The search for the Higgs boson has been a cornerstone of the physics program at the Large Hadron Collider in Geneva Switzerland. The ATLAS experiment successfully discovered the Higgs using the so-called ‘Golden Channels’ of H0 -> gamma gamma and H0 -> ZZ(∗) using data samples collected during the 2011 and 2012 run periods. In order to check if the discovered Higgs is consistent with purely Standard Model behaviour, it is necessary to further confirm the existence of the Higgs in each production mode and decay channel predicted by the Standard Model. For this dissertation, a search for the Higgs was conducted using the H0 -> b bbar decay channel, where the Higgs is produced by the inverse pair decay of two weak bosons exchanged by a scattered quark pair, also known as Vector Boson Fusion (VBF). This analysis uses data samples collected during the 2011 run period by the ATLAS detector totalling 4.2 /fb of proton-proton collisions at sqrt(s) = 7 TeV. No excess of events above background expectation is observed and 95% confidence level upper limits on the Standard Model Higgs cross section times branching ratio, sigma(VBF) x BR(H0 -> b bbar), are derived for Higgs masses in the range 115 < mH < 130 GeV. An observed 95% confidence level upper limit of 18.7 times the Standard Model cross section is obtained for a Higgs boson mass of 125 GeV. / Graduate / 0798

The search potential of a Standard Model Higgs boson in the Vector Boson Fusion production mechanism with Higgs boson decaying to two leptons and two neutrinos via decay to two Z bosons with the ATLAS detector is investigated. The ATLAS detector is a general purpose detector in operation at CERN measuring proton-proton collisions produced by the Large Hadron Collider. This channel has been shown to have high sensitivity at large Higgs mass, where large amounts of missing energy in the signal provide good discrimination over expected backgrounds. This work takes a first look at whether the sensitivity of this channel may be improved using the remnants of the vector boson fusion process to pro- vide extra discrimination, particularly at lower mass where sensitivity of the main analysis is reduced because of lower missing energy. Simulated data samples at centre of mass energy 7 Te V are used to derive signal significances over the mass range between 200-600 Ge V / c2. Because of varying signal properties with mass, a low and a high mass event selection were developed and optimized. A comparison between simulated and real data (collected in 2010) is made of variables used in the analysis and the effect of pileup levels corresponding to those in the 2010 data is investigated. Possible methods to estimate some of the main backgrounds to this search are described and discussed. The impact • of important theoretical and detector related systematics are taken into account. Final results are presented in the form of 95 % Confidence Level exclusion limits on the signal cross section relative to the SM prediction as a function of Higgs boson mass, based on an integrated luminosity of 33.4 pb -1 of data collected during 2010.

This paper presents an outline of the generation of mass for the massive Standard Model particles (fermions, $W^\pm$, $Z^0$) through electroweak symmetry breaking via the Higgs mechanism, and how the Higgs boson emerges from this framework. A Monte Carlo study was done on the decay $H\rightarrow\tau\tau$, with one leptonically and one hadronically decaying tau, with two different production channels for the $H$, gluon-gluon fusion (gg) and vector boson fusion (VBF), at $\sqrt s = 7\tev$ with a Higgs mass $m_H = 120\gev$. The kinematics of these two production channels were compared and it was found that the transverse momentum of muons produced in VBF were higher on average than those produced in gg. This differance was greater in muons originating from the leptonically decaying tau in the Higgs decay, than those produced by other processes in the underlying event. In the latter, however, the difference was still noticable. Jets were slightly more abundant in VBF than in gg, and were in VBF more distributed along the beam axis. The separation in pseudorapidity between the two jets with highest transverse momentum was found to be greater in VBF than in gg. An attempt to reconstruct the Higgs mass using Monte Carlo data run through a simulation of the ATLAS detector was done. The estimator used was the transverse mass of the system consisting of the visible part of the hadronically decaying tau, the lepton from the leptonically decaying tau and the total missing transvese energy. In gg the mean of the transvese mass distribution was found to be $89.26\gev$ with a root mean square uncertainty (RMS) of $23.86\gev$. In VBF the mean was found to be $85.57\gev$ with RMS $27.08\gev$.

