A new realisation of light thermal self-interacting dark matter and detection prospects

Light dark matter (DM) in the GeV ballpark faces weaker constraint from direct detection experiments. If such DM also has sufficient self-interactions due to a light mediator, it can alleviate the small-scale problems of the cold dark matter (CDM) paradigm while being consistent with the latter at large scales, as suggested by astrophysical observations. However, generating correct thermal relic for such light DM is challenging. While GeV scale CDM typically leads to thermal overproduction due to the absence of sufficient annihilation channels, self-interacting dark matter (SIDM) typically has under-abundant thermal relic due to its large annihilation rates into mediator particles. In this letter, we propose a minimal realisation of GeV scale SIDM with correct thermal relic, where one of the three singlet fermions, responsible for seesaw origin of light neutrino masses, assists in generating correct SIDM relic density via freeze-out. The setup thereby encompasses astrophysical as well as cosmological aspects of light DM while simultaneously providing a solution to the origin of light neutrino mass. Due to such connection to several frontiers, there exist exciting discovery prospects in terms of direct and indirect search of DM, laboratory and cosmology based dark photon signatures and effective light neutrino mass.