The Standard Model (SM) of particle physics provides the theoretical framework to describe the fundamental interactions among elementary constituents of matter. SM is supported by experiments to a high degree of accuracy, up to parts per-mil for the electroweak (EW) sector and parts-per-trillion for QED alone, but it still remains incomplete. Many observed phenomena lack explanation in the framework of the SM and its particles. They indicate the possibility of existence of particles and interactions beyond the SM (BSM). These phenomena include dark matter (DM), dark energy and baryonic asymmetry of the universe. In addition, a quantum description of gravity is still lacking. The top quark has the largest mass among the SM particles. Due to it’s heavy mass, top quark is the only colored particle which does not hadronize and hence its properties are directly accessible by studying it’s decay particles. The order one Yukawa coupling of the top quark also imbibes it with an important role in the behavior of the SM couplings at higher energy scales where possible BSM physics may contribute. As a result, precision measurements of top quark properties may provide a glimpse into BSM physics and hence making these measurements is one of the core aims of the Large Hadron Collider. In stark contrast with top quark physics is the elusive, dark matter (DM) of the universe. There exists a lot of observational evidence for it but, as of yet, with no clue with regards to its particle properties and interactions. Compelling evidence for the existence of DM comes from measurements based on cosmic microwave background radiation, astrophysical observations of distribution of visible matter in galaxy clusters, galactic cluster collisions (e.g. bullet cluster), gravitational lensing, galactic rotation curves, structure formation simulations, to name a few. It is interesting to investigate the possibility that there may be a connection between top quark and DM. In this thesis, we extend the SM with simplified models to study BSM physics at colliders and also to explain the DM puzzle. Here, we use the Top quark as a laboratory for constructing generic probes of BSM and also of the dark sector physics. In Chapter 1, we introduce some relevant background and salient aspects of the SM framework on which the following BSM theories are built. In Chapter 2 we explore an s channel and a t-channel simplified model in the context of top quark pair production using asymmetries constructed with kinematic variables of the top decay products. In Chapter 3, we then propose a simplified model which includes a colored scalar as the mediator between DM and SM particles, termed gluphillic scalar dark matter (GSDM). Monojet process is one of the primary channels to probe DM at hadron colliders. In Chapter 3, the discussion of monojet process at the Large Hadron Collider (LHC) is limited to the effective field theory (EFT) framework. In Chapter 4 we discuss collider processes in GSDM model with complete loop calculations for the diagrams involving the mediating colored scalar. We also compare the loop calculation with the EFT results to find the range of applicability of the EFT. The top quark study in Chapter 2 was initially inspired from an interesting observation made in 2008 which suggested a deviation from the SM in the forward-backward asymmetry (FBA) of a pair produced top quark. The value of FBA measured at the time was 18% ±12%. This value deviated by more than 1σ with respect to the SM leading order (LO) value of 5%. The deviation was observed by both the detectors at Tevatron, D0 and CDF, and it’s significance increased with additional data in 2012. Recent analyses of the data by D0 is now in better agreement with the latest effective-NNNLO calculations. However, the FBA measurements by CDF are still in tension with those by D0 and the value predicted by theoretical calculations. Inspired by this puzzle, which may be on its way to getting solved, we have been able to construct effective probes of BSM physics for the on-going and future searches of BSM in the top quark sector. In our analyses, we studied correlations among observables which can distinguish between different sources of BSM contributions in the top quark pair production. As a template, we use an s-channel and a t-channel mediator, both of which leave very different signatures in the kinematic asymmetry correlations. The simplified models considered by us also included parity breaking interactions which lead to polarized top quarks, providing another probe into the underlying production process. We find that all the kinematic distributions of the decay lepton get influenced by the polarization of the top quark. We show that these correlations can distinguish well between the template models of axigluon and diquark. In general, all of these observables also provide a probe into the polarization of the top quark and therefore any chiral couplings with the mediator. However, the lepton polar angle asymmetry measured in the lab frame is special in that it can not only probe the longitudinal polarization as other observables but is also sensitive to the transverse polarization of the top quark. We also show the effectiveness of the proposed top