Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Astronomical and cosmological observations suggest that 80% of the matter content of the universe is made up of dark matter. However, except for its gravitational interactions with ordinary matter particles, there is no evidence of its interactions via other known forces. A variety of extensions to the Standard Model of particle physics predict the existence of a weakly interacting massive particle which can account for the existence of dark matter. Even though these hypothetical particles can explain the abundance of dark matter in the universe, previous searches showed no direct evidence of their existence. In this thesis, a search for dark matter production in proton-proton collisions at the Large Hadron Collider (LHC) at CERN is presented. The observation of a new particle compatible with the Standard Model Higgs boson in 2012 by the ATLAS and CMS experiments at the LHC enabled new search channels for dark matter production at particle colliders. This thesis describes a search for dark matter produced in association with a Higgs boson decaying to a bottom quark and an anti-bottom quark (bb̅) with the ATLAS experiment. The search was conducted using proton-proton collision data gathered from 2015 and 2016 at a center-of-mass energy of 13 TeV at the LHC. The analysis is performed by selecting final states compatible with a Higgs boson decaying to a bb̅ pair recoiling against dark matter particles which are detected as large missing transverse energy. The observations show no significant deviation from Standard Model expectations without dark matter. The results are converted to limits on the parameter space of a Z'-2HDM signal model as well as limits on production cross sections of beyond the Standard Model processes with the same final state signature of a Higgs boson and large missing transverse energy, without any further assumptions.

The goal of this thesis is to measure the coherent + production cross section on carbon and oxygen nuclei, induced by muon neutrinos from the T2K beamline. This is performed using the tracker system of the off-axis near detector which consists of three argon gas Time Projection Chambers between which two Fine-Grained Detectors (FGD) are located. While the first FGD is made of plastic scintillator, the second one has alternate layers of scintillator and water which allows a measurement on oxygen target. The measurement on carbon target is reported as a function of the muon and pion momentum and angle, the sum of the pion and muon energies and the coplanarity angle between the muon and the pion. The statistics for the oxygen measurement remains too low and therefore an upper limit is set on both the differential cross sections and total flux-integrated cross section. The phase space considered is 0.2 < pμ− < 5 GeV/c , cos μ− > 0.7, 0.15 < p + < 1.5 GeV/c , cos + > 0.45, 0.5 < E + + Eμ− < 6.5 GeV, +μ− > 90 . The values found for the flux-integrated cross section in the reduced phase-space are: < C >= 3.23 ± 0.67(stat.) ± 0.82(syst.) × 10−40cm2 per carbon nucleus O 9.57 × 10−40cm2 per oxygen nucleus, with 95% probability It was found that the 95% upper limit on oxygen is always smaller than the Rein-Sehgal predictions, making this model unreliable for low neutrino energies. On the other hand, the limit remains compatible with the Berger-Sehgal model in every differential variables. The data for carbon target agree, within the systematic errors, with the Berger-Sehgal model except for pion momentum below 0.35 GeV/c and coplanarity angle below 155 degrees. In the latter cases an excess is observed in the data.

<p>The Duke electron storage ring, first commissioned in November of 1994, has been developed as a dedicated driver for storage ring free-electron lasers (SRFELs) operating in a wide wavelength range from infrared, to visible, to ultraviolet (UV) and vacuum ultraviolet (VUV). The storage ring has a long straight section for various insertion devices and can be operated in a wide energy range (0.25 GeV to 1.15 GeV).</p><p>Commissioned in 1995, the first free-electron laser (FEL) on the Duke storage ring was the OK-4 FEL, an optical klystron with two planar undulators sandwiching a buncher magnet. In 2005, the OK-5 FEL with two helical undulators was commissioned. Operating four undulators -- two OK-4 and two OK-5 undulators, the world's first distributed optical klystron FEL was brought to operation in 2005. Via Compton scattering of FEL photons and electrons in the storage ring, the Duke FEL drives the world's most powerful, nearly monochromatic, and polarized Compton</p><p>gamma-ray source, the High Intensity Gamma-ray Source (HI&#947;S). Today, a variety of configurations of the storage ring FELs at Duke have been used in a wide range of research areas from nuclear physics to biophysics, from chemical and medical research to industrial applications.</p><p>The capability of accurately measuring the storage ring electron beam energy spread is crucial for understanding the longitudinal beam dynamics and the dynamics of the storage ring FEL. In this dissertation, we have successfully developed a noninvasive, versatile, and accurate method to measure the energy spread using optical klystron radiation. Novel numerical methods based upon the Gauss-Hermite expansion have been developed to treat both spectral broadening and modulation on an equal footing. Through properly configuring the optical klystron, this energy spread measurement method has a large dynamic range. In addition, a model-based scheme has been developed for correcting the electron beam emittance related inhomogeneous spectral broadening effect, to further enhance the accuracy of measuring the electron beam energy spread. </p><p>Taking advantage of the direct measurement method of the electron beam energy spread, we have developed another novel technique to simultaneously measure the FEL power, electron beam energy spread, and other beam parameters. This allowed us to study the FEL power in a systematic manner for the first time. Based on the experimental findings and results of the theoretical predictions, we have proposed a compact formula to predict the FEL power using only the knowledge of electron</p><p>beam current, beam energy, and bunch length.</p><p>As part of the dissertation work, we have developed a self-consistent numerical model to study the storage ring FEL. The simulation program models the electron beam propagation along the storage ring, multi-turn FEL interaction in the undulators, gradual intra-cavity optical power buildup, etc. This simulation code captures the main features of a storage ring FEL at different time and space scales. The simulated FEL gain has been benchmarked against measured gain and calculated</p><p>gain with good agreement. The simulation package can provide comprehensive information about the FEL gain, optical pulse growth, electron beam properties, etc. In the near future, we plan to further improve the simulation model, by including additional physics effects such as microwave instability, to make it a more useful tool for FEL research.</p> / Dissertation

