Presupernovae Strongly Constrain Electrophilic MeV Bosons

Abstract

The years preceding a supernova (SN) can constrain new electrophilic bosons with MeV masses more than the explosion itself. Although cooler than SNe, the massive progenitors maintain consistently high temperatures much longer, injecting more particles whose decays to positrons contribute to the diffuse Galactic 511-keV emission. For dark photons, the increase is exacerbated by the lower plasma frequency compared to SNe, where MeV excitations are screened. We find preSNe bounds on dark photon couplings up to two orders of magnitude stronger than SNe for masses between 1 and 3~MeV, with similar improvements for other electrophilic species such as axion-like particles.