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 LHC (Large Hadron Collider) a produit en 2012 des collisions proton proton à une énergie de 8 TeV dans le centre de masse pour les expériences ATLAS et CMS. Ces deux expériences ont été conçues pour découvrir le boson de Higgs et pour rechercher de nouvelles particules prédites par des modèles théoriques. Le boson de Higgs a été découvert le 4 juillet 2012 par les expériences ATLAS et CMS. Cette découverte marque le début d'une nouvelle période de recherche dans le domaine. Avec la confirmation de l'existence du boson de Higgs, les recherches de nouvelle physique liées à ce boson sont devenues prioritaires. Par exemple, on peut chercher dans les données une nouvelle particule massive qui peut se désintégrer dans un boson de Higgs associé à d'autres particules du modèle standard. Une signature attendue est un boson de Higgs avec un quark top, les deux particules les plus lourdes du modèle standard. Le modèle standard prédit une section efficace pour la production du Higgs avec un quark top. Ainsi une mesure de cette section efficace montrant une valeur plus importante prouverait l'existence de physique au-delà du modèle standard. En outre, l'existence de physique au-delà le modèle standard pourrait montrer des résonances qui se désintègrent dans un quark top et un boson de Higgs. Dans la première partie de ce manuscrit, je présente les bases théoriques et expérimentales du modèle standard, ainsi que le dispositif expérimental. Dans le même chapitre théorique je discute une extension du modèle standard dans le cadre d'un modèle effectif englobant ce dernier. De plus, je détaille une étude de faisabilité d'une recherche d'une des nouvelles particules prédites par ce modèle, un quark vectoriel. Dans la deuxième partie, la recherche dans CMS de ce quark vectoriel T_, partenaire du quark top, est décrite. Ce partenaire du top est une nouvelle particule très similaire au quark top du modèle standard, mais beaucoup plus lourde. On considère le cas où ce nouveau quark se désintègre préférentiellement dans un quark top et un boson de Higgs. J'ai fait cette recherche dans le canal hadronique ou le Higgs se désintègre en deux quarks b et le quark top se désintègre en trois quarks, un quark b et deux quarks légers. J'ai reconstruit la masse du T_ à partir de l'identification de tous ses produits de désintégration. Le résultat obtenu est décrit sous forme des limites observées sur la section efficace de production du T_ déduites à partir de cette analyse / During 2012, the Large Hadron Collider (LHC) has delivered proton-proton collisions at 8 TeV center of mass energy to the ATLAS and CMS experiments. These two experiments have been designed to discover the Higgs boson and to search for new particles predicted by several theoretical models, as supersymmetry. The Higgs boson has been discovered by ATLAS and CMS experiments on July, 4th of 2012, starting a new era of discoveries in particle physics domain. With the confirmation of the existence of the Higgs boson, searches for new physics involving this boson are of major interest. In particular, data can be used to look for new massive particles that decay into the Higgs boson accompanied with other particles of the standard model. One expected signature is a Higgs boson produced with a top quark, the two heaviest particles in the standard model. The standard model predicts a cross section of top-Higgs production, then any enhancement of their associated production will be a clear signature of physics beyond the standard model. In addition, the existence of physics beyond the standard model can also be reflected by resonances that decay into a top-quark and a Higgs boson. In the first part of my work I describe the theoretical and experimental foundations of the standard model, as well as the experimental device. In the same theoretical chapter, I also discuss the formulation of an extension of the standard model. In addition, I describe a feasibility study of a search of one of the particles predicted by such model. The second part contains the realization of the search for a top partner, T_, within the CMS experiment. This top partner is a new particle very similar to the standard model top quark, but much heavier, that can decay into a top quark and a Higgs boson. The analysis looks for this particle in the full hadronic final state, where the Higgs boson decays into two b-quarks and the top quark decays into three standard model quarks, a b and two light quarks. In this channel, I reconstruct its mass from the identification of all its decay products. As a result of the analysis, I show the limits on the T_ production cross section from the number of observed events in the specific signature

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.