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Several applications of the Heavy Quark Symmetry are studied. They concern the extraction of the quark mixing parameter $V\sb{cb}$, the Standard Model prediction for the branching ratio of $B \to K\sp{*}\gamma$ and the extraction of the quark mixing parameter $V\sb{ub}$ from $B \to \pi e\nu\sb{e}$ decays. An effective theory incorporating both Heavy Quark and Chiral Symmetries is presented and some of its applications are discussed.

Based on a 87 fb−1 dataset, a search for D0-D¯0 mixing is made using the semileptonic decay modes D*+ → π +D0, D0 → [K/K*]eν (+c.c.) at the B-Factory facility at the Stanford Linear Accelerator Center. These modes offer unambiguous initial and final-state charm flavor tags, and allow the combined use of the D0 lifetime and D*+- D0 mass difference (ΔM) in a global likelihood fit. The high-statistics sample of reconstructed unmixed semileptonic D0 decays is used to model both the ΔM distribution and the time-dependence of mixed events directly from the data. Neural networks are used both to select events and to fully reconstruct the D0. A result consistent with no charm mixing has been obtained, Rmix = 0.0023 ± 0.0012(stat) ± 0.0004(sys ). This corresponds to an upper limit of Rmix < 0.0047 (95% C.L.) and Rmix < 0.0043 (90% C.L.). The lowest current published limit on semileptonic charm mixing is 0.005 (90% C.L.) (E791, E.M. Aitala et al., Phys.Rev.Lett. 77 2384 (1996)). The current best published limit using any analysis technique on the total rate of charm mixing is 0.0016 (95% C.L.) (Babar Kπ mixing, B. Aubert et al., Phys.Rev.Lett. 91 171801 (2003)).

In the first part of the thesis we study the formation of non-singular black holes in the collapse of magnetic monopoles. We employ a thin shell approximation model for a gravitating magnetic monople in which a false vacuum deSitter interior is matched to a Reissner-Nordström exterior. In the second part of the thesis we study solutions of D = 11 supergravity that correspond to M-branes wrapping supersymmetric cycles of Calabi-Yau manifolds. In the final chapter of the thesis we focus on the condition imposed by supersymmetry on the Kähler form of the Calabi-Yau manifold.

The Standard Model has been very successful over the last few decades in its agreement with experimental evidence; however there are some remaining puzzles in our understanding of the Universe which have yet to be solved. Even if the Higgs boson and Super Symmetry are discovered, questions still arise, such as why Nature is primarily made of matter when antimatter should have been produced in equal amounts at the beginning of the Universe, why the fundamental particles have the mass hierarchy that they do, what the nature of dark matter is, or whether or not quarks and leptons are themselves made of constituent parts, just to name a few. Theories Beyond the Standard Model attempt to tackle these questions, and also provide alternative explanations for electroweak symmetry breaking in case the Higgs mechanism in the Standard Model contradicts what is observed. The ATLAS detector was built to discover new physics from high-energy proton-proton collisions delivered by the Large Hadron Collider and to probe the electroweak scale with hard interactions at energies near ∼1 TeV. While searching for new physics processes occurring at a much higher invariant mass than available at previous colliders, understanding the performance of the detector is crucial, especially during the first few months of running. This thesis presents a motivation for using dimuons to search for new physics in early ATLAS data, a measurement of the Z0/γ* → μμ cross section as a first test of Standard Model theoretical predictions at [special characters omitted] = 7 TeV, and finally a search for new physics via a four-fermion contact interaction in the dimuon channel (qqμμ) using the full 2010 data set.

In the following discourse unpolarized cross sections and double helicity asymmetries of single inclusive positive and negative charged hadrons at mid-rapidity from p + p collisions at [special characters omitted] = 62.4 GeV are presented. Measurements for the transverse momentum range 1.0 < pT < 4.5 GeV/c are done with PHENIX detector at Relativistic Heavy-Ion Collider (RHIC) and are consistent with calculations based on perturbative quantum chromodynamics (pQCD) at next-to-leading order (NLO) in the strong coupling constant, α s. Resummed pQCD calculations including terms with next-to-leading log (NLL) accuracy, yielding reduced theoretical uncertainties, also agree with the data. The double helicity asymmetry, sensitive at leading order to the gluon polarization in a momentum fraction range of 0.05 [special characters omitted] xgluon [special characters omitted] 0.2, is consistent with recent global parameterizations disfavoring large gluon polarization.