Dissertations / Theses on the topic 'Higgs Sector'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student submitted PDF version of thesis.
Includes bibliographical references (p. 67-75).
In this thesis, we are investigating cosmological implications of hidden sector models which involve scalar fields that do not interact with the Standard Model gauge interactions, but couple directly to the Higgs field. We particularly focus on their relic particle density as a candidate for dark matter. For the case of hidden sector without a gauge field we have improved the accuracy of the bounds on the coupling constant and give bounds on the Lagrangian parameters. Models with Abelian and non-Abelian gauge fields are also studied with relic density bounds, BBN and galactic dynamics constraints. Several discussions on phase transitions and alternative dark matter candidates are included.
by Serkan Cabi.
Ph.D.

The Standard Model is regarded as one of the most successful scientific theories, but there is compelling evidence that it is an incomplete theory of particle physics. There is currently no understanding of the observed baryon asymmetry, the nature of dark matter, and dark energy. Field theoretic considerations indicate parameters in the Standard Model are extremely fine-tuned. This suggests the existence of new physics, accessible at higher energies, to explain these seemingly unnatural tunings. to solve these puzzles, and others not addressed by the Standard Model, many extensions of the Standard Model have been proposed. It is of great importance that we fully understand the effects these models have on Standard Model physics and how these theories can be tested. in this dissertation we explore the phenomenology associated with beyond the Standard Model physics, specifically focusing on models with extended Higgs sectors. in chapter 2, we study two classes of lepton flavor violating two Higgs doublet models. Chapter 3 explores a model where the Higgs is a pseudo-Goldstone boson of a non-abelian orbifold projection and all other low energy states carry no Standard Model charges. Chapters 4 and 5 study Higgs phenomenology in the context of a warped five dimensional space-time. The former analyzes the effects of Higgs-radion mixing in two Higgs doublet models. The latter reviews a previous proposal where the Higgs doublet takes the role of a bulk stabilizer. The result is a model containing a single Higgs-radion state. in the final chapter, we present a higher derivative extension of the type-I and II two Higgs doublet models. The resulting theory gives rise to unusual partner states containing negative kinetic energy terms.

El descobriment del bosó de Higgs és la culminació d’una recerca que ha durat 40 anys, i completa un marc teòric sota el qual gairebé totes del dades obtingudes de col·lisionadors de partícules poden ser explicades consistentment. Simultàniament, i paradoxalment, l’aparent con rmació d’una separacó d’escales entre l’electrofeble i la que suposadament estabilitza la masa del bosó de Higgs posa el relleu el problema de la jerarquia. En la recerca d’una descripció del món que simpli qui els patrons i simetries del Model Estàndard, l’exploració de l’escala dels TeVs i en particular l’estudi del bosó de Higgs tindran un paper central. En aquesta tesi presentem les possibilitats que el LHC i futurs col·lisionadors proveiran, amb énfasi en la determinació de l’auto-acoblament del bosó de Higgs. També considerarem l’estudi de la producció de bosons electrofebles com a eina per entendre les interaccions dels bosons de Golstone que formen part del doblet de Higgs, mostrant la seva rellevància al combinar-ho amb les dades del pol de la Z i altres cerques al LHC. Finalment, ens centrem en com el moment dipolar elèctric de l’electró posa fortes restriccions en models on el bosó de Higgs és descrit com un estat compost.
The discovery of the Higgs boson culminates a 40-year long hunt and completes a theoretical framework under which almost all collider data can be consistently explained. At the same time, paradoxically, the con rmation of an apparent mass gap above the electroweak scale exacerbates the problem of the electroweak hierarchy. In the search of a description of the world that simpli es the patterns and symmetries of the Standard Model, the exploration of the TeV scale and in particular the study of the Higgs boson will play a central role. In this thesis we explore the possibilities that the LHC and future colliders will bring, with particular emphasis on the determination of the Higgs self-coupling. We also consider the pair production of electroweak bosons as a probe of the Goldstone interactions in the Higgs doublet, and show its relevance as a way to improve the Z pole measurements and its interplay with other LHC searches. Finally, we focus on how the electron’s electric dipole moment can set strong constraints on models where the Higgs arises as a composite state.

After reviewing the reasons to believe that the road to shed light to Beyond Standard Model physics pass through a better understanding of the Higgs sector, we outline two complementary approaches along this main line: either tackling the phenomenological challenge to extract as much information as possible from collider experiments, or undertaking a theoretical investigation of new physics scenarios in order to gain knowledge on possible explanations to the Higgs mysteries. In the first part of the dissertation, we adopt a phenomenological approach and discuss the general problem of including infrared Standard-Model-like corrections in a model-independent new physics characterisation of Higgs decay and production processes. We deal first with infrared electromagnetic corrections (bremsstrahlung), providing a compact formalism to generally describe fermion bremsstrahlung in leading logarithmic approximation and then exemplifying its effectiveness through the “h -> 2e 2mu” case study. Subsequently we switch to a discussion of double-logarithmic mass singularities stemming from the electroweak sector, which affects among the others also Higgs production processes in the high-energy regime. We provide here a detailed analysis of such mass singularities in generic new physics scenarios for the case study of Higgsstrahlung. In the second part of the dissertation, we adopt a complementary approach and perform a theoretical study of high-scale supersymmetry. We exploit the highly constrained structure of the Higgs sector in such theories, together with other of their important features such as gauge coupling unification and radiative breaking of electroweak symmetry, to draw intriguing conclusions regarding the viability of such theories in the plane of Higgs and top-quark masses.
Per svelare i misteri della fisica oltre il Modello Standard si deve innanzitutto capire appieno il Modello Standard stesso ed in particolare il settore di Higgs, di gran lunga il meno compreso. In questa dissertazione delineiamo due possibili approcci a tale riguardo: si può assumere un’atteggiamento fenomenologico, cercando di parametrizzare in maniera ottimale gli esperimenti ad LHC, oppure un punto di vista più formale cercando di aumentare la nostra comprensione teorica del settore di Higgs (nella speranza di comprenderne i misteri). Nella prima parte di questa dissertazione, affrontiamo la questione delle correzioni infrarosse nella fisica degli acceleratori di particelle. Il nostro obiettivo è una descrizione generale di tali correzioni, da poter utilizzare in modo “model-independent” quando si cerca di caratterizzare la fenomenologia di nuova fisica ad LHC. Innanzitutto ci focalizziamo sulle correzioni elettromagnetiche (bremsstrahlung), fornendo un formalismo compatto e generale per descrivere la bremsstrahlung da correnti fermioniche neutre. Esemplifichiamo poi i nostri risultati tramite l’esempio del decadimento “h -> 2e 2mu”. In seguito ampliamo la nostra discussione per includere le correzioni elettrodeboli, limitandoci ai cosiddetti contributi doppio-logaritmici (importanti per i processi di produzione dell’Higgs) e concentrandoci sul caso di produzione associata Higgs-Z e Higgs-W. Nella seconda parte di questa dissertazione adottiamo un approccio complementare e effettuiamo uno studio teorico di una classe di teorie supersimmetriche chiamate “high-scale SUSY”. Sfruttando la struttura estremamente rigida del settore di Higgs in tali teorie, traiamo conclusioni molto interessanti riguardo la loro fattibilità nel piano (“massa dell’Higgs”, “massa del top”).

Complete one-loop results for the decay widths of neutral Higgs bosons ( h(_a)) into lighter neutral Higgs bosons ( h(_b), h(_c). ) arc presented for the MSSM with complex parameters. The results are obtained in the Feynman-diagrammatic approach, taking into account the full dependence on the spectrum of supersymmetric particles and all complex phases of the supersymmetric parameters. The genuine triple-Higgs vertex contributions are supplemented with two-loop propagator-type corrections, yielding the currently most precise prediction for this class of processes. The genuine vertex corrections turn out to be very important, yielding a large increase of the decay width compared to a prediction based on the tree-level vertex. One-loop propagator-type mixing between neutral Higgs bosons and Goldstone and Z bosons is also consistently taken into account. Complete one-loop results for the decay of a neutral Higgs boson into fermions are also presented, which include the full dependence on complex phases. The new results are used to analyse the impact of the experimental limits from the LEP Higgs searches on the parameter space with a very light MSSM Higgs boson. It is found that a significant part of the parameter space of the CPX benchmark scenario exists where channels involving the decay h(_2)→ h(_1) h(_1) have the highest search sensitivity, and the existence of an unexcluded region with M(_h1) ~ 45 GeV is confirmed. The public code Higgs Bounds is also presented, which can be used in conjunction with models with an arbitrary number of neutral Higgs bosons to determine whether parameter points have been excluded at the 95% CL by the LEP and Tevatron Higgs searches.