La découverte au LHC d'un boson scalaire possédant des propriétés fortement similaires à celles du boson de Higgs du Modèle Standard, indique certainement que l'acteur principal du mécanisme de la brisure de symétrie électrofaible a été trouvé. Cependant, plusieurs théories au-delà du Modèle Standard prédisent l'existence d'une particule similaire provenant d'un secteur plus riche. La mesure des propriétés du boson scalaire découvert nous permettra de savoir si celui-ci correspond ou non à la particule prédite par le Modèle Standard. Pour ce faire, nous utilisons une approche modèle-indépendante via le cadre d'une théorie des champs effective (TCE) pour le boson de Higgs, afin de paramétrer les déviations de ses couplages à la matière par rapport au Modèle Standard. Nous nous focalisons sur une théorie basée sur un Lagrangien effectif de dimension 6, qui inclut à la fois des opérateurs de Charge-Parité paire et impaire. Dans un premier temps nous tentons d'obtenir des contraintes sur une partie des coefficients effectifs de Wilson, pertinents pour la physique du boson de Higgs au LHC, en utilisant les dernières données de taux du Higgs provenant du Run-I des expériences ATLAS et CMS, ainsi que des données de précision électrofaibles du LEP, SLC et du Tevatron. Nous montrons que les données actuelles sont capables de contraindre de manière significative les opérateurs de CP paire ainsi que certains opérateurs de CP impaire du Lagrangien effectif. Dans un second temps nous étudions de possibles désintégrations exotiques du boson de Higgs, qui ne sont générées qu'en tant que conséquence des opérateurs effectifs de dimension 6 (générés par de la nouvelle physique inconnue) et non par le Modèle Standard seul. Les limites expérimentales actuelles nous permettent de placer des bornes supérieures sur ces opérateurs. Pour finir nous analysons certaines limitations de l'approche effective, par la comparaison de certains processus avec boson de Higgs à l'ordre des arbres dans la TCE, avec les prédictions pour les mêmes processus calculés à l'arbre et à une boucle, dans une classe simple d'extensions du Modèle Standard connue sous le nom de "Two-Higgs doublet models". / The discovery at the LHC of a scalar boson, the properties of which are strongly similar to the ones of the Standard Model Higgs boson, certainly indicate that the main actor of the electroweak symmetry breaking mechanism was found. However, many beyond-the-Standard Model theories predict the existence of such a similar particle coming from a richer sector. Measuring the properties of the discovered scalar will tell us whether or not it is the same particle as the one predicted by the Standard Model. To this aim we use a model-independent approach through a Higgs Effective Field Theory (EFT) framework to parametrize the deviations of its couplings to matter from the Standard Model. We focus on a Higgs EFT framework based on a dimension-6 effective Lagrangian, including both CP-even and CP-odd operators. We first attempt at putting constraints on a part of the effective Wilson coefficients relevant for Higgs physics at the LHC, using the latest Higgs rates data from the Run-I of the ATLAS and CMS experiments, as well as electroweak precision data from LEP, SLC and Tevatron. We show that the current data is able to significantly constrain CP-even and some CP-odd operators of the effective Lagrangian. We then move on to the study of possible exotic Higgs decays, that can only be generated as a consequence of the effective dimension-6 operators (generated from unknown new physics) and not from within the Standard Model alone, and derive upper bounds on those operators given the present experimental limits. Finally we analyze some of the limitations of the effective approach by comparing predictions on some Higgs processes at tree-level in EFT with respect to predictions at tree and 1-loop level on the same processes computed in a simple class of Standard Model extensions known as "Two-Higgs doublet models".

Pour la deuxième période de prise des données du LHC (Run 2) de 2015 - 2022, une quatrième couche de senseurs pixels a été installée dans le détecteur de l’existence ATLAS sur un tube de faisceau de plus petit diamètre afin d’ajouter de la redondance pour améliorer la reconstruction des trajectoires des particules chargées. Ce détecteur du pixel permettra d’assurer un suivi de la qualité de l’étiquetage des mesons b haute luminosité (b-tagging). Au cours des deux dernières années plusieurs composants ont été produits et assemblés sur des structures de soutien appelées échelles. Au total, 20 échelles ont été construites et qualifiées en cardere d’un procédé d’assurance qualité définie par ATLAS au CERN. Quatorze échelles ont été intégrées sur le tube de faisceau. Ceci constitue la première partie de la thèse dédiée à la partie construction du détecteur. La deuxième partie est consacrée à la recherche de boson de Higgs issu de collisions proton proton l’énergie du 8 TeV centre de masse de, se désintégrant en particules invisibles une luminosité integrée de 20.3 fb−1 enregistrées par le détecteur ATLAS au LHC. Les résultats sont interprétés dans les modèles de matière noire Higgs portail(Higgs portal Dark Matter). / For Run 2 of the LHC a fourth, innermost Pixel Detector layer on a smaller radius beampipe has been installed in the ATLAS Detector to add redundancy against radiation damage ofthe current Pixel Detector and to ensure a high quality tracking and b-tagging performance ofthe Inner Detector over the coming years until the High Luminosity Upgrade. State of the artcomponents have been produced and assembled onto support structures known as staves overthe last two years. In total, 20 staves have been built and qualified in a designated QualityAssurance setup at CERN of which 14 have been integrated onto the beam pipe. In the secondpart, A search for a Higgs boson produced via vector-boson fusion and decaying into invisibleparticles is discussed, using 20.3 fb−1 of proton proton collision data at the centre of massenergy of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a massof 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28is set on the branching fraction of H →invisible at 90% confidence level, where the expectedupper limit is 0.31. The results are interpreted in model of Higgs portal dark matter where thebranching fraction limit is converted into upper bounds on the dark matter nucleon scatteringcross section as a function of the dark matter particle mass, and compared to results from thedirect dark matter detection experiments.