One of the goals of the LHC is to test the existence of the Higgs Boson. This thesis presents a study of the potential to discover the Standard Model Higgs boson in the vector boson fusion (VBF) channel for the Higgs mass range 120-200 GeV/c2. The decay of Higgs bosons into WW* final state with both W-bosons decaying leptonically is considered. The main backgrounds are tt_+j and W+W-jj. This study, based on a full simulation of the CMS detector at the LHC, shows that a 5(Sigma) discovery can be done with an integrated luminosity of 12-72 fb-1 for 130-200 GeV/c2 Higgs bosons. Due to the uncertainties in the backgrounds, it is important to measure the backgrounds from data. This study shows that the major background can be measured directly to 7% with 30 fb-1. After discovering the Higgs boson mass using transverse mass template distributions is investigated in the VBF channel. The performance of the combined CMS electromagnetic and hadronic calorimeters (EB+HB) was measured at the H2 test beam at the CERN SPS during 2006 with various partivles in a large momentum range, 1-350 GeV/c. Another major contribution of this thesis is developing the method to optimize the energy reconstruction for the combined EB+HB system with which the corrected responses become 100% with 6% fluctuation and the stochastic resolution is improved from 111% to 94%.

MadGraph5 is used to generate events with Higgs boson pairs from vector boson fusion (VBF) at leading-order (LO) and next-to-leading-order (NLO) accuracy in QCD. The simulations are used to compute fiducial cross-sections in proton-proton collisions at a centre-of-mass energy of 13 TeV, using several kinematic cuts on the outgoing jets, e.g. the jet transverse momenta and pseudorapidity. The resulting cross-sections for NLO and LO are compared and their ratio,  the K-factor, is calculated for every kinematic cut. An attempt is made to extend the NLO simulation for non-Standard Model (SM) couplings between two vector bosons and two Higgs bosons (VVHH), however the corresponding model was found to be only compatible with LO accuracy in QCD.

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.

The nature of interactions of fundamental particles is governed by symmetries. These interactions are well described by an elegant and simple SU(3)c x SU(2)L x U(1)Y symmetric gauge theory that we call the Standard Model (SM) of particle physics. Very recently the CMS and ATLAS experiments at the Large Hadron Collider (LHC) confirmed the discovery of a boson of mass of about 125 GeV. Already, the data collected from these experiments seem to indicate that this particle is in fact the last missing piece and essential ingredient of the Standard Model : the Higgs boson. The Higgs has the very distinct role of providing a mechanism through which masses for other particles can be generated without destroying gauge invariance and hence the renormalizability of the theory. While this discovery completes the picture we have of the SM, the SM itself does not account for several experimentally observed phenomena , notably, dark matter (DM) and the baryon asymmetry in the universe (BAU). From a theoretical perspective a possibility for gauge coupling unification, an explanation for the quark flavour structure and the stability of the Higgs mass to radiative corrections are features that are absent in the framework of SM. This provides a strong basis to the hypothesis that there must be some intermediate scale (between the Planck scale and electroweak scale) of new physics, i.e. physics beyond the SM (BSM). The renormalizability of SM guarantees that various parameters of SM can be determined from the electroweak scale all the way up to the Planck scale. It is interesting to note that the RG evolution of the Higgs quartic coupling is driven to smaller values and can also become negative as the energy scale increases. Naively, a negative quartic coupling indicates destabilization of the EWSB vacuum. The energy scale at which the quartic coupling becomes negative would signify a break down of the theory and would set a scale for new physics. In principle the potential can be made stable through Planck scale dynamics and other vacua (other than the EWSB vacuum) may crop up. In this scenario the EWSB vacuum may decay to the deeper vacua. It is safe to say that, within experimental uncertainties of the Higgs and top quark masses the EWSB vacuum appears to be metastable. We are now left clueless: neither do we have any hints as to the nature of BSM physics nor the scale at which SM breaks down and new physics is assured. One should also note that although the evidence for BSM is compelling, data analysed from 7 and 8 TeV runs of the LHC have not produced any signals of BSM physics so far. Thus any indications of TeV scale BSM physics have been eluding us. In such a scenario the Higgs boson has assumed the role of a portal to study the possibilities of new physics. This is also motivated by the key role that the Higgs plays in generation of mass in a gauge symmetric theory. It is therefore reasonable to assume that the Higgs boson does in fact couple to particles predicted in BSM physics. Such couplings would play a role in modifying the properties of this boson. It is now essential to determine the properties of the Higgs as precisely as possible to search for signs of BSM. This thesis explores the idea of