Avec la découverte en juillet 2012 par les collaborations CMS et ATLAS d'un nouveau scalaire, aux propriétés similaires à celles d'un boson prédit plus de 50 ans auparavant, la phénoménologie au-delà du modèle standard (SM) a fait un grand pas. En effet, la découverte de cette pièce manquante du puzzle électrofaible a rendu certains modèles impossibles, tout en renforçant la crédibilité du SM. Cependant, on sait que le SM n'est pas le mot de la fin, car certaines de ses prédictions ne s'accordent pas avec les expériences, et ce boson est donc vu comme une porte vers de nouveaux secteurs de la physique. Après une introduction au SM et aux statistiques de la physique des particules, nous présenterons un paramétrage des couplages mesurés au LHC, afin de contraindre, via ces couplages, des modèles de physique au-delà du SM, plus fondamentaux. Ce paramétrage, s'intéressant particulièrement aux couplages à boucles, permettrait de lever une corrélation existant entre les paramètres actuellement utilisés. Nous étudierons ensuite la possibilité d'existence d'un scalaire, plus léger que celui observé, et ayant échappé aux recherches passées, et de sa recherche au LHC à l'aide de ce paramétrage. Nous donnerons des exemples de paramètres faisant apparaître un tel scalaire, dans le 2HDM et le NMSSM. Enfin, nous verrons que le scalaire peut influencer la production d'une paire de bosons de jauge faible, et qu'à l'aide de notre paramétrage, l'analyse de ce processus peut être utilisée de manière cohérente avec les mesures au pic de masse pour contraindre ces couplages. Nous finirons avec quelques limites de cette approche, ainsi que des propositions d'améliorations
With the discovery of a new particle in July 2012 by the CMS and ATLAS experiments, with properties resembling those of a boson predicted about 50 years earlier by theorists, the search for physics outside of the standard model of particle physics (SM) has made a big step. Indeed, the existence of this piece of the electroweak symmetry breaking proved wrong many theories, while giving more credit to the SM. However, because of some observations it fails to match, we know the SM cannot be the final word, and it is hoped that the study of this new particle will be a path to new sectors of physics. After a brief description of the SM and of the statistics of particle physics, we will present a way to parametrise the couplings measured at CERN, in order to constrain, through these couplings, more fundamental models describing physics beyond the standard model. This parametrisation, focusing on loop-induced couplings, would allow to lift a correlation existing between parameters currently used by experimentalists. We will then study the feasibility of a search for another scalar, lighter than the one discovered and which would have escaped previous searches, through the use of the same parametrisation. In two models, the 2HDM and the NSSSM, values of parameters yet unconstrained are shown.Finally, we will see that this scalar can also influence the joint production of two weak gauge bosons. We will show that the analysis of this process can be used in a coherent manner with on-shell Higgs measurements to constrain its couplings through our parametrisation. We will then end with a brief look at some limitation of this approach, before suggesting ways to overcome them

The Standard Model (SM) of elementary particles, a model experimentally verified to an un-precedented level of accuracy, foresees the existence a scalar particle, the so-called Higgs boson, that breaks the electroweak symmetry and explains the non-null value of fermion masses. The pursue of the Higgs boson has been carried out by several experiments at the LEP II and Tevatron colliders in the past, but unsuccessfully. However, during the year 2011, the ATLAS and CMS experiments at LHC began to observe some evidence of the elusive particle, finally confirming the discovering in 2012. Even if the Higgs boson and its observed properties represent a triumph for the Standard Model, there are many unresolved phenomena that the SM cannot explain. Several extensions have been proposed; some of the most relevant foresee several Higgs bosons instead of only one. After the commissioning of the LHC collider in 2011 and the first evidences of a light scalar boson, one of the fundamental questions to be answered was whether the new particle was the SM Higgs boson, or the lightest among the bosons foreseen by the theories extending the SM. The confirmation could come from the measurements of the Higgs couplings to the SM particles, or alternatively from the observation of new production processes and decays not predicted by the SM, or measured at a different rate. The associated production with b quarks of a Higgs boson, and its decay into pairs of b quarks, is a process with a small predicted cross section in the SM, and its observation would imply the presence of new physics. Previous searches at the Tevatron collider reported a slight excess in this channel. Afterwards the discovery of the Higgs boson in July 2012 by the CMS and ATLAS experiments, the knowledge of the properties of the Higgs boson and its mass become a valuable handle to increase the sensitivity of new physics searches. One of the most beneficial channels is the search of a heavy pseudoscalar A, and its decay into a Z boson and a light Higgs boson h, the latter assumed to be the recently-discovered 125 GeV boson. The Z boson is sought in its decay into a pair of electrons or muons, and the Higgs into a pair of b quarks. This channel, not predicted by the SM, probes a region in the parameter phase-space of beyond the SM theories which is, to some extent, complementary to the one of the multi-b search.
Il modello standard (SM) delle particelle elementari, verificato sperimentalmente ad alta precisione, prevede l’esistenza di una particella scalare, il bosone di Higgs, grazie al quale avviene la rottura di simmetria elettrodebole ed è possibile spiegare le masse non nulle dei fermioni. La ricerca del bosone di Higgs è stata perseguita in passato da diversi esperimenti ai collisori LEP II e Tevatron, ma sempre con esito negativo. Solo nell’anno 2011, all’acceleratore LHC gli esperimenti ATLAS ed CMS hanno cominciato ad osservare le prime evidenze dell’elusiva particella, per poi confermane definitivamente la scoperta nel 2012. Tuttavia, pur essendo l’esistenza e le caratteristiche osservate del bosone di Higgs l’ennesima verifica sperimentale della validità del modello standard, esistono fenomeni naturali che esso non è in grado di spiegare, come ad esempio la massa dei neutrini o la materia oscura. Diverse estensioni del modello standard sono state proposte; diverse tra le più accreditate prevedono non uno, ma più bosoni. Con l’entrata a regime di LHC nel 2011 e le prime evidenze di un bosone di Higgs, una delle domande fondamentali a cui dare risposta era se il segnale che si stava osservando fosse dovuto al bosone di Higgs dello SM, oppure al più leggero dei diversi bosoni previsti dalle teorie oltre il modello standard. La risposta poteva venire sia dalla misura degli accoppiamenti dell’Higgs alle particelle dello SM, sia osservando processi e decadimenti non previsti dallo SM, o misurati con una frequenza maggiore dell’atteso. La produzione di un bosone di Higgs in associazione con quarks b, e il suo successivo decadimento in una coppia di quark b, è un processo difficilmente osservabile nel modello standard, pertanto la sua osservazione averebbe significato la presenza di nuova fisica. Precedenti ricerche a Tevatron hanno evidenziato un lieve eccesso in questo canale. In seguito alla scoperta del bosone di Higgs nel Luglio 2012 da parte degli esperimenti CMS e ATLAS, la conoscenza delle proprietà di questa particella, ivi compresa la sua massa, diventano informazioni che possono essere utilizzate per aumentare la sensibilità a ricerche di processi di nuova fisica. Uno dei canali che beneficia di questa informazione è la ricerca di un bosone pseudoscalare A, in particolare nel suo decadimento in un bosone Z e un bosone di Higgs leggero h, che si assume essere quello di massa 125 GeV recentemente scoperto. Lo stato finale consiste in una coppia di elettroni o muoni originati dal decadimento del bosone Z, e una coppia di quark b dal decadimento dell’Higgs. Questo canale, non previsto dallo SM, permette di sondare una regione dello spazio dei parametri di teorie oltre il modello standard per certi versi complementare a quella del canale con molti b nello stato finale.

