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Le plasma de quarks et de gluons (QGP) est un état de la matière nucléaire apparaissant à hautedensité d’énergie. En laboratoire, il est possible de reproduire de telles conditions grâce aux collisionsd’ions lourds aux énergies ultra-relativistes. ALICE (A Large Ion Collider Experiment) estl’expérience du LHC dédiée à la mise en évidence du QGP.Différentes signatures ont été proposées et étudiées expérimentalement comme manifestations duQGP. Parmi celles-ci, le méson J/ψ joue un rôle central. Il fait partie de la famille des quarkonia,états mésoniques (Q¯Q) formées d’un quark lourd c ou b et de son anti-quark, liés par un potentield’interaction forte. En 1986, Matsui et Satz proposèrent la suppression des charmonia (états liés cc)et notamment du J/ψ comme signature de la formation du plasma de quarks et de gluons.ALICE peut détecter le J/ψ à grande rapidité (2.5 < y < 4) via le canal de désintégration en deuxmuons. Cette thèse porte sur la mesure de la production du J/ψ, via le canal muonique, dans lescollisions pp à une énergie dans le centre de masse de 2.76 et 7 TeV. Elle a exploité les donnéesacquises en 2010 et 2011 auprès du collisionneur LHC.Tenter d’appréhender le mécanisme de production du J/ψ (et plus généralement du quarkonium)dans les collisions pp est un préalable nécessaire avant d’aborder le degré de complexité suivantque constitue le cas des collisions noyau-noyau. Il est également un test important pour la QuantumChromo Dynamics (QCD), la théorie de l’interaction forte, aux énergies très élevées du LHC. / Quarkonia are meson states whose constituents are a charm or bottom quark and its correspondingantiquark (Q¯Q). The study of the production of such bound states in high-energy hadron collisionsrepresents an important test for the Quantum Chromo-Dynamics. Despite the fact that the quarkoniumsaga has already a 40-year history, the quarkonium production mechanism is still an open issue.Therefore, measurements at the new CERN Large Hadron Collider (LHC) energy regimes are extremelyinteresting.In this thesis, the study of inclusive J/ψ production in proton-proton (pp) collisions at √s = 2.76and 7 TeV, obtained with the ALICE experiment, is presented. J/ψ mesons are measured at forwardrapidity (2.5 < y < 4), down to zero pT, via their decay into muon pairs (μ+μ−).Quarkonium resonances also play an important role in probing the properties of the stronglyinteracting hadronic matter created, at high energy densities, in heavy-ion collisions. Under suchextreme conditions, the created system, according to QCD, undergoes a phase transition from ordinaryhadronic matter to a new state of deconfined quarks and gluons, called Quark Gluon Plasma(QGP). The ALICE experiment at CERN LHC has been specifically designed to study this state ofmatter. Quarkonia, among other probes, represents one of the most promising tools to prove the QGPformation. In order to correctly interpret the measurements of quarkonium production in heavy-ioncollisions, a solid baseline is provided by the analogous results obtained in pp collisions.Hence, the work discussed in this thesis, concerning the inclusive J/ψ production in pp collisions,also provides the necessary reference for the corresponding measurements performed in Pb-Pb collisionswhich were collected, by the ALICE experiment, at the very same center-of-mass energy pernucleon pair (√sNN = 2.76 TeV).

A measurement of J/\Psi polarization has been performed for 200 GeV proton+proton collisions with the PHENIX Muon Arms at RHIC. The results from the current data show no polarization within the PHENIX acceptance range. The results are consistent with the current model predictions and other experimental measurements.

Over the last few decades, the Standard Model (SM) of particle physics has been used as a means of understanding the world around us. However, there is an increasing amount of data that suggests the SM of particle physics only describes nature up to energies of the electroweak scale. Extensions to the SM have been developed as a means of explaining experimental observation. If these extensions are indeed the correct mathematical descriptions of nature, the Large Hadron Collider (LHC), located at the European Center for Nuclear Research (CERN) near Geneva, Switzerland, is expected to produce new and exciting physics signatures that can shed light on the evolution of our universe since the early hypothesized Big Bang. Of particular interest are models that may lead to events with highly energetic tau lepton pairs. In this dissertation, focus is placed on a possible search for new heavy gauge bosons decaying to highly energetic tau pairs using a data sample corresponding to an integrated luminosity of 36 pb^-1 of proton-proton collisions at sqrt(s) = 7 TeV collected with the CMS detector at the CERN LHC. The number of observed events in the data is in good agreement with the predictions for SM background processes. In the context of the Sequential SM, a Z0 with mass less than 468 GeV/c^2 is excluded at 95 percent credibility level, exceeding the sensitivity by the Tevatron experiments at the Fermi National Accelerator Laboratory.

