The Q-weak experiment at Thomas Jefferson National Accelerator Facility (USA) will make a precision determination of the proton weak charge with approximately 4% combined statistical and systematic uncertainties via a measurement of the parity violating asymmetry in elastic electron-proton scattering at very low momentum transfer and forward angle. This will allow an extraction of the weak mixing angle at Q^2=0.026 (GeV/c)^2 to approximately 0.3%. The weak mixing angle is a fundamental parameter in the Standard Model of electroweak interactions. At the proposed accuracy, a measured deviation of this parameter from the predicted value would indicate new physics beyond what is currently described in the Standard Model. Without deviation from the predicted value, this measurement would place stringent limits on possible extensions to the Standard Model and constitute the most precise measurement of the proton's weak charge to date. The key experimental apparatus include a liquid hydrogen target, a toroidal magnetic spectrometer and a set of eight Cerenkov detectors. The Cerenkov detectors form the main detector system for the Q-weak experiment and are used to measure the parity violating asymmetry during the primary Q-weak production runs. The Cerenkov detectors form the main subject of this thesis. Following a brief introduction to the experiment, the design, development, construction, installation, and testing of this detector system will be discussed in detail. This is followed by a detailed discussion of detector diagnostic data analysis and the corresponding detector performance. The experiment has been successfully constructed and commissioned, and is currently taking data. The thesis will conclude with a discussion of the preliminary analysis of a small portion of the liquid hydrogen data.

The goal of the Q-weak experiment is to make a measurement of the proton's weak charge ($Q^p_W = 1-4\sin^2\theta_W$) to an accuracy of ~4%. This would represent a ~0.3% determination of the weak mixing angle ($\sin^2\theta_W$) at low energy. The measurement may be used for a precision test of the Standard Model (SM) prediction on the running of $\sin^2\theta_W$ with energy scale. The Q-weak experiment operates at Thomas Jefferson National Accelerator Facility (Jefferson Lab). The experiment determines the proton's weak charge by measuring the parity violating asymmetry in elastic electron-proton scattering at low momentum transfer $Q^2 = 0.026 (GeV/c)^2$ and forward angles (~8 degree). The anticipated size of the asymmetry, based on the SM, is about 230 parts per billion (ppb). With the proposed accuracy, the experiment may probe new physics beyond Standard Model at the TeV scale. This thesis focuses on my contributions to the experiment, including track reconstruction for momentum transfer determination of the scattering process, and the focal plane scanner, a detector I designed and built to measure the flux profile of scattered electrons on the focal plane of the Q-weak spectrometer to assist in the extrapolation of low beam current tracking results to high beam current. Preliminary results from the commissioning and the first run period of the Q-weak experiment are reported and discussed.

Atomic parity violation measurements provide a way to probe physics beyond the Standard Model. They can provide constraints on conjectures of a massive Z′ bosonor a light boson, or searches of dark energy. Using the two-pathway coherent control technique, our group plans to make a new measurement of the weak interaction induced parity non-conservation (PNC) transition moment (<i>E_PNC</i>) on the cesium 6S→7S transition. We will coherently interfere a 2-photon transition with the Stark and PNC transitions to amplify and extract the PNC amplitude. Previously, our lab has measured the magnetic dipole transition moment on the same 6S→7S transition to about 0.4% uncertainty using this technique. In this dissertation, I discuss improvements made to the system, and review what future upgrades are needed for a new<i> E</i>_{<i>PNC </i>}measurement. Key systematics are also described. For an accurate determination of <i>E_PNC</i>, properties of cesium such as the scalar (<i>α</i>) and vector (<i>β</i>) transition polarizabilities are needed. I present improved determinations of keyelectric dipole matrix elements, and calculate new high precision determinations of <i>α</i> and <i>β</i>. Finally, using <i>β</i> and the previously measured value of <i>E_PNC/β</i>, I calculate new values for the weak charge of the cesium nucleus Q_w.<br>