AdS/CFT correspondence combines ideas of holography and duality in a way that allows one to make calculations in the otherwise hardly accessible (strongly coupled) regime of the target theory. In this thesis we are interested in describing strongly coupled sectors in the Standard Model and beyond within the holographic framework. In particular, we use the bottom-up holographic approach in application to the phenomenology of QCD and Composite Higgs. In the latter a new strong interaction binds hyper-fermions into composite states at TeV energies, and the Higgs is one of the pseudo-Goldstone bosons appearing as a result of new flavor symmetry breaking. In the bottom-up model building the conformal invariance of the AdS space must be broken by the manual introduction of the wall in the extra dimension. We favor the soft wall models, because they reproduce linear radial Regge-like spectra of the states. In application to QCD, we provide results on the determination of the QCD deconfinement temperature in the bottom-up models of different types, and the study of the low-energy QCD physics triggered by a novel implementation of the chiral symmetry breaking in the soft wall model. The phenomenological outcome of the latter model is given in terms of the fit to fifteen QCD observables with the RMS error of 30%. Concerning the New Physics, we specialize on the minimal Composite Higgs case. We calculate masses of the new composite resonances, estimate several couplings between the new sector states and the electro-weak bosons and analyse the holographic realization of the first and second Weinberg sum rules. Experimental limitations on the electro-weak precision observables and the misalignment angle, as well as the known value of the elctro-weak scale, constrain the parameter space of the holographic model and specify the scale of its predictions. We conclude that the model is able to accommodate new vector resonances with masses in the range 2 TeV to 3 TeV without encountering phenomenological difficulties.
En esta tesis estudiamos sectores fuertemente acoplados en el Modelo Estándar (SM) y más allá (BSM) dentro del enfoque holográfico. Nuestro propósito es sobresalir el método en la aplicación a la física mejor estudiada y luego utilizar el conocimiento acumulado en el ámbito que está fuera del alcance de los experimentos contemporáneos. Las ideas holográficas aparecieron después de que se hiciera la observación de Bekenstein y Hawking sobre la naturaleza de la entropía del agujero negro. El principio holográfico general se formuló de la siguiente manera: todo el contenido de información de una teoría de la gravedad cuántica en un volumen dado se puede codificar en una teoría eficaz sobre la frontera. La correspondencia anti-de Sitter / teoría conforme de campos (AdS/CFT) combinó holografía y dualidades (por esa razón también se conoce con el nombre de dualidad gauge/gravedad) en una interesante propuesta para la investigación de la teoría de cuerdas y los aspectos de la gravedad cuántica. Sin embargo, pronto se descubrió que la teoría supersimétrica $ N = 4 $ de Yang-Mills, por la que Maldacena ha encontrado un la teoría dual de cuerdas, es en cierto modo similar a la cromodinámica cuántica (QCD) en el régimen fuertemente acoplado. Eso desvió la atención a la parte gauge de la correspondencia. Sorprendentemente, la teoría de cuerdas es más fácil de resolver que la teoría mundana de quarks y gluones porque se conjetura que está en el límite semiclásico y débilmente acoplado. Existen pocas herramientas teóricas para estudiar regímenes fuertemente acoplados. Para QCD, el más conocido es la formulación lattice de primer principio. Reveló mucho sobre la naturaleza de QCD. Por ejemplo, señaló que la transición de desconfinamiento de los estados de hadrones ligados al plasma de quark-gluones ocurre como un cruce suave. Desafortunadamente, no es omnipotente: además de las restricciones tecnológicas y la complejidad cada vez mayor de los cálculos numéricos, existe, por ejemplo, un problema mucho más profundo relacionado con la inclusión del potencial químico de los quarks finito. Este último define uno de los ejes del diagrama de fase QCD, de modo que la lattice QCD no puede analizarlo en su totalidad. A su vez, la holografía se puede utilizar para abordar varios aspectos de QCD: espectros de mesón, glueball y barión, interacciones hadrónicas y el proceso de hadronización en colisionadores, confinamiento y ruptura de simetría quiral, materia hadrónica en condiciones externas extremas. Sin embargo, existen varias limitaciones en QCD que aparecen en construcciones similares a AdS/CFT. Primero, se supone que debe estar en el límite de $N_c$ grande. Independientemente de la holografía, se demostró que este límite puede verse como una versión exagerada de QCD tricolor común: el número de excitaciones radiales de un estado dado es infinito, cada una de ellas es infinitamente estrecha, el desconfinamiento representa una transición de fase real, etc. En segundo lugar, la propiedad de confinamiento no se puede combinar con CFT; uno debería encontrar una manera de romper la invariancia conforme e introducir una escala QCD $ \ Lambda_ {QCD} $. No hace falta decir que se desconoce la teoría dual de cadena exacta para QCD. En esta tesis utilizamos el llamado enfoque holográfico bottom-up (AdS / QCD) que fue desarrollado para superar las dificultades teóricas y que se centra en la descripción exitosa de la fenomenología QCD. En AdS / QCD se construyen modelos de cinco dimensiones siguiendo las reglas del diccionario AdS / CFT. Los campos 5D corresponden a los operadores de interpolación en 4D y pueden considerarse como la conexión entre la fuente sobre la frontera y un punto en el volumen. Las funciones de Green, que definen la teoría 4D, se pueden calcular a partir de la acción 5D debido a la correspondencia entre las funciones de partición. Al mismo tiempo, la representación de Kaluza-Klein de un campo 5D contiene los grados de libertad físicos reales 4D con los números cuánticos de los operadores duales. Se puede extraer el espectro de masas de estos modos. La invariancia conforme del espacio $ AdS_5 $ se rompe mediante la introducción manual de la pared en la quinta dirección ($ z $). Básicamente, se pueden considerar paredes de dos tipos: la pared dura, que es solo un corte a una distancia finita en la dirección $ z $, y la pared blanda, donde se introduce un perfil exponencial en la acción para suprimir cualquier contribución en $ z $ -infinito. Hay esfuerzos para aplicar la holografía en varios sistemas físicos, incluso tan desconectados de la teoría de cuerdas como la física de la materia condensada. Nuestro interés particular es la incorporación de modelos AdS / QCD en el ámbito de la física BSM. Hay varias ideas sobre cómo podrían producirse dinámicas fuertes allí. Nos centramos en los modelos de Higgs compuesto (CH), donde la nueva interacción fuerte une a los hiper-fermiones en estados compuestos a energías TeV, en paralelo a QCD que une quarks de SM en mesones y bariones. Este nuevo sector a la escala energética de varios TeV puede resolver el problema de naturalidad del SM al precio de una afinación relativamente pequeña. Además, proporciona predicciones BSM dentro del alcance experimental del futuro cercano. Por lo tanto, en esta tesis informamos sobre nuestras investigaciones de QCD y CH dentro del enfoque holográfico bottom-up. Una revisión de una amplia gama de temas es un requisito previo en este tipo de estudio que combina construcciones teóricas sofisticadas y análisis de datos fenomenológicos. En el Capítulo 2 revisamos el formalismo teórico detrás de las dualidades gauge/gravedad. Los capítulos 3 y 4 se refieren a las características teóricas y fenomenológicas relevantes de la QCD. En el Capítulo 5 presentamos modelos simples de AdS/QCD y delineamos el análisis holográfico de los espectros de partículas. El Capítulo 6 contiene nuestros resultados originales sobre la determinación de la temperatura de desconfinación de QCD en el marco de AdS/QCD. El Capítulo 7 corresponde al artículo dedicado al estudio de la física de QCD de baja energía desencadenada por la implementación particular de la ruptura de simetría quiral en el modelo con la pared blanda. En el Capítulo 8 se presenta un trabajo sobre Higgs compuesto holográfico. Comenzamos en el Capítulo 2 con una descripción del formalismo central, el de la correspondencia AdS / CFT. Las prescripciones holográficas generales se recopilan en el diccionario AdS / CFT. Luego, especificamos al caso la dualidad de Maldacena entre la teoría de cuerdas tipo IIB en $ AdS_5 \ times S_5 $ espacio-tiempo y la teoría supersimétrica $N=4$ de Yang-Mills en cuatro dimensiones. Discutimos en qué sentido la parte gauge de esta dualidad es similar a large- $ N_c $ QCD y qué aspectos faltan. En el Capítulo 3 discutimos algunos temas seleccionados de QCD. Revisamos varios métodos bien establecidos que permiten estudiar la QCD en diferentes regímenes: a gran número de colores, a pequeñas energías, centrándonos en las implicaciones de la simetría quiral y a temperatura finita. También cubrimos la expansión de productos del operador, la formulación lattice de QCD, sus predicciones e implicaciones. Los observables fenomenológicos de hadrones se consideran en el Capítulo 4 e incluyen los espectros de masas de resonancia, los acoplamientos y los factores de forma. Investigamos en detalle la idea de excitaciones de mesones radiales que pertenecen a las trayectorias radiales lineales de Regge, y enfatizamos la noción de trayectorias de mesones radiales universales. También se analizan los espectros de glueballs, estudiados sobre lattice. Habiendo resaltado las características fenomenológicas relevantes de QCD, pasamos a su implementación holográfica bottom-up. El formalismo general de incluir resonancias QCD en el volumen 5D se presenta en el Capítulo 5. Presentamos varios modelos simples de AdS/QCD: pared dura, pared blanda y pared blanda generalizada. Se evalúan sobre la base de su éxito en la descripción de los espectros fenomenológicos. Los modelos de paredes blandas demuestran la linealidad de las trayectorias radiales esperadas en las teorías con la realización adecuada del confinamiento. Además, mencionamos la posibilidad de adquirir familias de perfiles de paredes blandas que conduzcan a los mismos espectros. En AdS / QCD se asume que la transición de la fase de desconfinamiento es dual a la transición Hawking-Page entre diferentes geometrías en la teoría 5D. Eso nos permite estimar la temperatura (pseudo) crítica de desconfinamiento, $T_c$. En el Capítulo 6 exploramos varios modelos holográficos con diferentes opciones para fijar sus parámetros con el fin de obtener el valor fenomenológico de $T_c$. Los resultados dependen bastante de la elección de los parámetros del modelo. Concluimos que hay un subconjunto que proporciona $T_c$ cerca de las estimaciones de quenched lattice y grande-$ N_c $, y aquellas que predicen $ T_c $ en el rango físico (temperatura de freeze-out, estimaciones lattice con quarks dinámicos). El mecanismo holográfico AdS / QCD para la implementación dual de la ruptura de la simetría quiral se desarrolla en el Capítulo 7. Elegimos el marco de pared blanda e introducimos varias características novedosas para acomodar mejor los modos Goldstone. El modelo resultante no tiene muchos parámetros libres y debido a este minimalismo proporciona interesantes interrelaciones entre diferentes sectores. Se extraen varios observables de interés. El resultado fenomenológico se da en términos del ajuste a quince QCD observables con el error cuadrático medio de $ ~ 30%$. En el Capítulo 8 comenzamos con la motivación para la extensión de BSM con el sector fuertemente acoplado y enfatizamos las ventajas de los modelos CH. Además, nos especializamos en el caso de CH mínimo con el patrón de ruptura de la nueva simetría de sabor tipo $SO (5) -> SO (4) $. Allí aplicamos la técnica AdS / QCD y, en concreto, utilizamos la experiencia del capítulo anterior. Sin embargo, las diferencias con el caso QCD no son tan simples como el intercambio de los grupos de "sabor" y "color". La reconsideración de la realización holográfica y el papel de los bosones de Goldstone es un desarrollo importante del capítulo. Calculamos las masas de las nuevas resonancias compuestas, estimamos varios acoplamientos entre los nuevos estados del sector y los bosones electro-débiles y analizamos la realización holográfica de la primera y segunda reglas de suma de Weinberg. Las limitaciones experimentales de los observables de precisión electro-débil y el ángulo de desalineación, así como el valor conocido de la escala electro-débil, restringen el espacio de parámetros del modelo holográfico y especifican la escala de sus predicciones. Concluimos que el modelo es capaz de acomodar nuevas resonancias vectoriales con masas en el rango de 2-3 TeV sin encontrar dificultades fenomenológicas. En conclusión, aunque los métodos AdS / QCD están lejos de ser precisos, son útiles aunque solo sea para proporcionar un marco bastante simple donde se pueden diseñar y probar diferentes escenarios. Al mismo tiempo, es fundamental tratar conscientemente la cantidad de libertad que otorga este marco fenomenológico. Nos esforzamos por encontrar este equilibrio y elaboramos para motivar bien las suposiciones y elecciones tomadas a lo largo de la construcción del modelo holográfico.

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.

Cette thèse présente l’analyse des données enregistrées avec une énergie dans le centre de masse de 13 TeV, en 2015-2017, par le détecteur ATLAS au LHC. La première partie de la thèse concerne la mesure de l’efficacité des électrons avec une méthode Tag-and-Probe en utilisant des évènements Z->ee. L’efficacité de reconstruction des électrons est mesurée entre 97\% (15Hu (t->Hc) sont compatibles avec zéro et des limites supérieures valant 0.19\% (0.16\%) sont obtenues à 95\% de niveau de confiance. Ces résultats sont les meilleurs pour un canal donné aujourd’hui. La production du boson de Higgs avec une paire de quark top (ttH) permet un accès direct au couplage de Yukawa du top. Ce mode de production a été observé pour la première fois par les expériences ATLAS et CMS. Les défis et les efforts nécessaires pour mesurer la section efficace dans le canal multilepton sont discutés dans la dernière partie de la thèse
Proton-proton collisions at a center of mass energy of 13 TeV recorded during 2015-2017 with the ATLAS detector at LHC are analyzed in this thesis. The first part of the thesis details electron efficiency measurements with a Tag-and-Probe method in Z->ee events. The measured electron reconstruction efficiency varies between 97\% (15Hu or t->Hc) using the 2015-2016 data. These processes are strongly suppressed in the Standard Model (SM), however larger enhancements are predicted in new physics models. Multivariate analyses (MVA) are used to improve signal-background separation in multilepton final states. The measured branching ratio for the t->Hu (t->Hc) process is found to be compatible with zero and a 95\% confidence level upper limit is set at 0.19\% (0.16\%) with an expected limit of 0.15\% (0.15\%), the best limit in a single channel to date.The Higgs boson production in association with a top quark pair (ttH) allows direct access to the top Yukawa coupling. The ttH production mode was observed by the ATLAS and CMS experiments, independently, by combining several decay channels. Further efforts to measure ttH in the two leptons same-sign channel are discussed in the last part of the thesis