Die Suche nach dem Higgs-Boson des Standardmodells der Teilchenphysik stellte einen der Hauptgründe für den Bau des Large Hadron Colliders (LHC) dar, dem derzeit größten Teilchenphysik-Experiment der Welt. Die vorliegende Arbeit ist gleichfalls von dieser Suche getrieben. Der direkte Zerfall des Higgs-Bosons in Myonen wird untersucht. Dieser Kanal hat mehrere Vorteile. Zum einen ist der Endzustand, bestehend aus zwei Myonen unterschiedlicher Ladung, leicht nachzuweisen und besitzt eine klare Signatur. Weiterhin ist die Massenauflösung hervorragend, sodass eine gegebenenfalls vorhandene Resonanz gleich in ihrer grundlegenden Eigenschaft - ihrer Masse - bestimmt werden kann. Leider ist der Zerfall des Higgs-Bosons in ein Paar von Myonen sehr selten. Lediglich etwa 2 von 10000 erzeugten Higgs-Bosonen zeigen diesen Endzustand . Außerdem existiert mit dem Standardmodellprozess Z/γ∗ → μμ ein Zerfall mit einer sehr ähnlichen Signatur, jedoch um Größenordnungen höherer Eintrittswahrscheinlichkeit. Auf ein entstandenes Higgs-Boson kommen so etwa 1,5 Millionen Z-Bosonen, welche am LHC bei einer Schwerpunktsenergie von 8 TeV produziert werden. In dieser Arbeit werden zwei eng miteinander verwandte Analysen präsentiert. Zum einen handelt es sich hierbei um die Untersuchung des Datensatzes von Proton-Proton-Kollisionen bei einer Schwerpunktsenergie von 8 TeV, aufgezeichnet vom ATLAS-Detektor im Jahre 2012, auch als alleinstehende Analyse bezeichnet. Zum anderen erfolgt die Präsentation der kombinierten Analyse des kompletten Run-I Datensatzes, welcher aus Aufzeichnungen von Proton-Proton-Kollisionen der Jahre 2011 und 2012 bei Schwerpunktsenergien von 7 TeV bzw. 8 TeV besteht. In beiden Fällen wird die Verteilung der invarianten Myon-Myon-Masse nach einer schmalen Resonanzsignatur auf der kontinuierlichen Untergrundverteilung hin untersucht. Dabei dient die theoretisch erwartete Massenverteilung sowie die Massenauflösung des ATLAS-Detektors als Grundlage, um analytische Parametrisierungen der Signal- und Untergrundverteilungen zu entwickeln. Auf diese Art wird der Einfluss systematischer Unsicherheiten auf Grund von ungenauer Beschreibung der Spektren in Monte-Carlo Simulationen verringert. Verbleibende systematische Unsicherheiten auf die Signalakzeptanz werden auf eine neuartige Weise bestimmt. Zusätzlich wird ein bisher einzigartiger Ansatz verfolgt, um die systematische Unsicherheit resultierend aus der Wahl der Untergrundparametrisierung in der kombinierten Analyse verfolgt. Zum ersten Mal wird dabei die Methode des scheinbaren Signals auf einem simulierten Untergrunddatensatz auf Generator-Niveau angewendet, was eine Bestimmung des Einflusses des Untergrundmodells auf die Anzahl der ermittelten Signalereignisse mit nie dagewesener Präzision ermöglicht. In keiner der durchgeführten Analysen konnte ein signifikanter Überschuss im invarianten Massenspektrum des Myon-Myon-Systems nachgewiesen werden, sodass obere Ausschlussgrenzen auf die Signalstärke μ = σ/σ(SM) in Abhängigkeit von der Higgs-Boson-Masse gesetzt werden. Dabei sind Stärken von μ ≥ 10,13 bzw. μ ≥ 7,05 mit einem Konfidenzniveau von 95% durch die alleinstehende bzw. kombinierte Analyse ausgeschlossen, jeweils für eine Higgs-Boson-Masse von 125,5 GeV. Die erzielten Ergebnisse werden ebenfalls im Hinblick auf die kürzlich erfolgte Entdeckung des neuen Teilchens interpretiert, dessen Eigenschaften mit den Vorhersagen eines Standardmodell-Higgs-Bosons mit einer Masse von etwa 125,5 GeV kompatibel sind. Dabei werden obere Grenzen auf das Verzweigungsverhältnis von BR(H → μμ) ≤ 1,3 × 10^−3 und auf die Yukawa-Kopplung des Myons von λμ ≤ 1,6 × 10^−3 gesetzt, jeweils mit einem Konfidenzniveau von 95%.:1. Introduction 2. Theoretical Foundations 3. Experimental Setup 4. Event Simulation 5. Muon Reconstruction and Identification 6. Event Selection 7. Signal and Background Modeling 8. Systematic Uncertainties 9. Statistical Methods 10. Results 11. Summary and Outlook / The search for the Standard Model Higgs boson was one of the key motivations to build the world’s largest particle physics experiment to date, the Large Hadron Collider (LHC). This thesis is equally driven by this search, and it investigates the direct muonic decay of the Higgs boson. The decay into muons has several advantages: it provides a very clear final state with two muons of opposite charge, which can easily be detected. In