In this thesis a production of J/ψ meson in pp and heavy-ion collisions was investigated. A comparison between Monte Carlo generator and data by ATLAS experiment at CERN was done. PYTHIA version 8.2 was chosen for the Monte Carlo generator. In the first part, comparisons of several configurations of PYTHIA were used to establish the best matching of the Monte Carlo to the data using collisions with centre-of-mass energy of 7 TeV. Choice of parton distribution functions was found not to be important, while presence or absence of initial and final state radiation were proven to have an important impact on the result. In the second part, correlation of J/ψ meson and the jet production was quantified. Anti-kt algorithm with R = 0.4 was used to reconstruct jets. Pairs of muons were used to identify J/ψ meson candidates. Only less than 10% of J/ψ mesons were found to be associated with jets. 1

Longstanding evidence from observational astronomy indicates that non-luminous "dark matter" constitutes the majority of all matter in the universe, yet this mysterious form of matter continues to elude experimental detection. This dissertation presents a search for dark matter at the Large Hadron Collider using 139 fb\(^{-1}\) of proton-proton collision data at a centre-of-mass energy of \(\sqrt{s} = 13\,\)TeV, recorded with the ATLAS detector from 2015 to 2018. The search targets a final state topology in which dark matter is produced from the proton-proton collisions in association with a pair of W bosons, one of which decays to a pair of quarks and the other to a lepton-neutrino pair. The dark matter is expected to pass invisibly through the detector, resulting in an imbalance of momentum in the plane transverse to the beam line. The search is optimized to test the Dark Higgs model, which predicts a signature of dark matter production in association with the emission of a hypothesized new particle referred to as the Dark Higgs boson. The Dark Higgs boson is predicted to decay to a W boson pair via a small mixing with the Standard Model Higgs boson discovered in 2012. Collisions that exhibit the targeted final state topology are selected for the search, and an approximate mass of the hypothetical Dark Higgs boson is reconstructed from the particles in each collision. A search is performed by looking for a deviation between distributions of the reconstructed Dark Higgs boson masses and Standard Model predictions for the selected collisions. The data is found to be consistent with the Standard Model prediction, and the results are used to constrain the parameters of the Dark Higgs model. This search complements and extends the reach of existing searches for the Dark Higgs model by the ATLAS and CMS collaborations. / Graduate

The Standard Model of particle physics is a successful theory, yet it is incomplete. Supersymmetry is one of the favoured extensions of the Standard Model, elegantly addressing several unresolved issues. This thesis presents a search for the pair production of supersymmetric particles chargino one and neutralino two, where the neutralino two decays to the lightest neutralino and the 125 GeV Higgs boson. The final states considered for the search have large missing transverse momentum, an isolated lepton and two jets identified as originating from bottom quarks. The analysis is based on 20.3 inverse femtobarns of 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. No excess over Standard Model predictions is observed. The analysis has been combined with three independent searches that probe other decay modes of the Standard Model Higgs boson. Limits are set at 95% confidence level in the context of a simplified supersymmetric model. Common masses of chargino one and neutralino two are excluded up to 250 GeV for a massless neutralino one. The analysis of this dissertation has been reinterpreted in the context of a large scan of the phenomenological Minimal Supersymmetric Standard Model, along with 22 other ATLAS Run 1 searches. The resulting summary paper represents the most comprehensive assessment of the ATLAS constraints on Supersymmetry models to date. / Graduate / 0798 / cdavid@uvic.ca

This thesis presents three searches for new resonances in dijet invariant mass spectra. The spectra are produced using √s = 13 TeV proton-proton collision data recorded by the ATLAS detector. New dijet resonances are searched for in the mass range 200 GeV to 6.9 TeV in mass. Heavy new resonances, with masses above 1.1 TeV, are targeted by a high mass dijet search. Light new resonances, with masses down to 200 GeV, are searched for in dijet events with an associated high momentum object (a photon or a jet) arising from initial state radiation. The associated object is used to efficiently trigger the recording of low mass dijet events. All of the analyses presented in this thesis search for an excess of events, localised in mass, above a data-derived estimate of the smoothly falling QCD background. In each search no evidence for new resonances is observed, and the data are used to set 95% C.L. limits on the production cross-section times acceptance times branching ratio for model-independent Gaussian resonance shapes, as well as benchmark signals. One particular benchmark signal which is considered in all of the searches is an axial-vector Z' dark matter mediator model whose parameter space is reduced due to the results presented in this thesis.