On se place dans le cadre des modèles à dimension supplémentaire courbe adressant le problème de hiérachie de jauge. On considère ainsi le scenario suggéré par Randall et Sundrum sous l'hypothèse habituelle d'une symétrie de jauge custodiale étendue à l'espace total à 5 dimensions, permettant de protéger le modèle de trop larges corrections dans le secteur électrofaible, où le champ de Higgs est indépendament localisé sur ce qui est appelée la TeV-brane. Après avoir revu les bases de théorie des champs en dimensions supplémentaires, on introduit le cadre théorique des modèles RS, nous exposons en détail la construction de notre modèle phénoménologique, ainsi que les effets engendrés par la brisure de symétrie électrofaible sur notre modèle. Nous déterminons ensuite les diverses représentations minimales des multiplets de quarks sous la symétrie custodiale permettant de résoudre l'anomalie sur l'asymétrie avant-arrière du quark bottom. Il est ensuite montré en detail qu'il existe de fortes corrections à la valeur moyenne dans le vide du Higgs induites par de forts mélanges des bosons de jauge Z et W avec leurs exitations de KK. Le lien avec les tests de précision électrofaible est développé. Nous trouvons ainsi de substancielles corrections dûes à RS aux divers couplages au Higgs, pouvant affecter sa phénoménologie. La fin de cette thèse est dédiée aux extensions SupersSymétriques des modèles à dimension supplémenaire courbe, le champ de Higgs étant toujours confiné sur la TeV-brane
Ln the framework of warped extra dimension models addressing the gauge hierarchy problem, we consider the Randall-Sundrum scenario under the usual hypothesis of a bulk custodial symmetry protecting the model from large ElectroWeak corrections together with a Higgs field localized on the so-called TeV-brane. After reviewing the basics of field theory in extra dimensions, we introduce the RS framework, and show in details the construction of our phenomenological model as well as the effects of EW symmetry breaking. We determine the several minimal quark representations allowing to address the anomalies in the forward-backward b-quark asymmetry. It is then shown in details that there can exist large corrections to the Higgs boson Vacuum Expectation Value induced by mixings of the gauge bosons with their KK excitations. The connection with EW precision tests is developed. We find possibly substantial RS corrections to the various Higgs couplings able to affect its phenomenology. The end of this thesis is dedicated to SuperSymmetric extension of warped models, the Higgs field still being confined on the TeV-brane

This thesis is devoted to the study of the phenomenology of the Higgs sector of the minimal B −L extension of the Standard Model at present and future colliders. Firstly, the motivations that call for the minimal B −L extension are summarised. In addition, the model is analysed in its salient parts. Moreover, a detailed review of the phenomenological allowed Higgs sector parameter space is given. Finally, a complete survey of the distinctive Higgs boson signatures of the model at both the Large Hadron Collider and the future linear colliders is presented

The thesis is devoted to my Ph.D. research activities during last three years within the CMS collaboration. My primary field of interest was the investigation of the Higgs sector of the Standard Model and in connection with Beyond-Standard-Model New Physics searches.

The thesis is devoted to my Ph.D. research activities during last three years within the CMS collaboration. My primary field of interest was the investigation of the Higgs sector of the Standard Model and in connection with Beyond-Standard-Model New Physics searches.

Our current understanding of matter and its interactions is summarised in the Standard Model (SM) of particle physics. Many experiments have tested the predictions of the SM with great success, but others have brought our ignorance into focus by showing us there are new phenomena that we can not describe within the framework of the SM. These include the experimental observations of neutrino masses and dark matter, which confirms there must be new physics. What this new physics may look like at colliders motivates the original work in this thesis, which comprises three studies: the prospects of future electron-positron colliders in testing a model with an extended Higgs sector with a scalar triplet, doublet and singlet; the discovery potential at the Large Hadron Collider (LHC) of a non-minimal Supersymmetric model via conventional sparticle searches and via searches for displaced vertices; and the experimental search for long-lived massive particles via a displaced vertex signature using data of proton-proton collisions collected at a collider center of mass energy of 8 TeV in 2012 by the ATLAS detector operating at the LHC.

Orientador: Pedro Cunha de Holanda
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho apresentamos um estudo de alguns problemas que ainda não forem esclarecidos na física de neutrinos do modelo padrão (MP) e encaminhamos eles na direção de física nova. Para esse fim, estuda-se uma classe de extensões de Gauge quirais do MP com simetria SU(3)C x SU(3)L x U(1)X (modelos 331), onde estudamos a interação eletrofraca dos neutrinos com a matéria via correntes carregadas e neutras (CC e CN) através dos bósons Gauge do modelo para processos de espalhamento elástico coerente. Finalmente, encontramos vínculos entre as contribuições não padrão no modelo 331 com as interações não padrão (INP) em experimentos de seção de choque, onde encontramos que o modelo 331 apresenta INP diagonais universais da ordem e a M2 W,Z / M2 X <~ O(10-3). Mostramos que o valor esperado no vácuo (VEV) para o tão procurado bóson de Higgs neste modelo tem que ser ?2 <~174.105GeV e os VEV V > 1.3TeV e 142 GeV < ?1 < 201GeV. Os novos bósons do modelo deverão ter massas maiores a 610 GeV
Abstract: We present a study of some problems that are not yet clarified in Standard Model (SM) neutrino physics and guide them in the direction of new physics. To this aim, we study a class of extensions of the SM Gauge chiral symmetry SU(3)CSU(3)LU(1)X (331 model), where we study the electroweak interaction of neutrinos with matter via charged and neutral current (CC and NC) by the gauge bosons of the model for coherent elastic scattering process. We finally found connections between the non-standard contributions on 331 with non-standard interactions (NSI) in cross section experiments, where we find that the model 331 has diagonal universal NSI of order e a M2 W,Z / M2 X <~ O(10-3) . We show that the expected value in vacuum (VEV) for the quite searched Higgs boson in this model has to be ?2 <~174.105 GeV and the VEV V > 1.3TeV and 142 GeV < ?1 < 201 GeV. The new bosons of the model must have masses greater than 610 GeV
Mestrado
Física das Particulas Elementares e Campos
Mestre em Física

The Minimal Scale Invariant extension of the Standard Model (MSISM) is a model of low-energy particle physics which is identical to the Standard Model except for the inclusion of an additional complex singlet scalar and tree-level scale invariance. Scale invariance is a classical symmetry which is explicitly broken by quantum corrections whose interplay with the quartic couplings can be used to trigger electroweak symmetry breaking. The scale invariant Standard Model suffers from a number of problems, however the inclusion of a complex singlet scalar results in a perturbative and phenomenologically viable theory. We present a thorough and systematic investigation of the MSISM for a number of representative scenarios along two of its three classified types of flat direction. In these scenarios we determine the permitted quartic coupling parameter space, using both theoretical and experimental constraints, and apply these limits to make predictions of the scalar mass spectrum and the energy scale at which scale invariance is broken. We calculate the one-loop effective potential and the one-loop beta functions of the pertinent couplings of the MSISM specifically for this purpose. We also discuss the phenomenological implications of these scenarios, in particular, whether they realise explicit or spontaneous CP violation, contain neutrino masses or provide dark matter candidates. Of particular importance is the discovery of a new minimal scale invariant model which provides maximal spontaneous CP violation, can naturally incorporate neutrino masses, produces a massive stable scalar dark matter candidate and can remain perturbative up to the Planck scale. It can be argued that the last property, along with the classical scale invariance, can potentially solve the gauge hierarchy problem for this model.

In dieser Disseratation wird eine mögliche Erweiterung des Standardmodells der Elementarteilchen (SM) im Higgs-Sektor mithilfe von Topquarkpaarproduktion am Large Hadron Collider untersucht. Insbesondere wird dabei auf das sogenannte Zwei-Higgs-Duplettmodell eingegangen. Dieses Modell führt mehrere Spin-0 Bosonen (auch Higgsbosonen genannt) zusätzlich zum SM-Higgsboson ein. Dabei wird in dieser Arbeit von der Annahme ausgegangen, dass diese zusätzlichen Higgsbosonen schwer genug sind um in ein Top-Antitop-Paar zu zerfallen. Somit können die experimentellen Signaturen dieser neuen Teilchen mit Hilfe von Observablen der Topquarkpaarproduktion untersucht werden. Dazu wird die resonante Erzeugung von schweren Higgsbosonen und deren Zerfall in Topquarkpaare bis einschließlich Quantenkorrekturen in der nächst-zu-führenden Ordnung (NLO) in der QCD-Kopplungskonstanten berechnet. Weiterhin wird die volle Spininformation des Top-Antitop-Paares beibehalten, welche die Analyse von spinabhängigen Observablen erlaubt. Diese können, insbesondere in Falle von Top-Antitop-Spinkorrelationen, sehr sensitiv auf Effekte schwerer Higgsbosonen sein. Dies zeigt sich besonders in Vergleich zu spinunabhängigen Observablen. Die Sensitivität von spinabhängigen Observablen kann zudem noch durch entsprechende Schnitte auf den Phasenraum von Top- und Antitopquark verstärkt werden. In dieser Dissertation wird ein Verfahren vorgestellt, mit dessen Hilfe sich die Spinkorrelationen identifizieren lassen, welche die größte Sensitivität auf die Effekte schwerer Higgsbosonen aufweisen. Außerdem wird durch die Berechnung der Beiträge zur NLO u.a. gezeigt, dass diese Beiträge wichtig sind um aussagekräftige und robuste Observablen zu definieren. Die Ergebnisse der NLO, die in dieser Arbeit vorgestellt werden, sind die ersten ihrer Art für die resonante Erzeugung von schweren Higgsbosonen und deren Zerfall in Topquarkpaare.
In this dissertation a possible extension of the standard model of particle physics (SM) in the Higgs sector is investigated using top-quark pair production at the Large Hadron Collider as a probe. In particular, the so-called two-Higgs-doublet model (2HDM) is studied. The 2HDM introduces several spin-0 bosons (which are also called Higgs bosons) in addition to the SM Higgs boson. In this thesis these additional Higgs bosons are assumed to be heavy enough to decay into a top-antitop quark pair. Thus, the experimental signatures of these new particles can be studied through observables of top-quark pair production. To this end the resonant production of heavy neutral Higgs bosons and their decay into top-quark pairs in calculated up to next-to-leading order corrections in the QCD coupling constant retaining the full spin information of the top-antitop pair. This allows to analyse spin dependent observables which can be more sensitive to effects of heavy Higgs bosons than spin independent ones especially in the case of top-antitop spin correlations. The additional application of kinematical cuts on the phase space of top and antitop quarks can enhance the sensitivity further. In this thesis a method is presented that can be used to construct the spin correlation which is most sensitive to the effects of heavy Higgs bosons on top-quark pair production. Furthermore, it is shown that the next-to-leading order corrections are required to construct observables which entail robust predictions. The results for the next-to-leading order in the QCD coupling constant presented in this thesis were the first ones given for resonant heavy Higgs production and decay into top-quark pairs.

