From WIMPs to FIMPs: advanced methods and phenomenological studies

Abstract

The elusive nature of dark matter (DM) has prompted a variety of theoretical models and experimental searches aimed at uncovering its properties. This thesis explores three approaches to advance the understanding of DM production within simplified t-channel models, where DM interacts with a Standard Model (SM) fermion through a heavier gauge-charged mediator. First, we highlight the importance of non-perturbative effects from long-range interac- tions like the Sommerfeld effect and bound state formation in colored co-annihilations. We implement such effects in micrOMEGAs, a popular code to compute the DM relic abun- dance, and we find that previous exclusions of parameter space by direct detection and LHC searches may still be viable, emphasizing the potential of long-lived particle (LLP) and bound-state searches at the LHC. Second, we analyze thermal corrections to freeze-in production of feebly interacting DM using the Closed-Time-Path formalism. We perform a detailed analysis of the DM rate equation and we compute its relic abundance by using 1PI-resummed propagators derived from a 2PI effective action that includes the full dependence on relevant mass scales. Our results show significant deviations from the semiclassical Boltzmann approach and the Hard-Thermal-Loop approximation, demonstrating the role of theoretical uncertainties in determining DM properties. We also address the issue of negative spectral sums in models featuring gauge-charged scalar fields. Lastly, we study freeze-in production during non-instantaneous reheating after inflation, showing that low reheating temperatures and different inflationary potentials can alter DM production. We highlight the impact of inflationary dynamics on DM freeze-in and LLP searches, stressing the importance of accurately defining the reheating temperature.