addition, the muonic final state has an excellent mass resolution, such that an observed resonance can be pinned down in one of its key properties: its mass. Unfortunately, the decay of a Standard Model Higgs boson into a pair of muons is very rare, only two out of 10000 Higgs bosons are predicted to exhibit this decay. On top of that, the non-resonant Standard Model background arising from the Z/γ∗ → μμ process has a very similar signature, while possessing a much higher cross-section. For one produced Higgs boson, there are approximately 1.5 million Z bosons produced at the LHC for a centre-of-mass energy of 8 TeV. Two related analyses are presented in this thesis: the investigation of 20.7 fb^−1 of the proton-proton collision dataset recorded by the ATLAS detector in 2012, referred to as standalone analysis, and the combined analysis as the search in the full run-I dataset consisting of proton-proton collision data recorded in 2011 and 2012, which corresponds to an integrated luminosity of L = 24.8 fb^−1 . In each case, the dimuon invariant mass spectrum is examined for a narrow resonance on top of the continuous background distribution. The dimuon phenomenology and ATLAS detector performance serve as the foundations to develop analytical models describing the spectra. Using these analytical parametrisations for the signal and background mass distributions, the sensitivity of the analyses to systematic uncertainties due to Monte-Carlo simulation mismodeling are minimised. These residual systematic uncertainties are addressed in a unique way as signal acceptance uncertainties. In addition, a new approach to assess the systematic uncertainty associated with the choice of the background model is designed for the combined analysis. For the first time, the spurious signal technique is performed on generator-level simulated background samples, which allows for a precise determination of the background fit bias. No statistically significant excess in the dimuon invariant mass spectrum is observed in either analysis, and upper limits are set on the signal strength μ = σ/σ(SM) as a function of the Higgs boson mass. Signal strengths of μ ≥ 10.13 and μ ≥ 7.05 are excluded for a Higgs boson mass of 125.5 GeV with a confidence level of 95% by the standalone and combined analysis, respectively. In the light of the discovery of a particle consistent with the predictions for a Standard Model Higgs boson with a mass of m H = 125.5 GeV, the search results are reinterpreted for this special case, setting upper limits on the Higgs boson branching ratio of BR(H →μμ) ≤ 1.3 × 10^−3, and on the muon Yukawa coupling of λμ ≤ 1.6 × 10^−3 , both with a confidence level of 95 %.:1. Introduction 2. Theoretical Foundations 3. Experimental Setup 4. Event Simulation 5. Muon Reconstruction and Identification 6. Event Selection 7. Signal and Background Modeling 8. Systematic Uncertainties 9. Statistical Methods 10. Results 11. Summary and Outlook

The localized/delocalized duality of 5f electrons plays an important role in understanding the complex physics of actinides. Band-structure calculations based on the ad hoc assumption that 5f electrons are simultaneously localized and delocalized explained the observed dHvA experiments very well. This ad hoc assumption also gives the correct equilibrium volume for delta-Pu. Experimentally, the duality of 5f electrons is observed by inelastic neutron scattering experiments, or by soft X-ray angle-resolved photoelectron spectroscopy. It is worth recalling that the origin of partial localization in the 3d and 5f systems is quite different. In compounds with 3d electrons, the large crystalline electric field set up by the surrounding environment of transition metal ions plays a major role. On the other hand, in 5f systems, the Hund's rule correlations play the key role whilst the crystalline electric field is less important. In this thesis we have studied the effect of intra-atomic correlations on anisotropies in hopping matrix elements of different 5f orbitals. For that purpose, we used the effective model that includes on-site interactions that are responsible for Hund's rules and effective hopping terms that result from the hybridization of different 5f orbitals with the environment. Two different approximations, namely, rotationally invariant slave-boson mean-field (RISBMF) and infinite time-evolving block decimation (iTEBD), have been