using the Higgs as a portal to study BSM physics. The properties of the Higgs that have already been measured with data from the first two runs of the LHC are its mass, branching ratios, spin and CP. When placed in the framework of a particular new physics model, these properties impose restrictions on the couplings and masses of BSM particles. A strong candidate for a BSM scenario is a Supersymmetric extension of the SM. Supersymmetry is an extension of the Poincar´e group that describes space time symmetries. Fermionic and bosonic degrees of freedom are mixed through the generators of this extended symmetry. In the minimal supersymmetric extension of the SM (MSSM), each particle of SM has a corresponding superpartner with identical quantum numbers modulo its spin. Since we do not see, for example, a bosonic superpartner of the fermionic top quark of the same mass as that of the top quark, this must mean that the supersymmetry, even if it is realized in nature, is not exact and must be broken. Although the symmetry may be broken the MSSM has some very appealing features: stabilization of the Higgs mass to quantum corrections, gauge coupling unification and possible dark matter candidate if the lightest Supersymmetric particle happens to be both stable and neutral. It is interesting to note that in MSSM, the tree level Higgs mass is bounded from above by the Z boson mass ( ~90 GeV ). The measured value of the Higgs mass (~126 GeV ) is still achievable in the MSSM through quantum corrections, the largest contribution coming from the top quarks and stop squarks. One therefore sees that the mass of the Higgs can already provide information about top superpartners. The presence of additional charged and coloured scalars implies the possibility of existence of charge and colour breaking (CCB) minima which would affect the stability of the Electroweak Symmetry breaking (EWSB) minima generated by the Higgs potential. Stability of EWSB is then dependent on parameters in the scalar sector of MSSM. We explore the nexus between the Higgs mass and vacuum stability in this model and find restrictions on the MSSM parameter space. The lighter Higgs of the MSSM couples differently to SM particles than the SM Higgs boson. More specifically one expects the couplings of the MSSM Higgs to gauge bosons to be smaller than in SM and unlike the SM Higgs, up type quarks have couplings strengths that are different from that of down type quarks. In the decoupling regime these differences become negligible and the lighter MSSM Higgs behaves identically to the SM Higgs. The measured Higgs rates do not show any large deviations from the expectations of a SM Higgs. It is therefore reasonable to assume that MSSM, if realized, resides in the decoupling regime. While tree level processes are not altered significantly in this regime, the same cannot be said about loop induced processes such as (h→ γγ) or (gg → h). Such processes may be affected significantly by sparticles running in the loops. Higgs decays to two photons can be strongly affected by the stau sector of MSSM and we study this in connection with EWSB vacuum stability. In several models of dark matter, the dark matter candidate particle couples to the Higgs boson. It may well be that this candidate particle may be light enough so that the decay of the Higgs boson to these particles may be possible. For example, in the framework of the MSSM, the LSP (˜χ01) is the dark matter candidate and a decay of the form hχ˜→01χ˜01is possible depending on the mass and strength of coupling of such a particle. At the LHC this would show up as an branching ratio to particles that are invisible to the detectors. The dominant production mode of the Higgs at LHC proceeds through gluon fusion. In this channel a signal for an “invisibly” decaying Higgs would show up as missing energy plus jets at LHC. This has already been studied in quite some detail. We focus on other production modes, namely Vector Boson Fusion (VBF) and associated production (VH), in determining an invisible branching fraction at LHC. These two production channels are much less sensitive to any other BSM signals that may mimic an invisibly decaying Higgs and thus provide clean signals for the latter. A determination of the nature of interactions between the Higgs and gauge bosons is of paramount importance. An understanding of these interactions is closely tied to an understanding of the nature of EWSB. There are two aspects to probing these interactions. One is a determination of the Lorentz structure of the Higgs and gauge boson vertices and the second is to determine the strength of its couplings. The Higgs coupling to two gauge bosons (the hVV vertex) in SM is of the form ~ agµν . Under the assumption that BSM physics does not alter this Lorentz structure, information about possible new physics can be simply extracted through a determination of the strength of the coupling aV . However, the most general structure of this vertex is of the form (aV gµν + bV pµq ν + cV ɛ µνρσpρqσ) . Here p and q are the sum and difference of the two gauge boson momenta respectively and ɛµνρσ the completely antisymmetric Levi-Civita tensor. The term cV parametrizes CP-odd couplings while the rest are CP-even. The terms proportional to b V and cV may be generated by new physics. But which new physics model do we look at? There are a plethora of such models. Rather