Although astrophysical evidence supports the existence of dark matter (DM), it remains one of the unanswered questions left by the Standard Model (SM) of Particle Physics. However, under hypotheses of new interactions, the production of dark matter can be detected as an excess of events with large missing transverse momentum (p_T^miss) over the SM background process. This thesis documents a search for new particles at the Compact Muon Solenoid (CMS) at the Large Hadron Collider (LHC), targeting events where large p_T^miss and energetic jets are produced in a proton-proton collision at 13 TeV. The data were collected from 2017 to 2018 during the second half of LHC Run-II. The analysis also targets events where a jet is produced from W or Z bosons identified by a deep-neural-network-based tagger. Multiple control regions targeting specific background processes are defined which estimate background yield in the signal region through a simultaneous fit across control regions of all search channels. The result for the Run-II data corresponding to an integrated luminosity of 137 fb^-1 is obtained by combining this analysis with the previously published 2016 data. No excess of events is observed compared to the SM background expectations. The result of this search is interpreted in several new physics models, including simplified dark matter models, large extra dimension model (ADD), Higgs portal models, and leptoquark models. Limits are set on model parameters providing the most stringent direct constraints on dark matter search from colliders.

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This work presents results on the study of the scintillation of high-pressure Xenon gas irradiated by various sources. Noble gases such as Xenon give off characteristic scintillation light when irradiated. The goal of the study was to develop a characteristic based on the scintillation time response of Xenon gas that would reliably discriminate between events from different types of primary radiation (neutron or gamma). A reliable discrimination characteristic would enable the development of room temperature, gas phase detectors for use in the search for Galactic Dark Matter. The surprising result of the present work was that a reliable discrimination characteristic existed for distinguishing x-ray, gamma ray, and alpha particle events. Results for neutrons were negative. This was due to several factors: Ionization tracks in xenon generally form two roughly cylindrical regions. A region near the center of the track, called the core, has very dense ionization. An outer region, called the penumbra, has sparse ionization. In Xenon, recombination of ions and the subsequent scintillation from the penumbra region happens slowly and can be easily distinguished from scintillation that happens in the core region. Nuclear recoils resulting from neutron collisions that give recoil energies in the same range as that predicted for WIMP-nuclear collisions are of such low energy that they do not produce a significant penumbra region in Xenon gas. As such, the scintillation time response for these events is similar to that of high-energy gamma rays. Other results of the present work include: The amount of energy deposited in the gas needed to produce a scintillation photon was measured for gamma rays and was found to be in agreement with results from other experiments. Low-energy gamma rays appeared to produce more scintillation photons for an equal amount of energy deposited than high-energy gamma rays. The decay of the singlet and triplet molecular states of xenon was observed and the lifetimes of these states were measured. The singlet state lifetime was found to be independent of pressure while the triplet state lifetime was dependent on pressure. The lifetimes were measured and compared to previous results. A better understanding of the ionization, recombination, and scintillation processes of gaseous Xenon was achieved. Argon gas has been proposed as an alternative to Xenon gas for use in a high-pressure gas scintillation detector due to its lower mass and its property of forming a core ionization region that is much less dense than the core region of xenon. This substitution may allow for a reliable discrimination characteristic to be developed. / Physics

La présente thèse traite des propriétés de la Matière Noire (MN), en particulier dans le contexte du Next-to-Minimal Supersymmetric Standard Model (NMSSM). En premier lieu, est examinée la question de savoir si un neutralino dans le NMSSM pourrait expliquer un excès de photon monochromatique possiblement présent dans les données Fermi-LAT. Il est montré qu’un neutralino, associé à l’anéantissement d’une particule Higgs CP-impair échangée dans le canal s, peut, en principe, donner lieu à une section efficace suffisamment grande. Sont également étudiés les modèles asymétriques de matière noire aux fins d’expliquer la MN actuelle et la densité de baryons. Les limites supérieures de l’auto- anéantissement de la section efficace, qui peuvent potentiellement détruire la MN asymétrique, sont dérivées et appliquées à une variété de modèles. Enfin, est proposé un modèle supersymétrique qui prévoit des sneutrinos en tant que MN asymétrique viable et qui explique les petites valeurs de la masse des neutrinos. Sont ainsi étudiées des limites à ce modèle à partir de la physique des particules, de la cosmologie et des observations de la MN. / This thesis deals with Dark Matter (DM) properties, mainly in the context of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). First, it is examined whether a neutralino in the NMSSM could explain a monochromatic photon excess possibly present in the Fermi-LAT data. It is shown that neutralino pair annihilation with a CP-odd Higgs exchanged in s-channel can, in principle, give rise to a sufficiently large cross section. Asymmetric dark matter models, aiming at the explanation of the coincidence of present-day DM and baryon abundances, are also discussed. Upper bounds on DM self-annihilation cross section, which can potentially destroy the DM asymmetry, are derived and applied to a variety of models. Finally, a supersymmetric model is proposed, providing sneutrinos as viable asymmetric DM and explaining the smallness of neutrino masses. Bounds on this model from particle physics, cosmology and DM searches are studied.