This thesis concerns the study of extended Higgs sectors at future colliders. Such studies are well motivated since enlarged Higgs models are a necessity in many extensions of the Standard Model (SM), although these structures may be considered purely in the context of the SM, to be called the 'non-minimal SM'. The continuous theme of the thesis is the task of distinguishing between the (many) theoretically sound non-minimal Higgs sectors at forthcoming colliders. If a Higgs boson is found it is imperative to know from which model it originates. In particular, the possible differences between the Higgs sectors of the Minimal Supersymmetric Standard Model (MSSM) and the non-minimal SM are highlighted. Considered first are the detection prospects of light charged Higgs scalars (H(^±)) at e(^+)e(^-) colliders. The discovery of a H(^±) would provide unambiguous evidence for a non- minimal Higgs sector. We show that in certain (but not all)non-minimal models a light H(^±) may exist i.e. be within the mass range accessible at LEP2 (M(_H)(_ ±) ≤ Mw). In particular the MSSM requires M(_H±) ≥ Mw, and thus detection of a at LEP2 is strong evidence against the MSSM. We discuss ways of distinguishing between the models which may contain a light H(^±) by exploiting differences in the decay channels. Attention is then given to the neutral Higgs bosons of the non-minimal SM. It is possible that these particles may possess a greatly different phenomenology to that of the mammal SM and MSSM, and we explore the feasibility of observing these differences at LEP2 and the LHC It is found that distinct, sometimes spectacular signatures are possible. The thesis next considers a more exotic Higgs representation, namely that of Higgs triplets, and compares its phenomenology at LEP2 with that of the non-minimal models covered thus far The phenomenon of Higgs bosons decoupling from the fermions(fermiophobia) arises naturally in the above triplet model, and this concept is studied in more depth in the final chapter. It is emphasized that such particles are not possible in the MSSM.

Sonder la production du quark top et du boson de Higgs dans les évènements multi-jet avec expérience Atlas auprès du LHC.Le détecteur Atlas est une expérience généraliste placé auprès du collisionneur proton-proton circulaire de 27 km de circonférence, LHC. Le LHC est conçu pour produire des collisions avec une énergie nominale au centre de masse de l'ordre de racine carré8 = 14 Tev et une luminosité instantané de 10-34cm-2s-1
Probing top quark and Higgs boson production in multi-jet avents at the LHC with the Atlas detector.The Atlas detector is a general purpose detector located at the Large Hadron Collider (LHC). It aims at the discovery af new physics phenomena and improving our knowledge of the standart Model (SM)

Ce travail présente les mesures de section efficace de production de photons isoles dans les collisions proton-proton avec l'expérience ATLAS au LHC, a une énergie de 7 TeV dans le centre de masse. Les résultats sont d'abord obtenus pour une luminosité intégrée de 880 nb−1, puis dans un deuxième temps pour une luminosité de 36 pb−1. Les mesures faites avec 880 nb−1 et 36 pb−1 couvrent, respectivement, les intervalles en énergie transverse 15 ≤ E < 100 GeV et 45 ≤ E < 400 GeV. Les résultats sont compares aux prédictions de la chromodynamique quantique perturbative à l'ordre NLO. Les premières études pour la recherche du Higgs dans le canal de désintégration en deux photons sont également présentées pour 38 pb−1 de données collectées par l'expérience ATLAS en 2010 (et plus récemment pour 210 pb−1 accumules en 2011). Les limites d'exclusion observées sont citées en fonction d'une masse de Higgs se situant dans l'intervalle 110-140 GeV.

Cette thèse sera centrée sur les analyses des événements diphotons dans le détecteur ATLAS. L'activité évoluera au cours du temps et couvrira différents aspects: compréhension de la réponse du détecteur, participation a la prise de données, analyse physique et recherche de signaux de physique au-delà du modèle standard.

Le sujet principal de cette thèse porte sur les mesures de la masse de recul du boson de Higgs et de sa section efficace en utilisant la réaction de Higgs-strahlung avec Z → µ + µ− et e+e−, basées sur la simulation détaillée du détecteur ILD. L'étude a été conduite pour un Higgs de 120 GeV de masse, à une énergie de 250 GeV dans le centre de masse pour une luminosité intégrée de 250 fb−1 . La précision obtenue est de 28 MeV sur la mesure de la masse du Higgs et 2.0% sur celle de la section efficace en combinant les canaux de désintégration. Cette étude prouve que le bruit de fond peut être largement réduit et que les résultats sont sensibles à la configuration de l'accélérateur. L'analyse et ses résultats sont inclus dans le ILD Letter of Intent. Le second sujet de la thèse est la calibration en MIPs du prototype de ECAL Silicium - Tungstène d´eveloppé par la collaboration CALICE. Les constantes de calibration sont extraites des données des tests en faisceau effectués au FNAL en 2008 et sont stables par rapport aux données prises au CERN en 2006.

Le Modèle Standard (MS) de la physique des particules s’est imposé comme étant la description la plus précise des interactions fondamentales entre les particules élémentaires. La découverte d’un boson de Higgs, avec une masse de 125 GeV, en Juillet 2012 au Large Hadron Collider (LHC), en a marqué sa confirmation définitive. Cependant, de nombreux problèmes observationnels et théoriques sont au coeur du MS, la plupart liés au secteur de Higgs. Etant une particule scalaire, le boson de Higgs souffre de très grandes corrections radiatives, ce qui déstabilise l’échelle électro-faible et mène au problème de hiérarchie. L’un des buts principaux du LHC est d’explorer précisément le secteur de Higgs, afin de caractériser le mécanisme à l’origine de la brisure de la symétrie électro- faible et de tester de possibles solutions au problème de hiérarchie. Le secteur de Higgs est également responsable de la génération des masses des fermions dans le MS, par le biais des couplages de Yukawa. Ces couplages sont extrêmement non génériques et cela mène aux problèmes de la saveur au delà du MS.La première partie de cette thèse se concentre sur la caractérisation précise du secteur de Higgs. En particulier, le code public Lilith est présenté, il permet de dériver des contraintes sur des scénarios de nouvelle physique à l’aide des mesures des propriétés du boson de Higgs en collisionneurs. Une analyse des couplages du boson de Higgs dans le contexte de plusieurs scénarios est ensuite effectuée. Dans la seconde partie, la phénoménologie des modèles à deux doublets de Higgs est étudiée à la lumière des résultats de la première phase du LHC. La limite d’alignement, ainsi que la possible présence de bosons de Higgs légers, sont étudiées en détail. Finalement, dans la dernière partie de cette thèse, l’hypothèse de Violation Minimale de la Saveur est introduite comme une solution potentielle aux problèmes de la saveur au delà du MS. Appliquée au Modèle Standard Supersymétrique Minimal, l’évolution des couplages baryoniques violant la parité R sous le groupe de renormalisation est analysée en détail
The Standard Model (SM) of particle physics stands as the most successful description of the fundamental interactions between elementary particles. The discovery of a Higgs boson, at a mass of 125 GeV, in July 2012 at the Large Hadron Collider (LHC), marked its ultimate confirmation. However, various observational and theoretical problems lie in the heart of the SM, with the majority of them linked to the Higgs sector. Being a scalar, the Higgs boson is subject to very large radiative corrections and this ultimately leads to the electroweak hierarchy problem. One of the main goals of the LHC program is to precisely probe the Higgs sector, in order to characterize the mechanism at the origin of the breaking of the electroweak symmetry and test possible solutions to the hierarchy problem. The Higgs sector is also responsible for the generation of the fermion masses, as it induces the Yukawa couplings. The SM flavor sector is highly hierarchical and this leads to flavor puzzles in theories beyond the SM.The first part of this thesis is dedicated to the precise characterization of the Higgs sector. In particular, the public tool Lilith is presented, which allows to derive constraints on new physics models based on the Higgs measurements at colliders. It is then used to perform global fits of the Higgs couplings in the context of various scenarios. In the second part, the phenomenology of two-Higgs-doublet models is studied in the light of the results from the first run of the LHC. The so-called alignment limit is explored in detail, as well as the possible presence of light scalar states. Finally, in the last part of this thesis, the Minimal Flavor Violation hypothesis is introduced as a possible solution to the flavor puzzles beyond the SM. Enforcing it in the Minimal Supersymmetric Standard Model, the renormalisation group evolution of the baryonic R-parity violating couplings is then studied in detail

