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, axion-like particles (ALPs), and others. Radiative decays such as
$a\to2\gamma$ 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 $^{56}$Ni. Their low energies limit the energy
deposition from particle decays to less than about 0.1 B, where $1~{\rm
B~(bethe)}=10^{51}~{\rm erg}$. For 1-500 MeV-mass ALPs, this generic argument
excludes ALP-photon couplings $G_{a\gamma\gamma}$ in the
$10^{-10}$-$10^{-8}~{\rm GeV}^{-1}$ range.
