Constraints on Light Hidden Sector Gauge Bosons from Supernova Cooling

We derive new bounds on hidden sector gauge bosons which could produce new energy loss mechanisms in supernovae, enlarging the excluded region in mass-coupling space by a significant factor compared to earlier estimates. Both considerations of trapping and possible decay of these particles need to be incorporated when determining such bounds, as does scattering on both neutrons and protons. For masses and couplings near the region which saturates current bounds, a significant background of such gauge bosons may also be produced due to the cumulative effects of all supernovae over cosmic history.