quark kinematic observables, to distinguish between s and t-channel BSM physics models, in future searches for BSM particles at the run-II LHC. In a large verity of dark matter (DM) models the simplest candidate is the model of a singlet scalar particle. The scalar may couple to the standard model in a number of ways via any of the SM particles. Such models with BSM Yukawa interactions or gauge sector extensions are strongly constrained from both the direct detection and collider precision measurements. The remaining models either predict a very heavy dark matter, completely out of reach of collider searches or introduce an unnaturally weak coupling with the SM particles giving no justifications for the small numbers. An interesting corner of the space of possible DM models which has been under-explored so far includes interactions of DM particles with gluons. Although DM particles cannot themselves be charged or colored, a colored scalar mediator can allow this interaction. One such model arises when we consider the scalar DM in presence of a colored scalar particle, for example the one from t-channel model above. Such colored scalars are generically present in a number of BSM theories including SUSY and GUT. How-ever, without the need for any additional gauge symmetries, the two scalars would interact with each other via the marginal operators. In Chapter 3 we study a SM singlet scalar DM candidate which couples to SM via a colored scalar particle. In the GSDM model, DM and mediator interact via the quartic, marginal operator. DM annihilation cross-section of the order of weak interactions (∼ 0.1pb) is predicted to explain the observed dark matter relic density if arising from thermal production of a WIMP DM candidate of mass ∼ 100 GeV. On investigating the GSDM model, we find that it allows a large annihilation cross-section and is still compatible with direct detection bounds. This is so because the annihilation cross-section to a pair of colored scalars proceeds via a tree-level interaction, whereas the interaction with SM particles proceeds via loop diagrams involving the colored scalars. Our work shows that this model is compatible with the observed relic density of DM when the mediating particle is lighter than DM for a large range of the couplings. For masses of the DM and the mediator less then ∼ 50 GeV, the DM can also be lighter than the mediator where the annihilation then proceeds via loop interactions. This region of parameter space is strongly constrained from the collider physics bounds on a colored scalar particle. These bounds become much weaker in the case where the colored scalar does not couple to quarks and hence cannot decay. The bounds coming from long-lived colored scalars become relevant in those cases and also constrain the light mass window. A colored scalar interacting with quarks must do so without violating the strong flavor constraints. We consider the scalar in the framework of a class of models termed minimally flavor violating (MFV) and also assume that it couples only to the right handed up-sector quarks. Such a particle would couple to the top quark and would be observable at the LHC pair production of the top quark. We find constraints on a color triplet particle in such a case and show the coupling and mass regions allowed. Constraints from the decays to light quarks are interpreted from dijet process searches and limit the mass of a color-triplet scalar above 350 GeV. The primary process for direct search of stable particles produced at a collider is a single jet in association with missing transverse energy (MET). We find that in an effective field theory (EFT) framework, very weak bounds are obtained on the mediating scale. In Chapter 4, we perform complete loop calculations for processes involving colored scalar particles and DM at LHC in order to explore the GSDM model at LHC and FCC (Future Circular Collider). The EFT is valid only for mediator masses much heavier than the momentum transfer or the MET cuts. We show the region of applicability of the EFT by comparing it with respect to the loop induced calculation. We analyze the monojet + missing transverse energy (MET) process to find the expected bounds from LHC 13 TeV run-II. We calculate the reach of the LHC in the high luminosity run in the future and also the reach of the FCC to explore the GSDM model. We perform all our calculations for a number of representations of the colored mediator from a triplet to dimension 15. As expected, collider constraints are only significant when the dark matter is light enough (mDM ∼ 10 GeV) to be copiously produced at the LHC. We find that in the high luminosity run, LHC can probe the scalar triplet particle up-to 50 GeV mass in the monojet process though a dimension 15 particle can be probed up to 150 GeV. With an order of magnitude higher beam energy, FCC can rule out much larger parameter space or provide observational evidence for TeV scale mediating particles. In conclusion, this thesis adds to the growing body of literature which points towards BSM discoveries around the corner at high luminosity LHC in the top physics and in dark sector physics. We have also proposed avenues for precision BSM studies at the next generation colliders.

