The discovery of the Higgs boson by the LHC provided the last piece of the puzzle neces- sary for the Standard Model (SM) to be a successful theory of electroweak scale physics. However there exist various phenomenological reasons which serve as pointer towards the existence of physics beyond the Standard Model. For example the explanation for the smallness of the neutrino mass, baryon asymmetry of the universe, the presence of dark matter and dark energy etc. are not within purview of the Standard Model. Con- ceptual issues like the gauge hierarchy problem, weakness of gravity provide some of the theoretical motivation to pursue theories beyond the SM. We consider scenarios with warped extra-dimensions (Randall-Sundrum (RS) Model ) as our preferred candidate to answer some of the questions raised above. RS model gives an elegant geometric solution to address the hierarchy between the two fundamental scales of nature i.e. Planck scale and electroweak scale. In addition to this, the geometry of RS serves as a useful setup wherein the fermion mass hierarchy problem can also be solved. The goal of this thesis is to investigate whether RS model can be an acceptable theory of avour while at the same time serving as a solution to the hierarchy problem. In Chapter[1] we begin with a brief introduction of the SM, highlighting issues which pro- vides the necessary motivation for constructing new physics models. Various candidates of Beyond Standard Model (BSM) physics are introduced and a few preliminaries es- sential to understand frameworks with additional spatial-dimensions ( at and warped) is provided. In Chapter[2] we specialize to the case of warped extra-dimensions and motivate the need to have the SM elds in the bulk. Mathematical details related to the analysis of various spin elds (0; 12; 1 and 2) in a warped background necessary to understand relevant phenomenology is provided. The lack of knowledge of Dirac or Majorana nature of the neutrino leads to a wide variety of possibilities as far as neutrino mass generation is concerned. In Chapter[3] we focus on the leptonic sector where three cases of neutrino mass generation are consid- ered: a)Planck Scale lepton number violation (LLHH case) b) Dirac neutrinos c) Bulk Majorana mass terms. We then study the implications of each case on the charged lepton mass tting. The case with Planck scale lepton number violation in normal RS scenario requires large and negative values for the bulk mass parameters for the charged singlets cE. Dirac neutrinos and the case with Bulk Majorana mass terms give good t to data. For completeness, the ts for the hadronic sector is provided in the appendix. In Chapter[4] avour violation for each of three cases introduced in Chapter[3] is studied. For the case with Planck scale lepton number violation, the non-perturbative Yukawa coupling between the SM singlets and the KK states render the higher order diagrams incalculable. Lepton avour violation (LFV) is particularly large for the Dirac case and the bulk Majorana case for low Kaluza-Klein(KK) mass scales. We then invoke the ansatz of Minimal Flavour violation to suppress LFV with low lying KK scales and examples of avour group is provided for both cases. In Chapter[5] we present an example with a type II two Higgs doublet model applied to the LLHH case. The setup o ers a solution where LLHH scenario can be consistently realized in RS model, where the masses and mixing angles in the leptonic sector can bet with O(1) choice of bulk parameters. Assumption of global lepton number conservation (like in Dirac neutrinos) could lead to problems in theories of quantum gravity where it does not hold. This leads us to the question whether Dirac neutrinos can be naturally realized in nature. In Chapter[6] we consider the special case of bulk Majorana mass encountered in Chapter[3] where the bulk Dirac mass terms for the right handed neutrino is set to zero. We nd that this leads to a case where the e ective zero mode neutrino mass is of Dirac type with negligible e ects from the tower of Majorana states. In Chapter[7] we consider RS at the GUT scale which no longer serves as a solution to the hierarchy problem. SUSY is introduced in the bulk and the low energy SUSY serves as a solution to the hierarchy problem. Such models serve as a useful alternative to SUSY models with family symmetries (e.g. Froggatt-Nielsen Model). However the solutions to the Yukawa hierarchy problem are constrained due to anomaly cancellation conditions. In Chapter[8] supersymmetry breaking due to radion mediation in addition to brane localized sources is considered and detailed analysis of the running of soft masses and the low energy avour observables is considered for both cases separately. In Chapter[9] we conclude and present future directions.

