Low-energy supernovae severely constrain radiative particle decays

The hot and dense core formed in the collapse of a massive star is a powerful source of hypothetical feebly interacting particles such as sterile neutrinos, dark photons, axionlike particles (ALPs), and others. Radiative decays such as a -> 2 gamma deposit this energy in the surrounding material if the mean free path is less than the radius of the progenitor star. For the first time, we use a supernova (SN) population with particularly low explosion energies as the most sensitive calorimeters to constrain this possibility. These SNe are observationally identified as low-luminosity events with low ejecta velocities and low masses of ejected Ni-56. Their low energies limit the energy deposition from particle decays to less than about 0.1 B, where 1 B(bethe) = 10(51) erg. For 1-500 MeV-mass ALPs, this generic argument excludes ALP-photon couplings G(a gamma gamma) in the 10(-)(10) - 10(-8) GeV-1 range.