Unitarity bound on dark matter in low-temperature reheating scenarios
Nicolás Bernal (New York University Abu Dhabi, P.O. Box 129188, Saadiyat Island, Abu Dhabi, United Arab Emirates); Partha Konar (Physical Research Laboratory, Ahmedabad 380009, Gujarat, India); Sudipta Show (Physical Research Laboratory, Ahmedabad 380009, Gujarat, India)
The model-independent theoretical upper bound on the thermal dark matter (DM) mass can be derived from the maximum inelastic DM cross section featuring the whole observed DM abundance. We deploy partial-wave unitarity of the scattering matrix to derive the maximal thermally averaged cross section for general number-changing processes (with ), which may involve standard model particles or occur solely within the dark sector. The usual upper limit on the DM mass for -wave annihilation is around 130 TeV (1 GeV) for (3) and only applies in the case of a freeze-out occurring in the standard cosmological scenario. We consider the effects of two nonstandard cosmological evolutions, characterized by low-temperature reheating: (i) a kinationlike scenario and (ii) an early matter-dominated scenario. In the first case, early freeze-out strengthens the unitarity bound to a few TeVs for weakly interacting massive particles (WIMPs); while in the second case, the WIMP DM can be as heavy as due to a large entropy dilution.