Le sujet principal de la thèse est la mesure de la section efficace de production du boson de Higgs se décomposant en deux bosons Z dans l'état final à quatre leptons (H → ZZ * → 4l, l = e, μ), en utilisant 36.1 fb-1 de données collectées en 2015 et 2016 par l'expérience ATLAS au Grand Collisionneur de Hadrons (LHC) au CERN. La section efficace inclusive dans la chaîne de décomposition H → ZZ * → 4l est mesurée à 3.62 ±0.53 (stat) ± 0.25 (syst) fb, en accord avec la prédiction du Modèle Standard, de 2.91 ± 0.13 fb. Plusieurs sections efficaces fiducielles différentielles sont mesurées en fonction d'observables sensibles aux modes de production et de décomposition du boson de Higgs, incluant les variables cinématiques des jets produits avec le boson de Higgs. Les données et les prédictions du Modèle Standard sont en bon accord. Les sections efficaces extraites sont aussi utilisées pour contraindre des interactions anormales du boson de Higgs avec des particules du Modèle Standard.La deuxième partie de la thèse concerne l'isolation des muons. Les variables sont construites à partir des informations du tracker (isolation de trace) et des calorimètres (isolation calorimétrique). Elles mesurent l'activité ambiante autour de la trajectoire de la particule dans les détecteurs et constituent un outil puissant pour rejeter le bruit de fond. Avec les données collectées par ATLAS en 2015 et 2016,les distributions des variables sont comparées entre données et simulation. Des points de fonctionnement correspondant à des ensembles de coupures sur ces variables sont alors définis. En utilisant la décomposition du boson Z en deux muons, les efficacités du signal sont mesurées. Des facteurs d'échelle correspondant aux ratios entre données et simulation sont alors calculés. Le bon accord conduit à des facteurs d'échelle proches de l'unité
The main subject of the thesis is the measurement of the production cross section of the Higgs boson decaying to two Z bosons in the four-lepton final state (H →ZZ * → 4l, l = e, μ), using 36.1 fb- 1 of data recorded during 2015 and 2016 with the ATLAS experiment installed at the Large Hadron Collider (LHC) at CERN. The inclusive fiducial cross section in the H → ZZ * → 4l decay channel is measured to be 3.62 ± 0.53 (stat) ± 0.25 (syst) fb,in agreement with the Standard Model prediction of 2.91 ± 0.13 fb. Several differential fiducial cross sections are measured for observables sensitive to the Higgs-boson production and decay, including kinematic distributions of the jets produced together with the Higgs boson. Good agreement is found between the data and the predictions of the Standard Model. The extracted cross-section distributions are also used to constrain anomalous Higgs-boson interactions with Standard-Model particles.The second part of the thesis is dedicated to the muon isolation. Isolation variables, based on information from the tracker (track-based isolation) and the calorimeters (calorimeter-based isolation) measure the ambient activity surrounding a particle in the detectors and are a powerful tool to reject background objects. Using36.1 fb-1 of data recorded by ATLAS in 2015 and2016, the variable distributions are compared with simulation. Working points are then defined as sets of cuts applied to the isolation variables. Using theZ boson decays to a pair of muons, the signal efficiency of the working points is calculated. Scale factors are then derived as ratio of efficiencies measured in data over simulation. The good agreement leads to scale factors close to one

Complete one-loop results are presented for neutralino and Higgs decay processes of the form chi^0_i->chi^0_j h_a and h_a->chi^0_i chi^0_j in the MSSM with CP-violating parameters. An on-shell renormalisation scheme is developed for the chargino–neutralino sector that consistently takes into account imaginary parts arising from complex parameters and absorptive parts of loop integrals. The genuine vertex contributions are combined with two-loop Higgs propagator-type corrections to obtain the most precise prediction currently available for this class of processes. In the CP-violating CPX benchmark scenario, the corrections to the neutralino decay width are found to be particularly large – of order 45% for a Higgs mass of 40GeV. We find that in this unexcluded parameter region, which will be difficult to cover by standard Higgs search channels at the LHC, the branching ratio for the decay chi^0_2->chi^0_1 h_1 is large. This may offer good prospects of detecting such a light Higgs boson in cascade decays of supersymmetric particles. We also study the full Higgs production and decay processes in scenarios where the intermediate Higgs bosons are nearly mass degenerate and interference effects can have a significant impact. We find that an on-shell approximation gives results in good numerical agreement with the full momentum-dependent Higgs propagator matrix calculation and we develop a generalised narrow width approximation to be used in such a situation. We use these methods to study the asymmetry between the production of left-handed and right-handed neutralinos in Higgs decays at the LHC in the presence of CP-violating phases. Large asymmetries are found to be possible for large M_H^± > 500GeV and tan beta < 10, where the decay into neutralinos may be the only possibility to detect the heavy Higgs bosons.

Cette thèse étudie les collisions pp à sqrt{s} = 8 TeV avec des paires de photons dans l'état final, telles que recueillies par l'expérience ATLAS au LHC au cours de 2012. L'état final avec diphoton s'est avéré cruciale pour la découverte du boson de Higgs. La motivation pour la recherche de résonances supplémentaires se désintégrant en deux photons est très forte en raison l'existence de nombreux modèles prévoyant un secteur de Higgs étendu. Dans le même temps, des mesures détaillées des sections efficaces diphoton sont nécessaires pour établir la précision des prédictions théoriques actuellement disponibles pour ces processus.La mesure des photons par ATLAS repose sur un excellent étalonnage de la réponse du calorimètre électromagnétique. Pour cette raison, les études liées à l'étalonnage de la réponse de photons dans le calorimètre Argon Liquide (LAr) sont initialement réalisées, y compris la mesure de l'étalonnage par rapport à l'échelle de l'énergie de la couche calorimètre utilisant des photons, et la nécessité éventuelle d'un inter-étalonnage de la réponse en énergie des photons en fonction de l'angle azimutal $phi$. Une étude de l'amplitude du bruit du calorimètre dans des conditions de pile-up élevés est également présentée, aboutissant à des recommandations pour les configurations d'échantillonnage d'impulsion LAr au Run2.La recherche d'une résonance supplémentaire se désintégrant en deux photons est effectuée et aucun signal n'est observé: une limite est mise sur la section efficace fiducielle d'une résonance multipliée par son rapport de branchement de désintégration en diphoton. La mesure de la section efficace de production de diphoton isolés en association avec des jets est effectuée en fonction de la masse invariante de diphoton, l'impulsion transverse du système diphoton, la différence d'angles dans le plan azimutal de l'angle entre les deux photons et l'angle theta de coplanarité, en fonction de la multiplicité en jets et de différentes variables associés aux jets. Les résultats sont comparés aux dernières prévisions QCD Next-To-Leading-Order QCD. Une telle mesure est la première du genre effectuée au LHC
This thesis studies pp collisions at sqrt{s}=8 TeV with pairs of photons in the final state, as collected by the ATLAS experiment at the LHC during 2012. The diphoton final state has proven to be crucial to the discovery of the Higgs boson. The motivation for searching for additional resonances decaying into two photons is very strong due to many models predicting an extended Higgs sector. At the same time, detailed measurements of diphoton cross sections are necessary to establish the quality of the theoretical predictions currently available for these processes.The measurement of photons by ATLAS relies on an excellent calibration of the electromagnetic calorimeter response. For this reason, studies related to the calibration of the photon response in the ATLAS Liquid Argon (LAr) calorimeter are initially performed, including a measurement of the calorimeter layer energy scale relative calibration using photons, and the potential need of an inter-calibration of the photon energy response as a function of the azimuthal angle $phi$. A study of noise performance of the calorimeter in high pile-up conditions is also presented, resulting in recommendations for the LAr pulse sampling configurations for Run2.A search for an additional resonance decaying into two photons is performed, leading to no significant excess being observed. A limit on the fiducial cross section times branching fraction of a new resonance decaying into two photons is therefore computed. The measurement of the production cross section of isolated prompt diphoton in association with jets is performed as a function of the diphoton invariant mass, the transverse momentum of the diphoton system, the azimuthal angle difference of the two photons and the coplanarity angle theta, for jet multiplicity categories and as a function of various jet variables. The results are compared to the latest Next-To-Leading-Order QCD predictions. Such a measurement has not been previously published at the LHC

Dans ce document deux analyses des propriétés du boson de Higgs se désintégrant en 2 photons dans l'expérience CMS située auprès du LHC (Large Hadron Collider) sont présentées.Le document commence par une introduction théorique sur le Modèle Standard et sur la physique du boson de Higgs, suivie par une description détaillée de l'expérience CMS. En deuxième lieu, les algorithmes de réconstruction et identification des photons sont présentés, avec une attention particulière aux différences entre le premier et le deuxième run du LHC, le premier run (Run1) ayant été pris entre 2010 et 2012 avec une énergie dans le centre de masse de 7 puis 8 TeV, le deuxième (Run2) ayant commencé en 2015 avec une énergie dans le centre de masse de 13 TeV. Les performances des reconstructions du Run 1 et du Run 2 en ce qui concerne l'identification des photons sont comparées. Ensuite l'algorithme d’identification des photons pour l'analyse H->γγ et optimisé pour le Run2 est présenté. Pour ce faire une méthode d'analyse multivariée est utilisée. Les performances de l'identification des photons à 13 TeV sont enfin étudiées et une validation donnée-simulation est effectuée.Ensuite l'analyse H->γγ avec les premières données du Run2 est présentée. Les données utilisées correspondent à une luminosité intégrée de 12.9 fb⁻¹. Une catégorisation des événements est faite, afin de rendre maximale la signification statistique du signal et d’étudier les différents modes de production du boson de Higgs. La signification statistique observée pour le boson de Higgs du Modèle Standard est 1.7 sigma, pour une signification attendue de 2.7 sigma.Enfin une étude de faisabilité ayant pour but de contraindre les couplages anomaux du boson de Higgs aux bosons de jauges est présentée. Pour cette analyse les données à 8 TeV collectées pendant le Run 1 du LHC, correspondant a' une luminosité intégrée de 19.5/fb sont utilisées. Cette analyse exploite la production du boson de Higgs par fusion de bosons-vecteurs (VBF), avec le Higgs se désintégrant ensuite en 2 photons. Les distributions cinématiques des jets et des photons, qui dépendent de l'hypothèse de spin-parité, sont utilisées pour construire des discriminants capables de séparer les différentes hypothèses de spin-parité. Ces discriminants permettent de définir différentes régions de l'espace des phases enrichies en signal de différentes spin-parité. Les différents nombres d’événements de signal sont extraits dans chaque région par un ajustement de la masse invariante diphoton, permettant de déterminer les contributions respectives des différents signaux et permettant ainsi de contraindre la production de boson de Higgs pseudo-scalaire (spin-parité 0-)
In this document two analyses of the properties of the Higgs boson in the diphoton decay channel with the CMS experiment at the LHC (Large Hadron Collider) are presented.The document starts with a theoretical introduction of the Standard Model and the Higgs boson physics, followed by a detailed description of the CMS detector.Then, photon reconstruction and identification algorithms are presented, with a particular focus on the differences between the first and the second run of the LHC, the first run (Run1) took place from 2010 to 2012 with a centre-of-mass energy of 7 and then 8 TeV, while the second run (Run2) started in 2015 with a centre-of-mass energy of 13 TeV. Performances of Run1 and Run2 reconstructions from the photon identification point of view are compared. Then the photon identification algorithm for the H->γγ analysis optimised for Run2 is presented. To do that a multivariate analysis method is used. Performances of the photon identification at 13 TeV are finally studied and a data-simulation validation is performed.Afterwards, the H->γγ analysis using the first Run2 data is presented. The analysis is performed with a dataset corresponding to an integrated luminosity of 2.7/fb. An event classification is performed to maximize signal significance and to studyspecific Higgs boson production modes. The observed significance for the standard model Higgs boson is 1.7 sigma, while a significance of 2.7 sigma is expected.Finally a feasibility study, having the aim of constraining the anomalous couplings of the Higgs boson to the vector bosons, is presented. This analysis is performed using the data collected at 8 TeV during Run1 at the LHC, corresponding to an integrated luminosity of 19.5/fb. This analysis exploits the production of the Higgs boson through vector boson fusion (VBF), with the Higgs decaying to 2 photons. The kinematic distributions of the dijet and diphoton systems, which depend from the spin-parity hypothesis, are used to build some discriminants able to discriminate between different spin-parity hypotheses. These discriminants allow to define different regions of the phase-space enriched with a certain spin-parity process. The Higgs boson signal yield is extracted in each region from a fit to the diphoton mass, allowing to determine the contributions of the different processes and then constrain the production of a pseudo-scalar (spin-parity 0-) Higgs boson

