# Supernova 1987A Constraints on Sub-GeV Dark Sectors, Millicharged Particles, the QCD Axion, and an Axion-like Particle

## TL;DR

This paper uses Supernova 1987A data to set new constraints on various sub-GeV dark sector particles, including dark photons, millicharged particles, and axions, significantly improving previous bounds and exploring untested parameter spaces.

## Contribution

It introduces a new diagnostic for fermions with large mixing angles and updates bounds on multiple dark sector particles, extending the exclusion regions beyond prior work.

## Key findings

- Constraints on dark photon couplings and masses below 200 MeV.
- Exclusion of millicharged particles with charges between 10^(-9) and a few times 10^(-6).
- Ruling out hadronic axions with masses between 0.1 and a few hundred eV.

## Abstract

We consider the constraints from Supernova 1987A on particles with small couplings to the Standard Model. We discuss a model with a fermion coupled to a dark photon, with various mass relations in the dark sector; millicharged particles; dark-sector fermions with inelastic transitions; the hadronic QCD axion; and an axion-like particle that couples to Standard Model fermions with couplings proportional to their mass. In the fermion cases, we develop a new diagnostic for assessing when such a particle is trapped at large mixing angles. Our bounds for a fermion coupled to a dark photon constrain small couplings and masses <200 MeV, and do not decouple for low fermion masses. They exclude parameter space that is otherwise unconstrained by existing accelerator-based and direct-detection searches. In addition, our bounds are complementary to proposed laboratory searches for sub-GeV dark matter, and do not constrain several "thermal" benchmark-model targets. For a millicharged particle, we exclude charges between 10^(-9) to a few times 10^(-6) in units of the electron charge; this excludes parameter space to higher millicharges and masses than previous bounds. For the QCD axion and an axion-like particle, we apply several updated nuclear physics calculations and include the energy dependence of the optical depth to accurately account for energy loss at large couplings. We rule out a hadronic axion of mass between 0.1 and a few hundred eV, or equivalently bound the PQ scale between a few times 10^4 and 10^8 GeV, closing the hadronic axion window. For an axion-like particle, our bounds disfavor decay constants between a few times 10^5 GeV up to a few times 10^8 GeV. In all cases, our bounds differ from previous work by more than an order of magnitude across the entire parameter space. We also provide estimated systematic errors due to the uncertainties of the progenitor.