From WIMPs to FIMPs: advanced methods and phenomenological studies
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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.