than shooting in the dark at random BSM directions one could adopt the following approach. In the absence of BSM signals at the LHC so far, one could assume that the scale of physics is relatively high and BSM particles are more massive than SM particles and can therefore be integrated out of the Lagrangian. It is also prudent to assume that new physics respects the SU(3)c x SU(2)L x U(1)Y gauge symmetry of SM. With these two assumptions in hand, one could supplement the SM Lagrangian with additional operators. These operators which generally have mass dimensions greater than four would destroy the renormalizability of the theory, though an interpretation as an effective theory up to a scale Λ is still valid. The idea is to now study the consequences that this effective theory would have on measurable properties of the Higgs. The effective theory could affect both the Lorentz structure as well as the strength of the couplings of the Higgs to the gauge bosons. This thesis deals with the determination of the Lorentz structure of the Higgs coupling to two gauge bosons , i.e the trilinear vertex. An analysis of this for the hZZ vertex has already been performed by ATLAS and CMS using h → ZZ *decays. A pure pseudoscalar Higgs (cZ ≠0, aZ = bZ = 0) coupling has been ruled out at about 2 ~ 3 σ level. Bounds have also been placed on a mixed scalar-pseudoscalar coupling (a Z =0,cZ =0,bZ = 0). This however, is not the end of the story. There are two important points to note here. Firstly it is important to be able to verify these findings in other production modes. To this end, we investigate the ability of VBF production to probe such anomalous couplings and find strong effects on the pseudo-rapidity distributions of the tagging jets in VBF. Secondly it is important to also look for such anomalous couplings in the hWW vertex. At this point, one might argue that the hZZ vertex and hWW vertex are connected by Custodial symmetry. However this symmetry is violated in SM by gauging of the hypercharge. It follows that violations of this symmetry should arise naturally in BSM physics. A study of the anomalous vertex is not easily achieved in h→ WW ∗ decays due to backgrounds and difficulties in reconstructing momenta. The VBF channel can be quite effective here although there is significant contamination from VBF production through the Z boson. We find that a cleaner production mode to use would be associated production. Until recently the low cross-section of Vh made it difficult to analyse this channel at LHC. An analysis of Vh has been made possible by the use of modern jet substructure techniques using (h→ bb) decays. We use these techniques and study how one can probe anomalous couplings in the Vh production mode at LHC. One of the most important couplings of the Higgs is that to the top, the heaviest SM particle. Not only is this coupling responsible for the main production channel of the SM Higgs at the LHC but the interaction with the top also has important consequences on spontaneous symmetry breaking within the SM – notably, vacuum stability arguments – as well as beyond the SM – supersymmetry, for instance, where the top drives electroweak symmetry breaking in some scenarios. The strength as well as the CP property of the Higgs top coupling is therefore an important aspect of to study. more specifically we investigate terms of the form ψ¯t(at + ibtγ5)ψth. here ψt and h corresponds to the top quark and Higgs fields respectively. at and bt parametrize scalar and pseudoscalar couplings respectively. Since the dominant production mode of the Higgs at the LHC (gluon fusion) proceeds through a top quark loop as do decays of the Higgs to two photons, some information about these couplings may be extracted just by looking at Higgs production and decay rates. However, an unambiguous determination of these couplings is possible only through Higgs production with a top and anti-top pair. Although the production rates are very small at the LHC, such a study is of prime importance. We investigate t¯th production at the LHC and list some useful observable that can probe the couplings described above. The outline of the thesis is as follows. We start with brief introduction to SM and Electroweak Symmetry breaking (EWSB) also briefly reviewing SM Higgs production and decay at the LHC. We then investigate the information that the Higgs mass in conjunction with stability of the EWSB vacuum provides about the stop sector of the MSSM. We further investigate the information that Higgs decay rates in conjunction with the stability of the EWSB vacuum could provide about the stau sector in the MSSM. We move on to examining the extent to which an invisible branching ratio of the Higgs could be measured or excluded directly at the LHC. Coming to the second part of the thesis we examine in a model independent way the nature of the Higgs-gauge boson couplings. We first give a brief description of the Higgs gauge boson vertex and the effective theory approach following it up with a description of how this could be probed using Higgs decays. We then follow it up with a study on how the Lorentz structure could affect Higgs production in Vector Boson fusion and Higgs production in association with W or Z boson. Finally, we show how the CP properties of the Higgs coupling to the top quark can be investigated using tth production along with Higgs rates.