Le Modèle Standard, qui décrit les interactions entre les particules à l’échelle quantique, est une théorie imparfaite. Il possède plusieurs problèmes théoriques non-résolus et ne permet pas d’expliquer certaines observations astrophysiques comme celles de la matière noire et de l’asymétrie baryonique. Plusieurs théories, dites au-delà du Modèle Standard, ont été proposées afin de résoudre certains de ces problèmes, et une grande partie d’entre elles prévoient l’apparition de nouveaux phénomènes à haute énergie. L’objectif principal de cette thèse est de rechercher ces phénomènes dans les collisions proton-proton produites par le Large Hadron Collider à une énergie de centre de masse de 13 TeV. Une partie des travaux présentés dans cette thèse a en particulier été dédiée à la recherche de processus de production de quarks top de même signe, c’est-à-dire de même charge électrique, qui sont prédits par des modèles de supersymétrie à R-parité violée. Ces processus engendrent des évènements composés de leptons de même charge accompagnés de b-jets, lesquels ont l’avantage d’être faiblement contaminer par le bruit de fond provenant du Modèle Standard.Les travaux présentés dans cette thèse ont essentiellement porté sur deux analyses, chacune recherchant des phénomènes de nouvelle physique de nature différente dans les évènements composés de leptons de même charge dans les données enregistrées par le détecteur ATLAS. Une première analyse a porté sur la recherche de processus supersymétriques sur les données enregistrées en 2015 et en 2016 avec 36.1 fb$^{-1}$ de luminosité intégrée. Des signaux de production de quarks top de même signe ont été implémentés en se basant sur des processus supersymétriques violant la R-parité, et des régions de signal associées à ces processus ont été optimisées. Une deuxième analyse a porté sur la recherche de processus non-supersymétriques, dits exotiques, dans les données enregistrées en 2015 avec 3.2 fb$^{-1}$ de luminosité intégrée. Cette analyse a surtout été motivée par les résultats obtenus à 8 TeV, dans lesquels un modeste excès de nombre d’évènements par rapport aux prédictions du Modèle Standard a été observé dans deux régions de signal. Une partie des études relatives à cette analyse a été dédiée au développement et à la validation des méthodes d’estimation des différentes sources de bruit de fond.Aucune déviation par rapport aux prédictions du Modèle Standard n’a été observée dans chacune des régions de signal considérées dans les deux analyses. L’excès qui avait été observé dans les résultats obtenus à 8 TeV n’est donc pas confirmé. Des limites d’exclusion sur les modèles de nouvelle physique ont de plus été extraites à partir des résultats obtenus, en particulier sur les modèles de supersymétrie à R-parité violée utilisés pour produire les processus de production de quarks top de même signe. / The Standard Model, which describes the particle interactions at quantum level, is an imperfect theory. It has several theoretical problems and is unable to explain astrophysical observations like the dark matter and the baryonic asymmetry of the universe. Several beyond-standard models have been proposed to solve some of these issues, and predict new-physics phenomena at high energy. The aim of this thesis is to search for these phenomena in proton-proton collisions produced by the Large Hadron Collider at a center-of-mass energy of 13 TeV. Part of the studies presented in this thesis was dedicated to the search for production of same-sign top quarks based on R-parity violating supersymmetric models. These processes lead to a signature of same-sign leptons and b-jets, which have the advantage to be lowly contaminated by the Standard Model background.The studies presented in this thesis focused on two analyses, both searching for new-physics phenomena of different nature in same-sign leptons events in data recorded by the ATLAS detector. A first analysis focused on supersymmetric processes with data recorded in 2015 and in 2016 with 36.1 fb$^{-1}$ of integrated luminosity. Same-sign top quarks signals were implemented using R-parity violating supersymmetric processes, and signal regions associated to these processes were optimized. A second analysis focused on exotic (non-supersymmetric) processes with data recorded in 2015 with 3.2 fb$^{-1}$ of integrated luminosity. This analysis was motivated by a modest excess seen in two signal regions in the 8 TeV results. Several studies were focused on the development and the validation of background estimation methods.No deviations from the Standard Model predictions were observed the signal regions considered in both analyses. The 8 TeV excess is therefore not confirmed with the most recent data. Exclusion limits on new-physics models were extracted, in particular for the R-parity violating supersymmetric models that were used to produce the same-sign top quarks processes.

No description available.