The domain of a cannibal dark matter

Abstract

We consider a scenario in which the dark matter is alone in a hidden sector and consists of a real scalar particle with a manifest or spontaneously broken ℤ2 symmetry, at a temperature which differs from the one of the visible sector, T' ≠ T. While similar models with general couplings have already been studied in the literature, the special case of a model with spontaneous symmetry breaking constitutes a non-trivial limit of these results, since it features vanishing tree-level amplitudes for the processes k → 2 with k > 2 at threshold, thus making the cross-section governing dark-matter freeze-out velocity suppressed. We carefully determine the thermally averaged dark-matter annihilation cross-section in this scenario, including the possible effects of one-loop corrections and Bose-Einstein statistics, while also reporting our results in the domain of thermal dark matter candidates, T' fo/T vs. m DM with T' fo being the hidden-sector temperature at decoupling. We show that for fixed quartic coupling, due to entropy conservation, the thermal candidates lie along a curve T'/T ∝ m DM -1/3(1+κ) with anomalous scaling κ ∼ 𝒪(T' fo/m DM) > 0. Furthermore, we demonstrate that this scaling is valid for a broad class of cannibal DM scenarios, with or without symmetry breaking. In addition, we also discuss the relevant cosmological and astrophysical constraints.