This PhD thesis presents the measurement of the differential cross section for the production of a Z boson in association with jets in proton-proton collisions taking place at the Large Hadron Collider (LHC) at CERN, at a centre-of-mass energy of 8 TeV. A development of a data acquisition (DAQ) system for the Triple-Gas Electron Multiplier (GEM) detector in view of the Compact Muon Solenoid (CMS) detector upgrade is also presented.<p><p>The events used for the data analysis were collected by the CMS detector during the year 2012 and constitute a sample of 19:6 fb-& / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Le sujet de cette thèse porte sur la recherche de nouvelles résonances massives se désintégrant en une paire d’électrons ou une paire électron-muon avec le détecteur CMS, installé auprès du Grand Collisionneur du Hadrons (LHC) au CERN. Les données analysées correspondent à l’ensemble des collisions proton-proton enregistrées par le détecteur en 2012 à une énergie dans le centre de masse de 8 TeV. Après une brève introduction au modèle standard des particules élémentaires et à quelques unes des théories allant au-delà, le LHC et le détecteur CMS sont présentés. La reconstruction des différentes particules créées lors des collisions, en particulier des électrons et muons de haute énergie, est ensuite discutée. Deux analyses séparées sont menées.<p>La première consiste en la recherche d’une nouvelle résonance étroite, plus massive que le boson Z, dans le spectre de masse invariante des paires d’électrons, dont la principale contribution, dans le modèle standard, provient du processus de Drell–Yan. De telles résonances sont notamment prédites par des modèles dits de grande unification ou à dimensions spatiales supplémentaires. Le bruit de fond provenant des processus du modèle standard étant réduit dans la région étudiée, quelques événements localisés peuvent suffire pour mener à une découverte, et la sélection des électrons est optimisée afin de ne perdre aussi peu d’événements que possible. Les différentes contributions des bruits de fond sont partiellement estimées à partir de simulations. Une méthode basée sur le spectre de masse invariante des paires électron-muon mesuré dans les données est développée pour valider la contribution du second bruit de fond en terme d’importance. Aucun excès n’est observé par rapport aux prédictions du modèle standard et des limites supérieures à 95% de niveau de confiance sont placées sur le rapport entre la section efficace de production multipliée par le rapport de branchement d’une nouvelle résonance et celle au pic du boson Z. Ces limites sont ensuite converties en limites inférieures sur la masse de différentes particules hypothétiques de spin 1 ou de spin 2.<p>La seconde analyse consiste en une recherche de résonances massives et étroites dans le spectre de masse invariante des paires électron-muon. De telles résonances briseraient la conservation du nombre leptonique tel que prédit par le modèle standard. Cette possibilité existe cependant dans certains modèles de nouvelle physique. C’est notamment le cas pour un modèle à dimensions supplémentaires où apparaissent des nouveaux bosons neutres lourds. La sélection des événements demande un électron de haute énergie comme dans l’analyse précédente, et un muon de grande impulsion transverse. La stratégie de recherche est similaire au cas des paires d’électrons :le fait de rechercher un signal étroit rend l’analyse statistique très peu sensible aux erreurs systématiques affectant la normalisation absolue du spectre de masse électron-muon. Comme aucune déviation significative n’est observée par rapport aux prévisions du modèle standard, des limites supérieures sur la section efficace multipliée par le rapport de branchement sont établies pour le modèle à dimensions spatiales supplémentaires. Étant données les faibles valeurs théoriques de la section efficace de production des résonances violant la conservation de la saveur dans ce modèle, la quantité de données analysées ne permet pas d’en déduire une limite inférieure sur leur masse. Cette analyse représente néanmoins la première recherche directe avec l’expérience CMS, de bosons massifs, se désintégrant avec violation du nombre leptonique, en une paire électron-muon.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The analysis presented in this thesis focuses on two methods developed to estimate, from data, the multi-jet and the V+jets background processes for top quark pair production occuring during proton-proton at LHC. Top quark paires are reconstructed using the CMS detector, exploiting the semi-leptonic decay channel. Both methods have been developed and studied using Monte-Carlo simulated data. