Updated BBN constraints on electromagnetic decays of MeV-scale particles
  In this work, we revise and update model-independent constraints from Big
Bang Nucleosynthesis on MeV-scale particles $\phi$ which decay into photons
and/or electron-positron pairs. We use the latest determinations of primordial
abundances and extend the analysis in arXiv:1808.09324 by including all
spin-statistical factors as well as inverse decays, significantly strengthening
the resulting bounds in particular for small masses. For a very suppressed
initial abundance of $\phi$, these effects become ever more important and we
find that even a pure 'freeze-in' abundance can be significantly constrained.
In parallel to this article, we release the public code ACROPOLIS which
numerically solves the reaction network necessary to evaluate the effect of
photodisintegration on the final light element abundances. As an interesting
application, we re-evaluate a possible solution of the lithium problem due to
the photodisintegration of beryllium and find that e.g. an ALP produced via
freeze-in can lead to a viable solution.
