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 β 2Ξ³ 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 B (bethe) = 10 51 erg. For 1-500 MeV-mass ALPs, this generic argument excludes ALP-photon couplings GaΞ³Ξ³ in the 10 β10 -10 β8 GeV β1 range.