No description available.

The Compact Linear Collider (CLIC) is a proposal for a future linear electron--positron collider that could achieve collision energies of up to 3 TeV. In the CLIC concept the main high energy beam is accelerated using RF power extracted from a high intensity drive beam, achieving an accelerating gradient of 100 MV/m. This scheme places strict tolerances on the drive beam phase stability, which must be better than 0.2 degrees at 12 GHz. To achieve the required phase stability CLIC proposes a high bandwidth (>17.5 MHz), low latency drive beam "phase feedforward" (PFF) system. In this system electromagnetic kickers, powered by 500 kW amplifiers, are installed in a chicane and used to correct the phase by deflecting the beam on to longer or shorter trajectories. A prototype PFF system has been installed at the CLIC Test Facility, CTF3; the design, operation and commissioning of which is the focus of this work. Two kickers have been installed in the pre-existing chicane in the TL2 transfer line at CTF3 for the prototype. New optics have been created for the line to take these changes in to account, incorporating new constraints to obtain the desired phase shifting behaviour. Three new phase monitors have also been installed, one for the PFF input and two to verify the system performance. The resolution of these monitors must be significantly better than 0.2 degrees to achieve CLIC-level phase stability. A point by point resolution as low as 0.13 degrees has been achieved after a series of measurements and improvements to the phase monitor electronics. The performance of the PFF system depends on the correlation between the beam phase as measured at the input to the PFF system, and the downstream phase, measured after the correction chicane. Preliminary measurements found only 40% correlation. The source of the low correlation was determined to be energy dependent phase jitter, which has been mitigated after extensive efforts to measure, model and adjust the machine optics. A final correlation of 93% was achieved, improving the theoretical reduction in jitter using the PFF system from a factor 1.1 to a factor 2.7. The performance and commissioning of the kicker amplifiers and PFF controller are also discussed. Beam based measurements are used to determine the optimal correction timing. With a maximum output of around 650 V the amplifiers provide a correction range of ±5.5 ± 0.3 degrees. Finally, results from operation of the complete system are presented. A mean phase jitter of 0.28 ± 0.02 degrees is achieved, in agreement with the theoretical prediction of 0.27 ± 0.02 degrees for an optimal system with the given beam conditions. The current limitations of the PFF system, and possible future improvements to the setup, are also discussed.

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.

No description available.

Le Modèle standard de la physique de particules explique avec beaucoup de succès comment les particules fondamentales interagissent entre elles. Néanmoins, ce modèle n’est pas capable d’expliquer certains défis contemporains du domaine, comme la grande quantité de paramètres libres, le problème de la hiérarchie ou l’identité de la matière sombre. Pour expliquer ces phénomènes, il faut chercher la physique au-delà du Modèle standard. De sorte à satisfaire les contraintes provenant des mesures expérimentales, on peut considérer l’addition d’une seule nouvelle particule au Modèle standard qui se couple à deux quarks, le diquark. Sa contribution aux processus hadroniques offre une opportunité d’obtenir des limites dans ses paramètres à partir des données expérimentales contemporaines. Ceci nous permettrait d’évaluer son potentiel et d’identifier les processus auxquels cette particule peut contribuer. Parmi les états de diquarks possibles, on trouve les diquarks scalaires Dᵘ et Dᵈ, qui possèdent un couplage antisymétrique à des paires de quarks droitiers du type up ou du type down, respectivement. L’obtention des contraintes sur les valeurs de leur masse Mᴰ et leurs couplages aux quarks (x^q, y^q), où q = u, d, est l’objectif central de ce travail. Pour cela, on considère tout d’abord des recherches directes dans la production de dijet, la signature expérimentale des désintégrations de ces particules. Ensuite, on sonde l’influence indirecte du diquark Dᵘ dans les mesures expérimentales de la section efficace de production d’un seul quark top. Les données expérimentales obtenues dans le Grand collisionneur d’hadrons (LHC) pour ces processus à haute énergie sont utilisées pour comparer les prédictions théoriques de ces modèles et quantifier leur viabilité en fournissant de nouvelles contraintes sur ses paramètres. On est capable d’améliorer les contraintes trouvées dans la littérature de |xᵘ| ≤ 14,4 à |xᵘ| ≤ 0,13–0,15 pour Dᵘ et de |xᵈyᵈ| ≤ 0,022 avec |yᵈ| ≤ 0,17 à |xᵈ| ≤ 0,15–0,17 pour Dᵈ, les deux avec une masse de Mᴰ = 600 GeV. De plus, l’analyse des données pour la production d’un seul quark top montre que la réduction de l’espace de phase des pᵀ peut améliorer encore plus ces contraintes. / The Standard Model of particle physics explains with great success how fundamental particles interact. However, this model cannot explain some contemporary challenges of the domain, such as the large number of free parameters, the hierarchy problem or the identity of dark matter. To explain these phenomena, we need to search for physics beyond the Standard Model. In order to satisfy the existing constraints from experimental measurements, we can consider the addition of a single new particle to the Standard Model that couples to two quarks, a diquark. Its contribution to hadronic processes offers an opportunity to set limits on its parameters from contemporary experimental data. This would allow us to evaluate their potential and identify processes to which they can contribute. Among the possible diquark states, we find the scalar diquarks Dᵘ and Dᵈ, which have an antisymmetric coupling to pairs of right-handed up-type or down-type quarks, respectively. To obtain constraints on their mass Mᴰ and on their couplings to quarks (x^q, y^q), where q = u, d, is the main goal of this work. In order to do so, we start by considering direct searches in the production of dijet, the experimental signature of the decay of these particles. Afterwards, we probe the indirect influence of the Dᵘ diquark to the experimental measurements of the single-top-production cross section. Recent data for both of these processes from the Large Hadron Collider (LHC) is used to compare the theoretical predictions of these models and quantify their viability by providing new constraints on its parameters. We are able to improve the constraints found in the literature from |xᵘ| ≤ 14.4 to |xᵘ| ≤ 0.13–0.15 for Dᵘ and from |xᵈyᵈ| ≤ 0.022 to |yᵈ| ≤ 0.17 to |xᵈ| ≤ 0.15–0.17 for Dᵈ, both with masses of Mᴰ = 600 GeV. Other than that, the analysis of the data from single top production shows that the reduction of the pᵀ space can further improve these constraints.

No description available.