Search

A promising avenue to perform precision tests of the SM at the LHC is to measure differential cross-sections at high invariant mass, exploiting in this way the growth with the energy of the corrections induced by heavy new physics. We classify the leading growing-with-energy effects in longitudinal diboson and in associated Higgs production processes, showing that they can be encapsulated in four real "high-energy primary" parameters. We assess the reach on these parameters at the LHC and at future hadronic colliders, focusing in particular on the fully leptonic $WZ$ channel that appears particularly promising. The reach is found to be superior to existing constraints by one order of magnitude, providing a test of the SM electroweak sector at the per-mille level, in competition with LEP bounds. Unlike LHC Run-1 bounds, which only apply to new physics effects that are much larger than the SM in the high-energy tail of the distributions, the probe we study applies to a wider class of new physics scenarios where such large departures are not expected., Comment: 35 pages, 8 Figures, Fig.7 corrected

Aquesta tesi està basada en les publicacions fetes com a estudiant de doctorat a l’IFAE en els períodes 2012-2013 i 2015. Aquests estudis inclouen dos temes diferents, fenomenologia del Higgs al LHCk en el marc de models de Higgs Compost i Supersimetria, i la millora del Mètode de Truncació del Hamiltonià Renormalitzat (MTHR). La primera part de la tesis va ser motivada pel problema de la Naturalitat del Model Estàndard (SM), que es va fer manifest amb el descobriment l’any 2012 d’una partícula escalar amb una massa de 125 GeV i una senyal pels acoblaments compatible amb el Higgs del SM. L’objectiu d’aquests projectes era valorar l’impacte de la massa i senyal dels acoblaments mesurats en les dues extensions del SM més motivades per solventar el problema de la Naturalitat; aquests són models de Higgs Compost i models Supersimetrics. Aquesta part de la tesis està organitzada de la següent forma: En el Capítol 1 fem una breu introducció al Model Estàndard, les seves limitacions i motivem la necessitat d’anar a física més enllà del Model Estàndard (BSM). També incloem breus introduccions al trencament espontani de simetria (SSB) i al CCWZ, degut a la seva importància per entendre el SM i pel seu rol central en l’estudi de física més enllà del model estandard. En el Capítol 2 estudiem les implicacions del descobriment del Higgs per diversos models de Higgs compost. Això ho fem utilitzant les senyals dels acoblaments mesurades pels experiments just després del seu descobriment al 2012, que ens permeten reduir de forma significativa el espai de paràmetres d’aquests models. En el Capítol 3 ens centrem en l’estudi dels acoblaments del Higgs en models de Higgs compostos basats en el cose SO(5)/SO(4). En aquest apartat mostrem que els acoblaments del Higgs a gluons i fotons és poc sensible a resonàncies fermiòniques lleugeres si assumim que la simetria CP es conserva. També, trobem a primer ordre en els “mixings”, l’acoblament del Higgs a tops i gluons, i veiem que quan els normalitzem al Model Estàndard són iguals. En el Capítol 4, estudiem la relació entre els acoblaments del Higgs i la seva massa en models Supersimetrics, i trobem que la massa del Higgs de 125 GeV i la senyal dels acoblaments mesurats poden ser competitius per excloure parts del espai de paràmetres del sector de Higgs. Nosaltres ens centrem en el MSSM amb stops pesats, i possibles extensions que disminueixen el “petit” problema de les jerarquies. La segona part d’aquesta tesi va ser motivada per dos papers que van fer Rychkov et al. on presenten un nou mètode prometedor per poder solucionar sistemes fortament acoblats. El mètode consisteix en millorar el ja conegut Mètode de Truncació Hamiltoniana que s’utilitza per resoldre numericament sistemes fortament acoblats. La millora ve de integrar analiticament els modes més energètics de la teoria, cosa que fa millorar de forma substancial el resultat numèric, i permet reduir la demanda computacional per poder resoldre el sistema. Aquesta feina la presentem en el Capítol 5, on millorem el mètode a partir de trobar les expressions generals que són necessàries per dur a terme el procediment d’integrar els modes més energètics; a la vegada també resolem algunes de les qüestions que s’havien plantejat en articles anteriors a la nostra feina., This thesis is based on the publications done as a PhD student at IFAE during the periods 2012-2013 and 2015. These include two separate topics, Higgs phenomenology at the LHC within the frameworks of Composite Higgs and Supersymmetry and the improvement of the Renormalized Hamiltonian Truncation Method (RHTM). The first part of this thesis was motivated by the Naturalness problem in the Standard Model (SM), which was triggered by the discovery in 2012 of a scalar particle with a mass of 125 GeV and signal strengths compatible with the SM Higgs. The aim of these projects was to asses the impact of the measured mass and signal strengths on two of the best motivated frameworks that tackle the Naturalness problem; Composite Higgs and Supersymmetric extensions of the SM. This part of the thesis is organised as follows. In Chapter 1 we give an overview to the SM its shortcomings and motivate the need to go beyond the Standard Model (BSM). Brief introductions to Spontaneous Symmetry Breaking (SSB) and the CCWZ are also included due to their importance in the understanding of the SM and their pivotal role in the study of BSM physics . In Chapter 2 we study the implications of the Higgs discovery for different Composite Higgs models. This is done by using the signal strengths given by the experiments right after the Higgs discovery in 2012, which allow to greatly constrain the parameter space of some of these models. Chapter 3 is focused on the study of the Higgs couplings in Composite Higgs models based on the SO(5)/SO(4) coset. We show that the Higgs couplings to gluons and photons are insensitive to light fermionic resonances assuming that they preserve CP. Also, we find that at leading order in the mixings, the Higgs couplings to tops and gluons, when normalized to the Standard Model (SM), are equal. In Chapter 4 we study the relation between the Higgs couplings and the Higgs mass in Supersymmetric models while finding that the measured 125 GeV Higgs and the signal strengths can be competitive with direct searches in excluding some parts of the parameter space in the Higgs sector. We focus on the MSSM with heavy stops, and possible extensions that alleviate the small hierarchy problem. The second part of this thesis was motivated by two papers by Rychkov et al. where they present a promising new method to solve strongly coupled systems. The method consist in improving the already known Hamiltonian Truncation Method used to numerically solve strongly coupled systems. The improvement comes from analytically integrating out the high energy modes which results in a considerable improvement to the numerical result, and allows to reduce computational power needed to solve any given system. Our work, presented in Ch. 5, improves the method by finding the general expressions needed in the procedure to integrating out the high energy modes, and solves some of the issues left open in their work.

