Updated BBN constraints on electromagnetic decays of MeV-scale particles

TL;DR

This paper updates and strengthens constraints from Big Bang Nucleosynthesis on MeV-scale particles decaying into photons or electron-positron pairs, using latest primordial abundance data and including all relevant physical effects.

Contribution

It provides a comprehensive, model-independent reanalysis of BBN constraints on MeV particles, incorporating all spin-statistical factors and inverse decays, and introduces a public code for reaction network calculations.

Findings

Stronger bounds for small particle masses.

Freeze-in abundance scenarios are significantly constrained.

Re-evaluation of lithium problem solutions with ALPs.

Abstract

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.