Le travail présenté dans cette thèse a pour sujet la mesure du seul mécanisme de production électrofaible de quark top qui n'ait pas encore été observé au collisionneur hadronique du CERN (LHC): la voie-s. L'intérêt réside dans la confirmation des prédictions du Modèle Standard, mais le but est aussi de contraindre plusieurs scénarios de nouvelle physique à partir de la combinaison de la section efficace voie-s avec celle des autres canaux de production single top, la voie-t et Wt. Après une introduction générale sur la physique du quark top et les dispositifs expérimentaux visant à la détection et la reconstruction des objets physiques, on présente deux analyses réalisées des données de collisions proton-proton enregistrées par le détecteur ATLAS `a une énergie dans le centre de masse de 7 et 8 TeV. Etant donné que la production de quark top en voie-s est caractérisée par une tr`es faible pureté et que, en même temps, elle est défavorisée au LHC puisque elle a lieu par annihilation quark-antiquark, une technique multi variée a été appliquée dans les deux cas afin de discriminer le signal. Pour l' étude à 7 TeV, relatif à une luminosité intégrée de 4.7 $fb^{-1}$, deux classificateurs basés sur des arbres de décision boostés sont entrainés contre les événements de bruits de fond principaux; une coupure appliquée sur un des discriminateurs met en oeuvre une sélection des événements, la distribution issue de l'autre discriminateur est ensuite employée dans un fit de maximum de vraisemblance. Cela permet de d'eriver une limite supérieure sur la section efficace voie-s de 21.5 pb (14.3 pb attendu), correspondant à une portée de 0.6 déviations standard (0.8 attendu). L'analyse à 8 TeV, qui est basée sur une luminosité intégrée de 20.3 $fb^{-1}$, repose sur une strat'egie plus simple: seulement un classificateur BDT est optimisé pour isoler le signal, après une préalable sélection des événements grâce `a de simples coupures. Sa distribution discriminante est ensuite ajustée aux données selon la même approche qu' `a 7 TeV, en considérant les incertitudes systématiques et statistiques pendant la génération de pseudo expériences. La portée de la mesure étant encore inférieure à trois déviations standard (1.5 $sigma$ observé et 1.4 attendu), seule une limite `a 95% C.L. peut être établie sur la section efficace; le résultat correspond `a 15.7 pb (7.9 pb attendu). / The work reported in this thesis is aimed at measuring with the ATLAS detector theonly mechanism of top quark electroweak production which has not yet been observedat the Large Hadron Collider: the s-channel. Its interest relies in the confirmationof the Standard Model predictions but, as well, in the possibility of constraining severalnew physics scenarios when comparing the s-channel cross section with the oneof the other single top production modes (the Wt and t-channel). After a generalintroduction upon top physics and the experimental setup employed for the detectionand the reconstruction of the physics objects, we present two analyses realizedwith proton-proton collisions collected by the ATLAS detector at a center of massenergy of 7 and 8 TeV. Since the s-channel production is characterized by a verylow purity and, at the same time, is not favoured at the LHC because it proceedsvia quark-antiquark annihilation, a multivariate approach is applied in both cases todiscriminate the signal. For the 7 TeV study, based on an integrated luminosity of4.7 fb−1, two boosted decision trees classifiers are optimized against the main sourcesof background; a cut realized on one of such discriminants implements a tight eventselection, after which the second output distribution is used for a maximum likelihoodfit to the data. This allows to set an upper limit on the s-channel cross section of21.5 pb (14.3 pb expected), corresponding to a significance of 0.6 standard deviations(0.8 expected). The 8 TeV analysis, based on an integrated luminosity of 20.3 fb−1,has a simpler strategy: only one boosted decision trees classifier is trained to isolatesignal after a preliminary cut-based selection; its output distribution is then fitted tothe data with the same approach than at 7 TeV, taking into account the systematicand statistical uncertainty via the generation of pseudoexperiments. The significanceof the s-channel measurement being lower than 3 σ (1.3 standard deviation observed,1.4 expected), only a 95% C.L. limit can be set on the cross section; the result ishowever improved and corresponds to 14.6 pb (7.9 pb expected).3

La production de photons de grande impulsion transverse lors des collisions proton-proton (p-p), est décrite par la chromodynamique quantique perturbative (pQCD). Parmi ces photons, ceux issus directement d’un processus partonique énergétique (appelés photons directs) sont particulièrement intéressants car leur mesure permet de tester précisément les prédictions de la pQCD et offre la possibilité de mieux contraindre les fonctions de structure du proton. Ce travail de thèse a pour objectif l’étude et la mesure des photons directs produits dans les collisions p-p à 7 TeV avec le détecteur ALICE. Le calorimètre électromagnétique d’ALICE (EMCal), est utilisé pour réaliser cette mesure qui s’appuie notamment sur l’utilisation d’une procédure d’isolement permettant de réduire le bruit de fond provenant des autres modes de production (fragmentation, décroissance). Les aspects relatifs à la qualité des données dans EMCal, à l’identification des photons, mais aussi ceux liés à la correction du spectre ou encore à sa normalisation sont mis en avant. Finalement, la première section efficace de photons isolés mesurée dans les collisions p-p à 7 TeV avec le détecteur ALICE est présentée et comparée avec les prédictions théoriques avant d’être mise en regard avec les résultats issus des autres grandes expériences du LHC. / The high transverse momentum photon production inproton-proton collisions (p-p) is described by perturbativequantum chromodynamics (pQCD). Among thesephotons, those produced directly by an energetic partonicinteraction (called direct photons) are of great interestsince their measurement allows to test pQCDpredictions and it allows also the constraint of protonstructure functions. The work of this thesis aims atstudying and measuring direct photons produced in p-pcollisions at 7 TeV with the ALICE detector. The ALICEelectromagnetic calorimeter (EMCal) is used to achievethis measurement which is based on an isolation procedurethat allows to reduce background coming fromother photon production modes (fragmentation, decay).Multiple aspects like EMCal data quality, photon identificationas well as spectrum correction and its normalizationare highlighted. Finally, the first isolated promptphoton cross-section measured with ALICE detector ispresented, compared to theoretical predictions and tothe last results from other LHC experiments.