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 Modèle Standard Supersymétrique Minimal (MSSM), le plus étudié des modèles de Nouvelle Physique, contient un candidat à la matière noire : le neutralino. Un des mécanismes qui permet de réduire la densité relique de neutralino jusqu'à l'intervalle expérimental de WMAP et de Planck est la coannihilation entre le neutralino et le stop. Dans cette thèse nous étudions deux aspects différents liés à la prédiction de la densité relique dans la région de coannihilation neutralino-stop, ainsi qu'au calcul des sections efficaces d'annihilation et de coannihilation correspondantes. Nous présentons tout d'abord la matière noire en tant que WIMP ainsi que le Modèle Standard de la Physique des Particules, puis nous abordons le MSSM ainsi la phénoménologie de la densité relique de neutralino. Nous étudions ensuite la phénoménologie de la violation de saveur non minimale dans le secteur des squarks dans le contexte de la densité relique de neutralino. Nous considérons des termes violant la saveur dans le secteur des squarks up et down de chiralité droite et de troisième génération et montrons qu'ils peuvent avoir un impact important sur les sections efficaces d'annihilation et de coannihilation du neutralino, et en conséquence sur la densité relique. Finalement, nous nous intéressons à la possibilité d'améliorer la précision avec laquelle la densité relique est prédite, en calculant les sections efficaces d'annihilation et de coannihilation à l'ordre supérieur dans la théorie des perturbation. En se basant sur des travaux antérieurs qui ont montré que l'impact des corrections SUSY-QCD à une boucle pour l'annihilation de neutralino était supérieur à l'incertitude expérimentale, nous avons calculé de telles corrections dans le cas de la coannihilaiton neutralino-stop en bosons de jauge électrofaibles et bosons de Higgs. / The Minimal Supersymmetric Standard Model (MSSM), the most famous model of Physics beyond the Standard Model, provides a good dark matter candidate: the neutralino. One of the mechanisms which can reduce the predicted relic density of neutralino to the experimental range of WMAP and Planck is the coannihilation between the neutralino and the stop. In this thesis, we study two different aspects related to the prediction of the relic density in the neutralino-stop coannihilation region and the calculation of the corresponding annihilation and coannihilation cross sections. We first make short reviews of WIMP dark matter and the Standard Model of particle Physics, introduce the MSSM and discuss the phenomenology of neutralino relic density. We then study the phenomenology of Non Minimal Flavor Violation in the squark sector in the context of neutralino relic density. We consider flavor violating terms in the sectors of right handed third generation up and down squarks and show that they can have an important impact on the thermally averaged (co)annihilation cross section of the neutralino, and therefore on its relic density. Finally, we focus on the issue of improving the precision with which the relic density is calculated, by computing (co)annihilation cross sections at Next-to-Leading Order. Following earlier work in the calculation of one-loop SUSY-QCD corrections to the annihilation of neutralinos, which have shown that the impact of such corrections was larger than the experimental uncertainty, we have calculated similar corrections for the neutralino-stop coannihilation into electroweak gauge and Higgs bosons.

The Standard Model of particle physics attempts to unify the fundamental forces in the Universe (except gravity). Over the years it has been tested in numerous experiments. While these experimental results strengthen our understanding of the SM, they also point out directions for physics beyond the SM. In this thesis we assume supersymmetry (SUSY) to be the new physics beyond the SM. We have tried to analyze the present status of low energy SUSY after the recent results from direct (collider) and indirect (flavor, dark matter) searches .We have tried to see the complementarity between these apparently different experimental results and search strategies from the context of low energy SUSY. We show that such complementarity does exist in well-defined models of SUSY breaking like mSUGRA, NUHM etc. The first chapter outlines the present status of the SM and discusses about the unanswered questions in SM. Keeping SUSY as the new physics beyond the SM, we also detail about its present experimental status. Chapter1 ends with the motivation and comprehensive description about each chapter of the thesis. In chapter2, we present an introduction to formal structure of SUSY algebra and the structure of MSSM. One of the such complementarities we have studied is between flavor and dark matter. In general flavor violation effects are not considered when studying DM regions in minimal SUSY models like mSUGRA. If however flavor violation does get generated through non-minimal SUSY breaking sector, one of the most susceptible regions would be the co-annihilation region for neutralino DM. In chapter 3 we consider flavor violation in the sleptonic sector and study its implications on the stau co-annihilation region. In this work we have taken flavor violation between the right-handed smuon (˜µR) and stau (˜τR). Due to this flavor mixing the lightest slepton (ĺ1) is a flavor mixed state. We have studied the effect of such ĺ11’s in the ‘stau co-annihilation’ region of the parameter space, where the relic density of the neutralinos gets depleted due to efficient co-annihilation with the staus. Limits on the flavor violating insertion in the right-handed sleptonic sector mainly comes from BR(τ → µγ). These limits are weak in some regions of the Parameter space where cancellations happen with in the amplitudes. We look for overlaps in parameter space where both the co-annihilation condition as well as the cancellations with in the amplitudes occur. We have shown that in models with non-universal Higgs boundary conditions (NUHM) overlap between these two regions is possible. The effect of flavor violation is two fold: (a) It shifts the co-annihilation regions towards lighter neutralino masses and (b) the co-annihilation cross sections would be modified with the inclusion of flavor violating diagrams which can contribute significantly. In the overlap regions, the