BBN constraints on MeV-scale dark sectors. Part II. Electromagnetic
  decays
  Meta-stable dark sector particles decaying into electrons or photons may
non-trivially change the Hubble rate, lead to entropy injection into the
thermal bath of Standard Model particles and may also photodisintegrate light
nuclei formed in the early universe. We study generic constraints from Big Bang
Nucleosynthesis on such a setup, with a particular emphasis on MeV-scale
particles which are neither fully relativistic nor non-relativistic during all
times relevant for Big Bang Nucleosynthesis. We apply our results to a simple
model of self-interacting dark matter with a light scalar mediator. This setup
turns out to be severely constrained by these considerations in combination
with direct dark matter searches and will be fully tested with the next
generation of low-threshold direct detection experiments.