used to investigate the ground-state properties of the Hamiltonian. We have demonstrated that Hund's rule correlations enhance strongly anisotropies in hopping matrix elements. For a certain range of 5f bandwidth parameters this effect may result in a complete suppression of hopping processes for some of 5f orbitals, i.e., the system is in a partially localized phase. Within the RISBMF method, we calculated the ground-state properties and the phase diagram of the system. The suppression of hopping processes in some of 5f orbitals due to Hund's rule correlations can be seen through orbital-dependent quasiparticle weights. In a mean-field theory, a quasiparticle weight of zero for an orbital means a complete suppression of hopping processes in this orbital. Thus, quasiparticle weights and occupation numbers were used to classify partially localized phases. In the calculated phase diagram we obtain four partially localized phases that can be separated into two different sets. In the first set electrons in two orbitals are localized. In the second, electrons in one orbital are localized. The difference between the two sets is not simply the number of localized orbitals but the mechanism for the partial localization. For the first set, the Hund's rule mechanism applies: only those 5f electrons that enable the remaining ones to form a Hund's rule state will delocalize. This mechanism requires to have at least two localized orbitals, therefore it is definitely not applicable to those phases with only one localized orbital. For the second set, a situation similar to a single-band Mott-Hubbard transition applies. The direct on-site Coulomb interaction between jz and -jz electrons plays the key role for understanding the partial localization transition. In order to assess the validity of the RISBMF results we have used the iTEBD method to calculate the ground-state properties of a 1D system. Qualitatively, the two approaches agree with each other. However, we found an area where the RISBMF yields an artificial ground-state. Note that the mean-field method is worst for a 1D system. Therefore one shoud not judge from it the quality of the RISBMF method for the more general case.

The development of quantum machine learning is bridging the way to fault tolerant quantum computation by providing algorithms running on the current noisy intermediate scale quantum devices.However, it is difficult to find use-cases where quantum computers exceed their classical counterpart.The high energy physics community is experiencing a rapid growth in the amount of data physicists need to collect, store, and analyze within the more complex experiments are being conceived.Our work approaches the study of a particle physics event involving the Higgs boson from a quantum machine learning perspective.We compare quantum support vector machine with the best classical kernel method grounding our study in a new theoretical framework based on metrics observing at three different aspects: the geometry between the classical and quantum learning spaces, the dimensionality of the feature space, and the complexity of the ML models.We exploit these metrics as a compass in the parameter space because of their predictive power. Hence, we can exclude those areas where we do not expect any advantage in using quantum models and guide our study through the best parameter configurations.Indeed, how to select the number of qubits in a quantum circuits and the number of datapoints in a dataset were so far left to trial and error attempts.We observe, in a vast parameter region, that the used classical rbf kernel model overtakes the performances of the devised quantum kernels.We include in this study the projected quantum kernel - a kernel able to reduce the expressivity of the traditional fidelity quantum kernel by projecting its quantum state back to an approximate classical representation through the measurement of local quantum systems.The Higgs dataset has been proved to be low dimensional in the quantum feature space meaning that the quantum encoding selected is not enough expressive for the dataset under study.Nonetheless, the optimization of the parameters on all the kernels proposed, classical and quantum, revealed a quantum advantage for the projected kernel which well classify the Higgs boson events and surpass the classical ML model. / Utvecklingen inom kvantmaskininlärning banar vägen