An abundance of cosmological evidence suggests that cold dark matter exists and makes up 83% of the matter in the universe. At the same time, this dark matter has eluded direct detection and its identity remains a mystery. Many large international collaborations are actively searching for dark matter through its potential annihilation in high-density regions of the universe, its creation in particle accelerators, and its interaction with Standard Model particles in low-background detectors. One of the most promising dark matter candidates is the weakly interacting massive particle (WIMP) which falls naturally out of extensions of the Standard Model. A variety of detectors have been employed in the search for WIMPs, which are expected to scatter with atomic nuclei, yet none have been more successful than dual-phase liquid xenon time projection chambers (TPCs). The first ton-scale liquid xenon TPC, XENON1T, began operating in 2016 and with only 34.2 days of data has set the most strict limits on the WIMP-nucleon interaction cross sections for WIMP masses above 10 GeV/c^2, with a minimum of 7.7 × 10−47 cm^2 for 35 GeV/c^2 WIMPs. One of the major keys to success for liquid xenon TPCs is our understanding of interactions in the medium through myriad measurements. Given that the expected WIMP scattering rate increases with decreasing interaction energy, there has been more focus in recent years in pushing our understanding of interactions in liquid xenon to lower energies. Additionally, as liquid xenon TPCs operate with a large electric field in the medium, an effort has been made to understand how the signal response of xenon changes as a function of the applied electric field. In this thesis, I describe the details of XENON1T, its calibration and characterization, with a special emphasis on the electronic and nuclear recoil calibrations, and the inaugural WIMP search of XENON1T. I then discuss a dedicated measurement, made in the calibration-optimized liquid xenon TPC neriX, of the signal response of low energy nuclear recoils in liquid xenon at electric fields relevant to the dark matter search. The measurements of signal response in XENON1T and neriX were performed using an analysis framework that I developed to allow a more sophisticated examination of recoil responses using GPU-accelerated simulations.

We present two different interacting dark sector models: one in which the dark matter particle is charged under a non-abelian dark gauge group, whose gauge bosons constitute a dark radiation component; and one in which a fraction of the dark matter has efficient number-changing self-interactions that keep it warm. We find that in general the structure formation is slowed down in these models, which addresses a discrepancy in the measurement of the σ8 parameter of large-scale structure. We also perform fits to cosmological data for a generalization of the non-abelian model (in which only a fraction of the dark matter interacts with the dark gauge bosons) and show that it can ease the current experimental tension in the measurement of the Hubble expansion rate H0.

Flat cosmology with collisionless cold dark matter (CDM) and cosmological constant ([Lambda]CDM cosmology) may have some problems on small scales, even though it has been very successful on large scales. We study the effect of Self-Interacting Dark Matter (SIDM) hypothesis on the density profiles of halos. Collisionless CDM predicts cuspy density profiles toward the center, while observations of low mass galaxies prefer cored profiles. SIDM was proposed by Spergel & Steinhardt [161] as a possible solution to this cuspy profile problem on low-mass scales. On the other hand, observations and collisionless CDM agree on mass scales of galaxy clusters. It is also known that the SIDM hypothesis would contradict with X-ray and gravitational lensing observations of cluster of galaxies, if the cross section were too large. Our final goal is to find the range of SIDM scattering cross section models that are consistent with those astrophysical observations in two different mass scales. There are two theoretical approaches to compute the effect of self-interacting scattering -- Gravitational N-body simulation with Monte Carlo scattering and conducting fluid model; those two approaches, however, had not been confirmed to agree with each other. We first show that two methods are in reasonable agreement with each other for both isolated halos and for halos with realistic mass assembly history in an expanding [Lambda]CDM universe; the value of cross section necessary to have a maximally relaxed low-density core in [Lambda]CDM is in mutual agreement. We then develop a semianalytic model that predicts the time evolution of SIDM halo. Our semianalytic relaxation model enables us to understand how a SIDM halo would relax to a cored profile, and obtain an ensemble of SIDM halos from collisionless simulations with reasonable computational resources. We apply the semianalytic relaxation model to CDM halos, and compare the resulting statistical distribution of SIDM halos with astrophysical observations. We show that there exists a range of scattering cross sections that simultaneously solve the cuspy core problem on low-mass scales and satisfy the galaxy cluster observations. We also present that other potential conflicts between [Lambda]CDM and observations could be resolved in Part II and III. / text

<p>We examine the Einstein equation coupled to the Klein-Gordon equation for a complex-valued scalar field. These two equations together are known as the Einstein-Klein-Gordon system. In the low-field, non-relativistic limit, the Einstein-Klein-Gordon system reduces to the Poisson-Schrödinger system. We describe the simplest solutions of these systems in spherical symmetry, the spherically symmetric static states, and some scaling properties they obey. We also describe some approximate analytic solutions for these states.</p><p>The EKG system underlies a theory of wave dark matter, also known as scalar field dark matter (SFDM), boson star dark matter, and Bose-Einstein condensate (BEC) dark matter. We discuss a possible connection between the theory of wave dark matter and the baryonic Tully-Fisher relation, which is a scaling relation observed to hold for disk galaxies in the universe across many decades in mass. We show how fixing boundary conditions at the edge of the spherically symmetric static states implies Tully-Fisher-like relations for the states. We also catalog other ``scaling conditions'' one can impose on the static states and show that they do not lead to Tully-Fisher-like relations--barring one exception which is already known and which has nothing to do with the specifics of wave dark matter.</p> / Dissertation

We consider the sensitivity of high luminosity neutrino experiments to light stable states, as arise in scenarios of MeV-scale dark matter. To ensure the correct thermal relic abundance, such states must annihilate to the Standard model via light mediators, providing a portal for access to the dark matter state in colliders or fixed targets. This framework implies that neutrino beams produced at a fixed target will also carry an additional “dark matter beam”, which can mimic neutrino scattering off electrons or nuclei in the detector. We therefore develop a Monte Carlo code to simulate the production of a dark matter beam at two proton fixed-target facilities with high luminosity, LSND and MiniBooNE, and with this simulation determine the existing limits on light dark matter. We find in particular that MeV-scale dark matter scenarios motivated by an explanation of the galactic 511 keV line are strongly constrained. / Graduate

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