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Cette these a pour sujet la recherche de nouvelle physique et l'etude de la production Drell-Yan dans le canal di-electron a l'aide du detecteur CMS au LHC. Certaines theories au dela du Modele Standard (extra dimensions, theories de grande unification) predisent l'existence de particules massives pouvant se desintegrer en une paire d'electrons. La selection des evenements recherches est presentee et etudiee. La strategie d'analyse est introduite et testee. Enfin, l'analyse des premieres donnees a 7 TeV est decrite et les resultats sont commentes. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Détection et identification de la réaction quark + anti-quark -> e+ + e- à l'aide du détecteur CMS (Compact Muon Solenoid) auprès du Grand Collisionneur de Hadrons du CERN, le LHC. Cette réaction permet de tester avec précision le Modèle Standard et de rechercher d'éventuelles nouvelles particules (Z') prédites par les théories de grande unification (GUT) et par les modèles à dimensions spatiales supplémentaires. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In recent years, there has been a large interest in the subject of extra dimensions in particle physics. In particular, a number of models have been suggested which provide solutions to some of the problems with the current Standard Model of particle physics, and which could be tested in the next generation of high-energy experiments. Among the most important of these models are the large extra dimensions model by Arkani-Hamed, Dimopoulos, and Dvali, the universal extra dimensions model, and models allowing right-handed neutrinos to propagate in the extra dimensions. In this thesis, we study phenomenological aspects of these three models, or simple modifications of them.   The Arkani-Hamed-Dimopoulos-Dvali model attempts to solve the gauge hierarchy problem through a volume suppression of Newton's gravitational constant, lowering the fundamental Planck scale down to the electroweak scale. However, this solution is unsatisfactory in the sense that it introduces a new scale through the radius of the extra dimensions, which is unnaturally large compared to the electroweak scale. It has been suggested that a similar model, with a hyperbolic internal space, could provide a more satisfactory solution to the problem, and we consider the hadron collider phenomenology of such a model.   One of the main features of the universal extra dimensions model is the existence of a potential dark matter candidate, the lightest Kaluza-Klein particle. In the so-called minimal universal extra dimensions model, the identity of this particle is well defined, but in more general models, it could change. We consider the indirect neutrino detection signals for a number of different such dark matter candidates, in a five- as well as a six-dimensional model.   Finally, right-handed neutrinos propagating in extra dimensions could provide an alternative scenario to the seesaw mechanism for generating small masses for the left-handed neutrinos. Since extra-dimensional models are non-renormalizable, the Kaluza-Klein tower is expected to be cut off at some high-energy scale. We study a model where a Majorana neutrino at this cutoff scale is responsible for the generation of the light neutrino masses, while the lower modes of the tower could possibly be observed in the Large Hadron Collider. We investigate the bounds on the model from non-unitarity effects, as well as collider signatures of the model. / QC 20110324

The beam intensity of the Large Hadron Collider will be significantly increased during the Phase-II long shut down of 2024-2026. Signal processing techniques that are used to extract the energy of detected particles in ATLAS will suffer a significant loss in performance under these conditions. This study compares the presently used optimal filter technique to alternative machine learning algorithms for signal processing. The machine learning algorithms are shown to outperform the optimal filter in many relevant metrics for energy extraction. This thesis also explores the implementation of machine learning algorithms on ATLAS hardware. / Graduate

