Reconciling Thermal Freeze-Out with Results from Dark Matter Searches: The Role of Forbidden Channels

The role of Dark Matter is fundamental in the understanding of the dynamics and the evolution of the Universe. Even though its existence is established by observing its gravitational properties, there are still no probes of its interactions with visible matter. In this context, thermal freeze-out has played a key role in explaining how Dark Matter is produced in the early Universe and the lack of Dark Matter detection in the past decades can be used as a hint to either look for alternative Dark Matter candidates or to extend the freeze-out mechanism itself by taking into account "new" available annihilation channels: the forbidden channels. This thesis investigates scenarios where the relic density is set by Dark Matter annihilations which are kinematically forbidden at zero kinetic energy but become accessible at finite temperature. After presenting the general formalism and deriving the corresponding Boltzmann equation, which tracks the number density evolution, we will explore the viable parameter space of a model which appears to be a natural extension of the forbidden regime: the Dark Matter particle annihilates into heavier Standard Model leptons via scalar- or vector-mediated interactions. We then analyse the possibility of annihilations into heavier third-generation quarks and we derive the couplings to the Standard Model and the mediator mass that reproduce the observed relic abundance, by working under the boundary condition m(DM) = m(SM) and by considering the annihilations into Standard Model particles to be the dominant contribution. At this point, we ask ourselves whether one-loop induced processes happening in the early Universe might become irreducible when going beyond the boundary region of the parameter space, i.e. when studying annihilations such that m(DM) < m(SM).