flavor violating cross sections become comparable and in some cases even dominant to the flavor conserving ones. A comparison among the different flavor conserving and flavor violating channels, which contribute to the neutralino annihilation cross-section, is presented. One of the challenges of addressing quantitatively the complementarity problems is the lack of proper spectrum generator (numerical tools which computes SUSY sparticle spectrum in the presence of flavor violation in the sfermionic sector). For the lack of a publicly available code which considers general flavor violating terms in the renormalization group equations (RGE) we have developed a SUSY spectrum calculator, named as SuSeFLAV .It is a code written in FORTRAN language and calculates SUSY particle spectrum (with in the context of gravity mediation) in type I seesaw, in the presence of heavy right handed neutrinos (RHN). SuSeFLAV also calculates the SUSY spectrum in other type of SUSY breaking mechanisms (e.g. gauge mediation). The renormalization group (RG) flow of soft-SUSY breaking terms will generate large off-diagonal terms in the slepton sector in the presence of this RHNs, which will give rise to sizable amount of flavor violating (LFV) decays at the weak scale. Hence, in this code we also calculate the different rare LFV decays like, µ → eγ, τ → µγ etc. In SuSeFLAV the user has the freedom to choose the scale of the RHNs as well as the mixing matrix in neutrino sector. It is also possible to choose the values of the SUSY breaking input parameters at the user defined scale. The details of this package is discussed in chapter 4. Many of the present studies of complementarity between the direct and indirect searches are inadequate to address realistic scenarios, where SUSY breaking could be much more general compared to the minimal models. The work in this thesis is a step to wards this direction. Having said that, in the present thesis we have considered modifications of popular models with either explicit flavor violating terms (in some sectors) or sources of flavor violation through new particles and new couplings motivated by strong phenomenological reasons like neutrino masses. It should be noted however, the numerical tool which has been developed during the thesis can be used to address more complicated problems like with complete flavor violation in models of SUSY breaking. One of the popular mechanisms of neutrino mass generation is the so called Seesaw Mechanism. Depending on the extra matter sector present in the theory there are three basic types of them. The type I seesaw, which has singlet bright-handed neutrinos, the type II seesaw contains scalar triplets and type III seesaw has additional fermionic triplets. One of the implications of the seesaw mechanism is flavor violation in the sfermionic sector even in the presence of flavor universal SUSY breaking. This leads to a complementarity between flavor experiments and direct SUSY searches at LHC. With the announcement of the results from the reactor neutrino oscillation experiments, the reactor mixing angle (θ13) in the neutrino mixing matrix (PMNS matrix) gets fixed to a rather large non-zero value. In SO (10) GUT theories neutrino Yukawa couplings of type I seesaw gets related to the up-type fermion sector of the SM. In chapter 5 we update the status of SUSY type I seesaw assuming SO (10)- like relations for neutrino Dirac Yukawa couplings and two cases of mixing, one large, PMNS-like, and another small, CKM-like, are considered. It is shown that for the large mixing case, only a small range of parameter space with moderate tan β is still allowed. It is shown that the renormalization group induced flavor violating slepton mass terms are highly sensitive to the Higgs boundary conditions. Depending on the choice of the parameters, they can either lead to strong enhancements or cancellations with in the flavor violating terms. We have shown that in NUHM scenario there could be possible cancellations which relaxes the severe constraints imposed by lepton flavor violation compared to mSUGRA. We further updated the flavor consequences for the type II seesaw in SUSY theories. As mentioned previously in type II seesaw neutrino mass gets generated due to exchange of heavy SU (2) L triplet Higgs field. The ratio of lepton flavor violating branching ratios (e.g. BR(τ → µγ) /BR (µ → eγ) etc.) are functions of low energy neutrino masses ans mixing angles. In chapter 6 we have analyzed how much these ratios become, after the experimental measurement of θ13, in the whole SUSY parameter space or in other words how much these ratios help to constrain the SUSY parameter space. We compute different factors which can affect this ratios. We have shown that the cMSSM-like scenarios, in which slepton masses are taken to be universal at the high scale, predict 3.5 BR(τ → µγ) / BR(µ → eγ) 30 for normal hierarchical neutrino masses. We Show that the current MEG limit puts severe constraints on the light sparticle spectrum in cMSSM-like model for seesaw scale with in1013 - 1015 GeV. These constraints can be relaxed and relatively light sparticle spectrum can be still allowed by MEG result in a class of models in which the soft mass of triplet scalar is taken to be non-universal at the GUT scale. In chapter 7 we have analyzed the effect of largen eutrino Yukawa couplings on the supersymmetric lightest Higgs mass. In July 2012, ATLAS and CMS collaboration have updated the Higgs search in LHC and found an evidence of a scalar particle having mass around 125 GeV. The one-loop contribution to Higgs mass mainly depends on the top trilinear couplings (At), the SUSY scale and the top Yukawa (Yt). Thus in models with extra large Yukawa couplings at the high scale like the seesaw mechanism ,the renormalization scaling of the At parameter can get significantly affected. This in turn can modify the light Higgs mass at the weak scale for the same set of SUSY parameters. We have shown in type I seesaw with (Yν ~ 3Yu) the light Higgs mass gets reduced by 2 - 3 GeV in most of the parameter rspace. In other words the SUSY scale must be pushed high enough to achieve similar Higgs mass compared to the cMSSM scenario. We have got similar effect in SUSY type III seesaw scenario with (Yν ~Yu) at the GUT scale. In chapter 8 we summarize the results of the thesis and discuss the possible future directions.