MicroBooNE is a liquid argon time projection chamber that collected neutrino data at Fermilab's Booster Neutrino Beam from 2015 to 2020. One of its primary goals is to investigate the “Low Energy Excess” of neutrino events observed by the MiniBooNE experiment, for which candidate photon-like interpretations include an underestimation of neutrino neutral current (NC) resonant Δ production with subsequent radiative decay or another anomalous source of single photon production in neutrino interactions. In particular, NC Δ radiative decay is poorly constrained background process to electron neutrino measurements and could be a sizable contribution to the “Low Energy Excess.” This thesis will present the analysis developed to search for NC Δ → N𝛾 events in MicroBooNE, consisting of a boosted decision tree based event selection with an NC neutral pion background constraint, using data from the first three years of operations corresponding to 6.9 × 10²⁰ POT.

The hunt for Dark Matter plays a truly critical role in contemporary physics. At both the largest and smallest scales, deep questions are being raised about the fundamental nature of the universe - questions that confirmation and then characterisation of particle dark matter will provide many answers to. This thesis presents some of the world's most sensitive searches to date for certain types of axion dark matter, axion-like particles, and two-neutrino double electron capture. These have been conducted using the Large Underground Xenon (LUX) experiment. Evidence for dark matter and physics beyond the Standard Model of particle physics is described in Chapter 1, while Chapter 2 gives an overview of proposed candidates for particle dark matter. The various experimental approaches being used to detect particle dark matter are presented in Chapter 3. Direct detection with time projection chambers plays a major role in this thesis, with particular interest in the LUX detector, that is described in its components and operations. Chapter 4 presents LUX direct searches for weakly interacting massive particles. Although I have contributed to these analyses, they are included for completeness only, as they are not part of my central work. The LUX collaboration's searches for axion dark matter and axion-like particle have delivered world-leading results on the axion-electron coupling constant. These results, that I personally led and which have been published in Physics Review Letters, are presented in Chapter 5, along with sensitivity studies, also led by me, made for the future LUX-ZEPLIN experiment. Finally, a search for two-neutrino double electron capture of 124Xe, that I performed using LUX data to extract a limit on the half life of the process, is presented in Chapter 6. Although being allowed by the Standard Model, two-neutrino double electron capture shares the matrix element calculation framework with the neutrinoless channel of the same process, becoming of great interest in the scope of neutrino physics. Conclusions follow and close the thesis.

Le Modèle Standard de la physique des particules décrit les interactions des particules sub-atomiques à travers les interactions fortes, faibles et électromagnétiques.Toutefois, il est clair que cette théorie ne constitue pas une description complète de la Nature. Mais le fait expérimental que les neutrinos changent de saveur au cours de leur propagation(oscillation des neutrinos) implique qu'ils ont une masse non nulle. Le travail de cette thèse se concentre sur l'expérience d'oscillation T2K : une expérience à longue ligne de base installée au Japon. % qui est détaillés dans le Chapitre 2. Pour la première fois T2K a pu observer l'apparition de neutrinos de saveur électronique dans le faisceau principalement composé de neutrinos muoniques etobtenir les premières contraintes sur la violation de la symétrie Charge-Parité. Il permet également de mesurer précisément les paramètres d'oscillations θ₁₃ , θ₂ ₃ et Δm²₂ ₃ .Cela nécessite une excellente compréhension du détecteur et des modèles d'interactions des neutrinos. Cette thèse se concentre sur ces deux objectifs : réduire les systématiques liées audétecteur et approfondir nos connaissances sur l'interaction des neutrinos avec la matière.Le groupe CEA, dans lequel je suis impliqué, est responsable de la maintenance et de l'opération des Chambres à Projections Temporelles (TPC) du détecteur proche (ND280).Elles permettent l'identification des particules chargées produites par l'interaction des neutrinos dans le detecteur et la mesure de leur impulsion.Une partie de mon travail a consisté en l'étude des TPCs et en particulier de l'alignement des modules MicroMegas installés sur les plans de lecture. En effet tout défaut d'alignement entre différents modules peut causer un biais sur la mesure del'impulsion des particules traversant la TPC. Les neutrinos sont étudiés grâce à l'observation de l'état final de leur interaction avec la matière. Les modèles d'interactions doivent alors être parfaitement bien compris,car les expériences d'oscillation présentes et futures approchent d'une phase où notre connnaissance des interactionsdes neutrinos devient un facteur limitant pour la détermination des paramètres d'oscillations. L'interaction quasi élastique par courant chargé (CCQE) est le processus dominant pour T2K. Ce manuscrit décrit la mesure durapport des sections efficaces d'interaction des neutrinos sur les noyaux de Carbone et d'Oxygène. Cette mesure contribue à la réduction des incertitudes pour l'analyse d'oscillation, liéesà l'utilisation d'une différente cible pour le détecteur proche et le détecteur lointain. / The Standard Model of particle physics describes the interactions of subatomic particles through the strong, weak and electromagneticinteractions. However, it is known that this theory is not a complete description of the nature. Indeed the observation that neutrinos can change their flavor alongtheir propagation path (neutrino oscillation) proves that they actually have a mass. The work of this thesis has been performed in the T2K oscillation experiment: a long baseline experiment located in Japan. % detailed in Chapter ref{sec_t2k_res}.Using the data collected so far, T2K has been able to observe for the first time the $nu_{mu} rightarrow nu_e$ appearance and to give first results on the Charge-Paritysimmetry in neutrino oscillation. It provides also precise measurements of the oscillation parameters θ₁₃ , θ₂ ₃, Δm²₂ ₃.The precise measurement of oscillation parameters requires a good understanding of the detector and of the neutrino interaction model. This thesis is thus focused on boththese aspects: reduce the detector uncertainties and improve our knowledge of neutrino interactions with matter.The CEA Saclay group where I am involved, is in charge of the maintenance and operation of the Time Projection Chambers (TPCs) of the T2K near detector (ND280)which are used to identify and measure the kinematics of the charged particles produced in neutrino interactions.Part of my work was focused on the study of the TPC and in particular on the alignment of the MicroMegas modules instrumenting their readout planes.Indeed, misalignments between modules may cause a bias on the momentum measurement of the particles crossing the TPC.Neutrinos are studied through the observation of the final state of their interactions with matter. Therefore, the interactionmodels need to be extremely well understood to infer the neutrino properties correctly.Indeed current and next future oscillation experiments are approaching the phase of precise measurements of the mixing parameters becominglimited by our knowledge of neutrino interactions.The charged current quasi-elastic (CCQE) interaction is the most relevant process at T2K.This manuscript describes the measurement of CCQE cross-section ratio between oxygen and carbon nuclei. It can contribute to reduce the uncertainties on the oscillation analysis arisingfrom the different target between the near and far detector.