We present a class of composite Higgs models arising from a warped extra dimension that can satisfy all the electroweak precision tests in a significant portion of their parameter space. A custodial symmetry plays a crucial role in keeping the largest corrections to the electroweak observables below their experimental limits. In these models the heaviness of the top quark is not only essential to trigger the electroweak symmetry breaking, but it also implies that the lowest top resonance and its custodial partners, the custodians, are significantly lighter than the other resonances. These custodians are the trademark of these scenarios. They are exotic colored fermions of electromagnetic charges 5/3, 2/3 and -1/3, with masses predicted roughly in the range 500-1500 GeV. We discuss their production and detection at the LHC., Comment: 23 pages, 2 figures

Theories where the Higgs boson is a composite particle elegantly solve the hierarchy problem. This idea has been recently investigated in the framework of 5-dimensional warped models that, according to the AdS/CFT correspondence, have a 4-dimensional holographic interpretation in terms of strongly coupled field theories. We present a minimal model in which the Higgs arises as a pseudo-Goldstone boson and the electroweak symmetry is dynamically broken. This model can successfully solve the flavor problem and pass all the electroweak precision tests. To cite this article: R. Contino, A. Pomarol, C. R. Physique 8 (2007).

The AdS/CFT correspondence allows to relate 4D strongly coupled theories to weakly coupled theories in 5D AdS. We use this correspondence to study a scenario in which the Higgs appears as a composite pseudo-Goldstone boson (PGB) of a strongly coupled theory. We show how a non-linearly realized global symmetry protects the Higgs mass and guarantees the absence of quadratic divergences at any loop order. The gauge and Yukawa interactions for the PGB Higgs are introduced in a simply way in the 5D AdS theory, and their one-loop contributions to the Higgs potential are calculated using perturbation theory. These contributions are finite, giving a squared-mass to the Higgs which is one-loop smaller than the mass of the first Kaluza-Klein state. We also show that if the symmetry breaking is caused by boundary conditions in the extra dimension, the PGB Higgs corresponds to the fifth component of the bulk gauge boson. To make the model fully realistic, a tree-level Higgs quartic coupling must be induced. We present a possible mechanism to generate it and discuss the conditions under which an unwanted large Higgs mass term is avoided., 31 pages, 6 figures