Phenomenological Explorations in Dark Matter

Abstract

The previous decade has seen an explosive increase in explorations into the nature of dark matter (DM) encompassing astrophysical, particle and cosmological probes. We face today a large body of gravitational evidences supporting the existence of dark matter and yet must grapple with ambiguity about its particle nature. This makes it one of the most important and challenging questions in physics, with a wide range of implications. Of particular interest is the interplay between the particle nature of dark matter and its astrophysical and cosmological manifestations. In this thesis, we address a few important questions regarding the possible properties of dark matter.

The thesis consists of two parts. The first part explores lepton flavored dark matter (LFDM). One of the main results of this part is the connection between the stability of dark matter and the symmetries it possesses. We systematically show that many representations of lepton flavored dark matter are stable under the minimal flavor violation (MFV) hypothesis as long as there are no lepton number violating interactions. As a special case of the stability condition, we show that DM carrying certain charges under lepton number are trivially stable from lepton number conservation alone. We then study the cases of freeze-in mechanisms for relic density production and their detection phenomenology. We see that the LFDM in the MFV framework naturally accommodates a freeze-in production. Additionally, the notoriously difficult to detect freeze-in mechanism leads to some observable signatures at present and future direct detection experiments in minimal models of LFDM. In the second part of the thesis, we explore two important questions related to direct and indirect searches for dark matter. The latest results from the direct detection experiment XENON1T achieved unprecedentedly low background rates in electron recoil events for O(keV) recoil energies, with the future experiment XENONnT projected to lower it even further. Motivated by this, we explore the reach of XENON1T experiment in probing inelastic dark matter. We consider a dark sector consisting of two Majorana fermions χ1 and χ2 that form a pseudo-Dirac state with O(keV) mass splitting. We study the freeze-in production of the DM along with constraints from lepton colliders, flavor factories, beam dumps, and supernova cooling in the relevant parameter space. We then study the direct detection of the DM by assuming the lighter state makes up the full DM abundance in current epoch. We show then that direct detection process is enabled only by up-scattering of the dark matter in the Sun, followed by a down-scattering in electron recoil events at direct detection experiments on Earth. This leads to constraints from the current results from XENON1T experiment. We also find that hitherto unconstrained parameter space will be probed at the XENONnT experiment. The second question addresses whether neutron stars can be used as reliable probes of particle dark matter. For concreteness, we focus on the case where dark matter is captured by muons leading to kinetic heating in old neutron stars. The temperatures of old neutron stars can be probed at near-future telescopes like the James Webb Space Telescope (JWST). Our results show that the capture rates and subsequently the temperatures of the neutron stars are crucially dependent on the dark matter properties as opposed to the astrophysical properties of the neutron stars, like equation-of-state, velocity of the neutron star, dark matter halo distribution, etc. This, we believe, sets the path for neutron stars to become reliable laboratories of dark matter properties.