Sub-MeV dark photon as dark matter in multi-temperature universe*

Dark photon as dark matter is subject to several constraints such as it should have larger lifetime than the lifetime of the universe, satisfy the relic density limit, and be consistent with the BBN constraints. Here we discuss a sub-MeV dark photon model which survives these stringent constraints. The model includes two hidden sectors, one of which interacts directly with the visible sector while the second has only indirect coupling with the visible sector. Since the visible and the hidden sectors are in general in different bath temperatures, we use a formalism for the evolution of three bath temperatures: one for the visible sector and two for the hidden sectors and involves solution of Boltzmann equations coupling the three sectors. We will discuss the parameter region where the sub-MeV dark photon can exist and be consistent with all the current experimental constraints. Further, our analysis can be extended to a multi-temperature universe with multiple hidden sectors and multiple heat baths and may find application for a wider class of phenomena involving multiple sectors.

Based on:  JHEP 06 (2021) 086, arXiv: 2103.15769[hep-ph], by Amin Aboubrahim, Wan-Zhe Feng, Pran Nath, and Zhu-Yao Wang.