La théorie effective à basse énergie du secteur électrofaible sans particule de Higgs utilise les méthodes de la Théorie de perturbation Chirale. La formule de comptage de Weinberg permet un développement cohérent en boucles et la renormalisation correspondante. On trouve que certains opérateurs effectifs, supprimés par une échelle de masse dans le cas du Modèle Standard, ne le sont plus dans le cas sans particule de Higgs. De plus, ils apparaissent à l'ordre dominant du développement chiral, en conflit avec les expériences. Pour décrire leur suppression, on demande l'invariance sous la symétrie obtenue lorsque le secteur composite (produisant les trois modes de Goldstone) et le secteur élémentaire (quarks, leptons et champs de Yang-Mills) sont découplés. On introduit alors les couplages en réduisant la symétrie à
SU(2)xU(1) via des spurions. Les opérateurs indésirables n'apparaissent qu'aux ordres supérieurs du développement simultané en puissances des impulsions et des spurions. De plus, la brisure de l'isospin faible est traitée de façon systématique et la formule de comptage de Weinberg peut être reproduite. Les spurions permettent de rendre compte des faibles masses des neutrinos actifs. Les trois neutrinos droits, légers par rapport au TeV, introduits afin de préserver une symétrie custodiale, sont quasi-stériles et stables. On étudie également le raccordement des anomalies dans le secteur composite, généralisant la construction de Wess-Zumino: on obtient une contrainte sur la théorie sous-jacente. Appliquant le formalisme des spurions aux modèles "moose" linéaires ouverts, on démontre des règles de somme de Weinberg généralisées.

La théorie effective à basse énergie du secteur électrofaible sans particule de Higgs utilise les méthodes de la Théorie de Perturbation Chirale. La formule de comptage de Weinberg montre la cohérence du développement en boucles, avec la renormalisation correspondante. En revanche, on trouve que certains opérateurs effectifs, supprimés par une échelle de masse dans le cas du Modèle Standard, ne le sont plus dans le cas sans Higgs. De plus, ils apparaissent à l'ordre dominant du développement chiral, en conflit avec les expériences. Pour décrire leur suppression, on demande l'invariance sous la symétrie obtenue lorsque le secteur composite (produisant les trois modes de Goldstone) et le secteur élémentaire (quarks, leptons et champs de Yang Mills) sont découplés. On introduit alors les couplages en réduisant la symétrie à SU(2)xU(1), via des spurions. Les opérateurs indésirables n'apparaissent qu'aux ordres supérieurs du développement simultané en puissances des impulsions et des spurions. De plus, la brisure de l'isospin faible est traitée de façon systématique et la formule de comptage de Weinberg peut être reproduite. Les spurions permettent de rendre compte des faibles masses des neutrinos actifs. Trois neutrinos droits, légers par rapport au TeV, sont introduits afin de préserver une symétrie custodiale : ils sont quasi-stériles et stables et peuvent contribuer à la matière sombre. On étudie également le raccordement des anomalies dans le secteur composite, généralisant la construction de Wess-Zumino : on obtient une contrainte sur la théorie sous-jacente. Appliquant le formalisme des spurions aux modèles moose, on démontre des règles de somme de Weinberg généralisées
The low-energy effective theory of electroweak symmetry-breaking without a Higgs particle is constructed using the methods of Chiral Perturbation Theory. Weinberg's power-counting formula demonstrates the consistency of the loop expansion, with the corresponding renormalization. We find that the suppression of effective operators by a mass scale, which was automatic in the case of the Standard Model, no longer holds in the Higgs-less case for a specific class of operators. Moreover, the incriminated operators appear at leading order in the chiral expansion, in conflict with experiments. To account for their suppression, invariance under a larger symmetry is required, corresponding to the composite sector (which produces the three Goldstone modes) being decoupled from the elementary sector (quarks, leptons and Yang-Mills fields). The couplings are introduced via spurions: this reduces the symmetry to SU(2)xU(1). Ln the simultaneous expansion in powers of momenta and spurions, the aforementioned operators are relegated to higher orders. Ln addition, the method allows for a systematic treatment of weak isospin breaking. The Weinberg power-counting formula can be recovered, and small neutrino masses accounted for. The three right-handed neutrinos (lighter than the TeV) are introduced in connection with the custodial symmetry: they are quasi-sterile and stable and may thus contribute to dark matter. We also obtain a constraint on the underlying theory by studying the anomaly-matching in the composite sector and generalizing the Wess-Zumino construction. The spurion formalism is also applied to open linear moose models, for which generalized Weinberg sum rules are derived

One of the main motivating factors for the construction of the Large Hadron Collider (LHC) was the search for the Higgs boson, postulated to explain the origin of fundamental gauge boson masses. This thesis presents the results of the first search for the Higgs boson at the LHC, using 35 pb^−1 of proton-proton collision data with a centre of mass energy of 7 TeV collected by the ATLAS experiment throughout 2010. The search is performed in the H -> WW channel, since the branching ratio for Higgs boson decays to W bosons is large for a wide range of Higgs boson masses. Two different search methods are presented: a straightforward cut-based method and a method using a matrix-element-based discriminant to provide additional separation between signal and background. The matrix element method is shown to give better expected sensitivity at all Higgs boson masses. Using these methods, a SM-like Higgs boson with a mass of 160 GeV with a production rate of 1.2 times the SM rate is excluded at 95% Confidence Level and limits are placed on the production rate of the SM Higgs boson in the range of masses from 120 < mH < 200 GeV. In addition, a measurement of the SM WW cross section is performed. It is essential to understand this channel since it is the major background to the H -> WW search. SM WW production is also sensitive to new physics processes, which would enhance its cross section. The SM WW cross section is measured to be σ(WW) = 40+20−16(stat.)±7(syst.) pb, which is consistent with the NLO SM expectation of 46 ± 3 pb.

The highly celebrated discovery of a new particle with a mass of 125 GeV in proton-proton collisions by the ATLAS and CMS experiments at the CERN Large Hadron Collider in 2012 has been shown to be compatible with the Standard Model description of the Higgs boson. However, in order to fully verify the Standard Model nature of the Higgs boson, most of its properties still remain to be measured. Such measurements include differential cross section measurements, which are shown here for the H→WW decay channel and the coupling of the Higgs boson to bottom quarks, for which a study of future prospects is presented. Differential fiducial cross section measurements of the Higgs boson were performed in the H → WW*→lvlv channel at the ATLAS detector with 20 fb−1 of √s = 8 TeV collision data. For Higgs bosons produced by gluon-gluon fusion, the cross section is measured as a function of kinematic variables, including transverse momentum and rapidity of the Higgs boson, as well as the number of jets associated with the Higgs event. The obtained distributions are unfolded to a fiducial volume using a two-dimensional iterative Bayesian algorithm. The measured fiducial differential cross sections are compared to predictions from Monte Carlo generators. The total cross section measured in the fiducial volume defined by the charged lepton and neutrino kinematic properties is 36.0 ± 9.7 fb. Additionally the jet-veto efficiency in the fiducial volume is extracted from the differential cross sections. An analysis is presented of Higgs boson production and decay into bottom quarks in association with a vector boson at the ATLAS detector for the future high-luminosity LHC with proton-proton collisions at √s = 14 TeV. The vector bosons are reconstructed from Z→l+l− or W→lv final states, where l is an electron or muon. The analysis uses generator-level Monte Carlo samples to which efficiency and resolution smearing functions are applied. These reproduce the expected resolution of the upgraded ATLAS detector for the foreseen amount of pile-up due to multiple overlapping proton-proton collisions. The analysis of the ZH(→ l+l−b¯b) channel is presented and results are combined with the WH(→lvb¯b) channel from a corresponding study. For an integrated luminosity of 300 fb−1 using an average pile-up of 60, the expected significance is 3.9 σ with an expected error on the signal strength of 25%. Likewise, for 3000 fb−1 using an average pileup of 140 the expected significance is 8.8 σ , and the error on the signal strength is expected to be about 15%.

