Low-Energy Supernova Constraints on Millicharged Particles

The hot and dense conditions of the supernova core provide an ideal environment for the production of new feebly-interacting particles. Low-energy supernovae, characterized by low explosion energy, are particularly intriguing due to their stringent constraints on energy transfer from the core to the mantle by new particles. We investigate low-energy supernova constraints on millicharged particles by considering three production channels in the core: plasmon decay, proton bremsstrahlung, and electron-positron annihilation. We compute the energy deposition due to Coulomb scatterings of millicharged particles with protons in the mantle and find that low-energy supernovae impose the most stringent constraints on millicharged particles in the mass range of $\sim(12 - 170)$ MeV. Furthermore, we find that the electron-positron annihilation process, previously omitted in supernova studies on millicharged particles, is the dominant production channel in the high-mass region. This leads to new constraints from both supernova cooling calculations and low-energy supernova analyses. We also investigate MCP production via processes involving thermal pions and find that these processes could dominate over electron-positron annihilation, albeit with significant uncertainties.