Stellar cooling limits on KK gravitons and dark dimensions

A bstract We revisit cooling bounds on light Kaluza-Klein (KK) gravitons, as arise in the dark dimension scenario, considering globular clusters, neutron stars, and supernovae. In addition to bremsstrahlung, we account for two novel production channels: resonant mixing with the in-medium photon and a pion-induced process in supernovae. The strongest limits arise from SN 1987A, with the emissivity from the pion process exceeding that from bremsstrahlung by a factor of a few albeit with substantial uncertainties, while resonant production is heavily suppressed. We obtain a bound on the KK mass scale of m KK ≳ 0.6 eV (≳ 500 eV) for 2 (3) extra dimensions, which, having accounted for these previously neglected processes, is broadly compatible with existing analyses. Improved understanding of the properties of pions in supernovae could strengthen these limits to roughly eV (keV). For 1 extra dimension, the bounds are weaker than those from laboratory searches. We also show that constraints from KK graviton decays to Standard Model particles are less stringent than the cooling bounds if there is KK number violation at the level typically assumed in the dark dimension scenario, although these bounds could be strengthened by future observations.