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.

Here is presented the top-quark pair plus photon production cross section measured in pp collisions at a centre-of-mass energy of √ s = 8 TeV with the CMS detector at the Large Hadron Collider, using data recorded in 2012 corresponding to an integrated luminosity of Lint = 19:6 fb−1. The measurement is performed in the dilepton decay channel. The fiducial region is defined by the final state of the process pp → W⁺W⁻b bl γ, with a minimum photon transverse energy of ET( γ) > 25 GeV and lies within a pseudorapidity range of η ( γ)| < 1:4442, with a separation of ∆R(γ; all) > 0:3 between the photon and quarks or leptons in η - space. Signal events are simulated using the MadGraph event generator. Data-driven templates are constructed in order to measure the photon identification efficiency using the supercluster footprint removal technique, and a fit to the charged hadron isolation is performed. The ratio of tt + γ cross section relative to the inclusive t t cross section, R = t t+ t t, is exploited in order to cancel various sources of systematic uncertainties. The combination of dilepton final state decay channels ratio in the fiducial area is measured to be R = 0:00115 ± 0:00023(stat:). Using a recent CMS t t cross section measurement at 8 TeV in the dilepton channel of t t = 245:6± 1:3(stat:)± +6:6/5:5 (syst:) ± 6:5(lumi:) pb, the top pair plus photon production cross section is calculated to be CMS / t t+γ = 944 154(stat:+syst:) fb. Being in agreement with the t t+γ SM expectation of SM / t t+γ = 861 ± 71(scale) ± 30(PDF) fb, this is the most accurate measurement of the t t +γ process to date, and the first at a centre-of-mass energy of 8 TeV in the dilepton channel.

Thesis (M.S.)--Oregon State University, 1980. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--Oregon State University, 1980. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.S.)--Oregon State University, 1979. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.S.)--Oregon State University, 1978. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--Oregon State University, 1978. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.\

Thesis (M.S.)--Oregon State University, 1977. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.