ → τν) are set to 2.0 pb–8 fb in the range 200–2000 GeV.

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.

In this thesis I explore the possibilities of detecting doubly charged Higgs bosons from the Higgs Triplet Model (HTM) at the Large Hadron Collider (LHC) at CERN. Higgs bosons are included into the Standard Model (SM) of particle physics in order to explain the origin of mass of the elementary particles. Even though the SM is considered to be a reliable starting point for any particle theory, no Higgs particles have to this day been found. There are therefore high expectations for the record-breaking energies of the LHC to lead to a Higgs discovery. The HTM produces seven different Higgs bosons, and among these we find the doubly charged ones. Because of their exotic charge, I focus on studying these Higgs particles in particular. To this purpose, I implement the full HTM theory as an alternative model option in standard particle physics Monte-Carlo software and then simulate LHC proton-proton collisions at a center of mass energy of 14 TeV. The investigated signal is defined as two like-signed leptons, four hard jets and missing energy in the final state. The main production mechanisms are pair-production and associated production with a singly charged Higgs. Since I choose to study a region of parameter space where the triplet vacuum expectation value is relatively large, the doubly charged Higgs decays into W's and the singly charged Higgs into WZ or tb. The results of the simulations show that the LHC could probe Higgs masses up to at least 300 GeV with an integrated luminosity of about 300 fb.1.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Propomos um processo alternativo para a produção difrativa do bóson de Higgs inspirado no Modelo KMR. Neste modelo, exploramos a produção do bóson de Higgs através da interação γ*p em alta energia importando do Modelo KMR a proposta de Troca Dupla de Pomerons entre as partículas do espalhamento. Em ordem dominante, modelamos esta interação através da troca de glúons, como o Pomeron da QCD. Utilizando este vértice de produção, inserimos os fatores de forma de Sudakov a fim de suprimir a emissões de glúons por bremsstrahlung. Com isso, aplicamos este modelo para interações γp, subprocesso de interesse para o estudo de produção em colisões periferais entre prótons. Neste sentido, estimamos a seção de choque de produção do bóson de Higgs para energia de Tevatron e LHC, comparando diversas opções de distribuições de glúons no próton. Através destes resultados efetuamos uma comparação com aqueles obtidos previamente pelo Modelo KMR. Como resultado, observamos um comportamento distinto daquele observado no Modelo KMR, mostrando um crescimento da seção de choque com a massa do bóson de Higgs. Pela ação dos fatores de forma de Sudakov, este crescimento é saturado em um determinado valor de massa dependendo da energia de centro-de-massa do processo. No intervalo de massa onde se espera observar o bóson de Higgs, a taxa de eventos do processo γp se mostrou superior aquela observada no Modelo KMR. / We propose an alternative process for the diffractive Higgs boson production inspired in the KMR Model. In this model, we explore the Higgs boson production through the γ∗p interaction in the high-energy limit, importing from the KMR Model the proposal of Double Pomeron Exchange between the scattering particles. In leading order, we shape this interaction through gluon exchange, as the QCD Pomeron. Utilizing this production vertex, we insert the Sudakov form factors to supress the bremsstrahlung gluon emission. Therefore, we apply this model do γp interactions, a subprocess of interest in the study of peripheral collisions beetwen protons. In this sense, we estimate the production cross section of the Higgs boson for energies of Tevatron and LHC, comparing several options of parton distributions in the proton. Thus, through this results we make a comparison with those obtained previously with the KMR model. As a result, we observe a distinct behavior as those observed in the KMR model, showing a growth of the cross section with the mass of the Higgs boson. With the action of the Sudakov form factors this growth is saturated at a determined value of mass depending of the center-of-mass energy of the process. In the range of mass where the observation of the Higgs boson is expected the event rate of the γp process showed a greatest value those observed in the KMR model.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Apresenta-se neste trabalho um estudo sobre a largura de decaimento total do bóson de Higgs através do canal H&#8594; ZZ &#8594; (4e, 4, 2e2). Segundo o Modelo Padrão da Física de Partículas Elementares, um bóson de Higgs com massa de 126 GeV deve ter uma largura de decaimento total &#915;H = 4.15 MeV, muito abaixo da resoluções dos experimentos instalados no LHC. Isto impede uma medida direta sobre os eventos da ressonância. Recentemente foi proposto limitar &#915;H a partir da relação entre a taxa de eventos observados na região da ressonância e na região off-shell. Utilizando o pacote de análise desenvolvido pela colaboração CMS obteve-se um limite de &#915;H < 31.46(12.82) MeV em 95(68.3)% CL combinando os dados coletados pelo LHC em colisões pp em &#8730;s = 7 TeV (5.1fb-1) e em &#8730;s = 8 TeV (19.7fb -1). / We present in this work a study about the Higgs boson total width using the channel H&#8594; ZZ &#8594; (4e, 4, 2e2. According to the Standard Model of Elementary Particle Physics, the Higgs boson with mass around 126 GeV should have a total decay width of &#915;H = 4.15 MeV, very below the resolution of the experiments installed at the LHC. This fact prevents a direct measurement on the events of the Higgs resonance. Recently it was proposed limit &#915;H from the relationship between the rate of events observed in the resonance and the off-shell regions. Using the package of analysis developed by CMS collaboration was obtained a limit of &#915;H < 31.46(12.82)MeV in 95(68.3)% CL combining the data collected by the LHC in pp collisions at &#8730;s = 7 TeV (5.1fb-1)and at &#8730;s = 8 TeV (19.7fb -1).

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