för nya algoritmer att lösa krävande kvantberäkningar på dagens brusfyllda kvantkomponenter. Däremot är det en utmaning att finna användningsområden för vilka algoritmer som dessa visar sig mer effektiva än sina klassiska motsvarigheter. Forskningen inom högenergifysik upplever för tillfället en drastisk ökning i mängden data att samla, lagra och analysera inom mer komplexa experiment. Detta arbete undersöker Higgsbosonen ur ett kvantmaskinsinlärningsperspektiv. Vi jämför "quantum support vector machine" med den främsta klassiska metoden med avseende på tre olika metriker: geometrin av inlärningsrummen, dimensionaliteten av egenskapsrummen, och tidskomplexiteten av maskininlärningsmetoderna. Dessa tre metriker används för att förutsäga hur problemet manifesterar sig i parameterrummet. På så vis kan vi utesluta regioner i rummet där kvantalgoritmer inte förväntas överprestera klassiska algoritmer. Det finns en godtycklighet i hur antalet qubits och antalet datapunkter bestämms, och resultatet beror på dessa parametrar.I en utbredd region av parameterrummet observerar vi dock att den klassiska rbf-kärnmodellen överpresterar de studerade kvantkärnorna. I denna studie inkluderar vi en projicerad kvantkärna - en kärna som reducerar det totala kvanttillståndet till en ungefärlig klassisk representation genom att mäta en lokal del av kvantsystemet.Den studerade Higgs-datamängden har visat sig vara av låg dimension i kvantegenskapsrummet. Men optimering av parametrarna för alla kärnor som undersökts, klassiska såväl som kvantmekaniska, visade på ett visst kvantövertag för den projicerade kärnan som klassifierar de undersöka Higgs-händelserna som överstiger de klassiska maskininlärningsmodellerna.

Doctor of Philosophy / Department of Physics / Tim Bolton / This thesis represents the first study of the helicity distributions of Z[gamma] di-boson production at hadron colliders. I use 5 fb⁻¹ of [radical]s = 7 TeV center of mass energy proton-proton collision data, collected by the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC), to look at the angular distribution of the Z[gamma] [right arrow] e⁺e⁻[gamma] / [mu]⁺ [mu]⁻ [gamma] process and measure the helicity amplitudes that govern it. This study provides sensitivity to the interference terms between different quantum states and through the interference terms to the possible new physics. The final state is comprised of leptons (muon-antimuon or electron-positron pairs) with transverse momentum over 20 GeV and a photon with transverse energy over 30 GeV. Helicty amplitudes are measured for the total angular momentum of the quark-antiquark system up to J[subscript]q[subscript bar]q = 2. Four-dimensional multivariate analysis of the 2011 CMS data shows no significant deviations from the standard model prediction for the measured amplitudes.

Dans le Modèle Standard de la physiques des particules, le boson de Higgs permet de générer la masse des particules élémentaires. Les contraintes théoriques et expérimentales actuels imposent au boson de Higgs d'avoir une masse comprise entre 114.4 et 158 GeV à 95% de niveau de confiance. De plus, le Tevatron a récemment exclu la zone de masse entre 100 et 109 GeV, 158 et 175 GeV à 95% de niveau de confiance. Ces résultats confortent la recherche du boson de Higgs à basse masse dans la fenêtre encore ouverte. Le détecteur DØ est situé près de Chicago, au Tevatron, collisionneur protons-antiprotons avec une énergie dans le centre de masse de 1.96 TeV. Le sujet de cette thèse est la recherche du boson de Higgs produit en association avec un boson Z. C'est un canal sensible au boson de Higgs de basse masse (<135 GeV) qui a un rapport de branchement H->bb de l'ordre de 80% dans cette région en masse. Le canal d'étude ZH->nu nu bb a un état final composé de 2 jets de saveurs lourdes et de l'énergie transverse manquante emportée par les neutrinos. L'identification des jets de saveur lourde ("b-tagging") est réalisée à l'aide d'un nouvel algorithme que l'on a développé (SLTNN) : la méthode est basée sur la désintégration semi-leptonique des quarks b. L'analyse de recherche du boson de Higgs a été menée avec 3 fb−1 de données. L'utilisation de SLTNN a permis d'améliorer de 10% l'efficacité d'identification de boson de Higgs. En revanche, la sensibilté globale de l'analyse, une fois les bruits de fond et erreurs systématiques prises en compte, est très peu améliorée (<1%).