The LHC (Large Hadron Collider) will have a long shut down in the years of 2019 and 2020, referred to as LS2. During this stop the LHC injector complex will be upgraded to increase the luminosities, which will be the first step of the high luminosity LHC program (which will be realized during LS3 that takes place in 2024-2026). The LHCb experiment, whose main purpose is to study the CP-violation, will during this long stop be upgraded in order to withstand a higher radiation dose, and to be able to read out the detector at a rate of 40MHz,compared to 1MHz at present. This change will improve the trigger efficiency significantly. One of the LHCb sub-detectors the Trigger Tracker (TT), will be replaced by a new sub-detector called UT. This report presents the early stage design (preparation for mock-up building) of the box that will be isolating the new UT detector from the surroundings and to ensure optimal detector operation. Methods to fulfill requirements such as light and gas tightness, Faraday-cage behavior and condensation free temperatures, without breaking the fragile beryllium beam pipe, are established. / LHC (Large Hadron Collider) kommer under åren 2019-2020 att ha ett längre driftstopp. Under detta driftstopp så kommer LHC's injektionsanordningar att uppgraderas för att kunna sätta fler protoner i circulation i LHC, och därmed öka antalet partikelkollisioner per tidsenhet. Denna uppgradering kommer att vara första steget i "High Luminocity LHC"-programmet som kommer att realiseras år 2024-2026. LHCb-experimentet, vars främsta syfte är att studera CP-brott, kommer också att uppgraderas under stoppet 2019-2020. Framför allt så ska avläsningsfrekvensen ökas från dagens 1MHz till 40MHz, och experimentet ska förberedas för de högre strålningsdoser som kommer att bli aktuella efter stoppet 2024-2026. En av LHCb's deldetektorer, TT detektorn, kommer att bytas ut mot en ny deldetektor som kallas UT. Den här rapporten presenterar den förberedande designen av den låda som ska isolera UT från dess omgivning och försäkra optimala förhållanden för detektorn. Kraven på den isolerande lådan och tillvägagångssätt för att uppfylla dessa krav presenteras. / LHCb, LS2 and LS3 Upgrade

Dans le Modèle Standard de la physique des particules, le mécanisme Cabibbo-Kobayashi-Maskawa (CKM) décrit le phénomène du mélange des quarks. De ses paramètres, l'angle gamma est celui connu avec la moins grande précision. Les mesures directes donnent une incertitude d'environ 15º, importante comparée à celle sur la valeur extraite des ajustements globaux, de 3º. Pour vérifier la cohérence du Modèle Standard, gamma doit être mesuré précisément. Cela est possible en utilisant des processus au niveau des arbres, où seules des contributions du Modèle Standard sont attendues, ou avec des processus impliquant des boucles, qui peuvent être sensibles à des effets au-delà. Des différences entre la mesure de gamma avec des diagrammes en arbres et avec des boucles pourraient être donc une indication de nouvelle physique. Cette thèse présente la première mesure des observables CP dans la désintégration B0 -> DK*0. Celle-ci est sensible à gamma du fait de l'interférence entre l'amplitude des diagrammes b -> u et b -> c, au niveau des arbres. L'asymétrie CP dans le mode B0 -> D(K+K-)K*0 et le rapport des largeurs partielles avec B0 -> D(K+pi-)K*0 sont mesurés avec 1 /fb de données récoltées par l'expérience LHCb en 2011,A_KK_d = -0,452 +/- 0,230 +/- 0,025 = A_CP+,R_KK_d = 1,360 +/- 0,366 +/- 0,075 = R_CP+. L'asymétrie CP du mode supprimé B0 -> D(K-pi+)K*0 et le rapport des largeurs partielles avec le favorisé B0 -> D(K+pi-)K*0 sont mesurés avec 3 /fb de données récoltées en 2011 et 2012,A_sup_d = -0,094 +/- 0,318 = A_ADS,R_d = 0,075 +/- 0,023 = R_ADS. Les études réalisées sur le système de déclenchement hadronique de l'expérience LHCb sont aussi présentées. / In the Standard Model of particle physics, the Cabibbo-Kobayashi-Maskawa (CKM) mechanism describes the quark mixing effect. The CKM gamma angle is one of the parameters of the Standard Model that are known less accurately. Direct measurements give an uncertainty of around 15º, large with respect to the uncertainty on the value extracted from global fits, of 3º. In order to test the Standard Model consistency, the gamma angle needs to be measured precisely. This can be done using processes at the tree-level, where only Standard Model contributions are expected, or using processes involving loop diagrams, which can be sensitive to physics beyond the Standard Model. Differences in the gamma measurement from tree and loop diagrams would be an indication of new physics. This thesis presents the first measurement of the CP observables in the B0 -> DK*0 decay. Sensitivity to gamma arises from the interference of the b -> u mediated amplitude with the b -> c one, at the tree-level. The CP asymmetry of the B0 -> D(K+K-)K*0 mode and the partial width ratio of this channel with respect to B0 -> D(K+pi-)K*0 are measured using 1 /fb of data collected by the LHCb experiment in 2011,A_KK_d = -0.452 +/- 0.230 +/- 0.025 = A_CP+,R_KK_d = 1.360 +/- 0.366 +/- 0.075 = R_CP+. The CP asymmetry of the suppressed B0 -> D(K-pi+)K*0 mode and the partial width ratio with respect to the favoured B0 -> D(K+pi-)K*0 are measured using the total 3 /fb of data collected in 2011 and 2012,A_sup_d = -0.094 +/- 0.318 = A_ADS,R_d = 0.075 +/- 0.023 = R_ADS. In addition, the studies performed on the hardware hadronic trigger of the LHCb experiment are also presented.