This thesis presents a cross-section measurement of the Z -+ TeTh process performed using the ATLAS detector at the Large Hadron Collider at CERN. The data used were collected at a centre-of-mass energy of 7 TeV during 2010 and corresponds to an integrated luminosity of 36 pb-I. This measurement uses the final state where one T lepton decays leptonically, to an electron or muon, plus neutrinos, and the other T lepton decays hadronically. The "(* / Z -+ a, It and di-boson background processes are estimated using Monte Carlo simulation, the W -+ TV Monte Carlo background prediction is estimated using a semi data-driven method. The multijet background is estimated with a fully data-driven 'ABCD' method using three exclusive control regions built by inverting uncorrelated requirements on the isolation of the electron or muon and the product of the electric charge of the identified electron or muon and hadronically decayed T lepton. The measured cross-sections in each channel are corrected for the individual branching ratios and yield the total cross-section a(Z -+ TT, minv 66 - 116 GeV) = 1142 ± 135.5(stat.) ± 206.2(syst.) ± 40. 19(1umi.) ± 3.6(theo.) pb for the electron channel and a(Z -+ TT, minv 66 - 116 GeV) = 857.6 ± 81.4(stat.) ± 132.5(syst.) ± 30.19(lumi.) ± 2.8(theo.) pb for the muon channel, where the invariant mass of the Z boson is between 66 and 116 GeV. These are in good agreement with the theoretical prediction of960 ± 49.5 pb. Performing a measurement of a well-known Standard Model process using the ATLAS de- tector in this new high energy regime is essential for validating the T lepton identification in particular, which is essential for any new physics search such as H -+ TeTh' A cut-based analysis for the Vector Boson Fusion produced Higgs boson decaying to two T leptons channel is introduced and the possibility of using the more complex analysis technique, the Matrix Element method, to increase sensitivity is discussed.

Les prédictions du Modèle Standard de la physique des particules ont été testées dans des expériences à un degré de précision considérable. Aujourd'hui, les collisionneurs de particules recherchent l'unique élément au sein du Modèle Standard inobservé à ce jour : le boson de Higgs, responsable de la brisure de la symétrie électrofaible. La liberté asymptotique de la chromodynamique quantique (QCD), la théorie des interactions fortes du Modèle Standard, rend possible les calculs perturbatifs en QCD. En revanche, à cause de la valeur de la constante de couplage forte, plusieurs ordres de la série perturbative sont nécessaires pour que les prédictions QCD soient précises et comparables aux mesures des collisionneurs. Le spectre de masse du boson de Higgs a été contraint par des recherches directes et des ajustements de précision. Dans la partie basse de la fenêtre de masse autorisée, les recherches du boson de Higgs se désintégrant en deux photons sont parmi les plus efficaces. L'étude au LHC de paires de photons isolés, appelées diphotons, permet de tester la QCD perturbative à des échelles d'énergie jusqu'alors inexplorées. De plus, la mesure de la section efficace de production de diphotons est capitale car les paires de photons produites dans les interactions dures de QCD représentent le plus important bruit de fond dans les recherches du boson de Higgs se désintégrant en deux photons. Cette thèse présente une étude détaillée de la production de paires de photons isolées, son dernier chapitre détaille la première mesure de la section efficace différentielle de production de paires de photons avec le détecteur CMS à une énergie de centre de masse de 7 TeV
The predictions made by the Standard Model of particle physics have been tested experimentally to a considerable degree of precision. The quest is now on, at high energy particle colliders, for the only remaining pièce within the framework of the Standard Model to be discovered, namely the Higgs boson, which is responsible for the electroweak symmetry breaking. The asymptotic freedom of Quantum Chromodynamics (QCD), the theory of strong interactions in the Standard Model, makes QCD predictions possible in perturbation theory. The comparatively high value of the strong coupling constant however makes several orders of the series necessary for predictions to be accurate and comparable to measurements. The mass of the Higgs boson is known from direct searches and by precision fits of electroweak data. In the lower part of this mass window, searches of the Higgs boson decaying to two photons are among the most sensitive. Studies of isolated photon pairs at the LHC allow the testing QCD in its perturbative regime at energy scales never probed before. Moreover, the measurement of the cross section of diphotons is crucial, as photon pairs produced in hard QCD interactions form the largest irreducible background to the search for a low mass Higgs boson that decays to diphotons. This thesis presents a comprehensive study of the production cross section of pairs of prompt isolated photons. Its last chapter gives the details of the first measurement of the production cross section of pairs of isolated photons with the CMS detector at a centre-of-mass energy of 7 TeV

A search for the Higgs boson produced with large transverse momentum and decaying as H -> bb is presented, using the 80 fb^{-1} from the dataset collected by the ATLAS detector at sqrt{s}=13 TeV. The Higgs boson is reconstructed as a large-R jet with two b-tagged variable radius track-jets. The work focuses on a peculiar event topology in which the Higgs is produced in association with another Large-R jet. Considering events with reconstructed pT above 480 GeV and with a reconstructed mass from 70-230 GeV, a signal significance of 1.6 sigma for the Higgs and of 5 sigma for the V (Z boson + W boson) is observed. Furthermore, the new Full Run2 analysis is presented, using 136 fb^{-1} from the dataset collected by the ATLAS detector at sqrt{s}=13 TeV. The strategy up to the computation of the expected sensitivity for the Higgs boson is presented in this thesis. The validation of the background modeling is described in details, including tests on data. A preliminary extraction of the Z boson signal is performed and the measured signal strength corresponds to muZ = 0.82 +/- 0.09.

A search for the Higgs boson produced with large transverse momentum and decaying as H -> bb is presented, using the 80 fb^{-1} from the dataset collected by the ATLAS detector at sqrt{s}=13 TeV. The Higgs boson is reconstructed as a large-R jet with two b-tagged variable radius track-jets. The work focuses on a peculiar event topology in which the Higgs is produced in association with another Large-R jet. Considering events with reconstructed pT above 480 GeV and with a reconstructed mass from 70-230 GeV, a signal significance of 1.6 sigma for the Higgs and of 5 sigma for the V (Z boson + W boson) is observed. Furthermore, the new Full Run2 analysis is presented, using 136 fb^{-1} from the dataset collected by the ATLAS detector at sqrt{s}=13 TeV. The strategy up to the computation of the expected sensitivity for the Higgs boson is presented in this thesis. The validation of the background modeling is described in details, including tests on data. A preliminary extraction of the Z boson signal is performed and the measured signal strength corresponds to muZ = 0.82 +/- 0.09.

The Higgs boson and the top quark have been a focus in modern elementary particle physics research because of their special roles in the Standard Model (SM) of particle physics. The studies of both particles are crucial for revealing the unsolved puzzles of modern particle physics. The coupling between the Higgs boson and the top quark, the top-Yukawa coupling, is one of the fundamental parameters in the SM that can potentially direct the future development of the theory of elementary particle physics. This thesis presents two analyses on the Higgs boson and the top quark, using proton-proton (pp) collision data collected by the ATLAS detector during 2012 and 2015. A search for the SM Higgs boson produced in association with a top quark pair (tt̄H) was performed using 20.3 fb⁻¹ of pp collision data at a centre-of-mass energy of √s = 8 TeV. The search is designed to be primarily sensitive to the H → bb decay mode. Events with one of two electrons or muons are used in the search. No significant excess of events is observed above the background predicted by the SM. An observed (expected) upper limit on the signal strength of 3.4 (2.2) times the SM prediction is obtained at 95% confidence level. The tt̄H signal strength, represented by the ratio of the measured tt̄H cross-section to the SM prediction, is found to be μ = 1.5 ± 1.1 for a Higgs boson mass of m_H = 125 GeV. A measurement of the top quark pair (tt̄) production cross-section was performed using 3.2 fb⁻¹ of pp collision data at √s = 13 TeV. The measurement uses events with an opposite-charge-sign electron-muon pair and exactly one and two jets originating from b quarks. The inclusive tt̄ production cross-section is measured to be σ_tt̄ = 818 ± 8(stat) ± 27(syst) ± 19(lumi) ± 12(beam) pb, where the uncertainties arise from data statistics, analysis systematic effects, the integrated luminosity and the LHC beam energy. The total relative uncertainty is 4.4%. The result is consistent with the theoretical prediction at the next-to-next-to-leading order accuracy in the strong coupling constant αs of QCD, with the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms. A fiducial cross-section corresponding to the experimental acceptance of leptons is also measured.

This thesis presents two measurements performed in the $Htautau$ channel and based on the proton-proton collisions data collected by the ATLAS experiment during 2015 and 2016 and corresponding to an integrated luminosity of 36.1~fb$^{-1}$. The $Htautau$ is a considerably important decay channel because it allows to directly measure the Higgs Yukawa coupling and to probe the Higgs boson properties. Measurements of the Higgs production cross section in the $Htautau$ decay channel are presented. The observed (expected) significance of the $H ightarrow au au$ signal excess over the expected background amounts to 4.4 (4.1) standard deviations. This result, combined with the data taken at $sqrt{s}=7$ and 8 TeV, leads to an observed (expected) significance of 6.4 (5.4) and constitutes the first ATLAS observation of $H ightarrow au au$. The measured total cross section of $H ightarrow au au$, using the data collected at $sqrt{s}=13$~TeV, is $3.77^{+0.60}_{-0.59}~( ext{stat.})~^{+0.87}_{-0.74}~( ext{syst.})$~pb which is consistent with the Standard Model expectation. In addition, total cross sections for the vector boson fusion and gluon-gluon fusion processes have been measured separately and similar results are reported based on the simplified template cross section framework. All of these measurements are in agreement with the Standard Model predictions. The second measurement presented in this manuscript concerns a CP violation study in the same decay channel. The coupling structure of the vertex where the Higgs couples to two electroweak gauge bosons in its vector boson fusion production is investigated. The Optimal Observable method is used to directly test CP invariance in this vertex. No sign of CP violation is observed and constraints on the $ ilde{d}$ parameter, which governs the strength of CP violation, have been set. A 68% CL interval is observed for $ ilde{d}$ of [-0.090, 0.035], compared to an expected interval of $ ilde{d} in [-0.035, 0.033]$. No constraints can be set at 95% CL despite an expected interval $ ilde{d} in [-0.21, 0.15]$ at 95% CL was determined.