Supernova constraint on self-interacting dark sector particles

We examine the constraints on sub-GeV dark sector particles set by the proto-neutron star cooling associated with the core-collapse supernova event SN1987a. Considering explicitly a dark photon portal dark sector model, we compute the relevant interaction rates of dark photon (A') and dark fermion (chi) with the Standard Model particles as well as their self-interaction inside the dark sector. We find that even with a small dark sector fine structure constant alpha(D) << 1, dark sector self-interactions can easily lead to their own self-trapping. This effect strongly limits the energy luminosity carried away by dark sector particles from the supernova core and thus drastically affects the parameter space that can be constrained by SN1987a. We consider specifically two mass ratios m(A') = 3m(chi) and 3m(A') = m(chi) which represent scenarios where the decay of A' to chi(chi) over bar is allowed or not. For m(A') = 3m(chi), we show that this effect can completely evade the supernova bounds on widely examined dark photon parameter space for a dark sector with alpha(D) greater than or similar to 10(-7). In particular, for the mass range m(chi) less than or similar to 20 MeV, supernova bounds can only be applied to weakly self-interacting dark sector with alpha(D) less than or similar to 10(-15). For 3m(A') = m(chi), bounds in regions where alpha(D) greater than or similar to 10(-7) for m(chi) less than or similar to 20 MeV can be evaded similarly. Our findings thus imply that the existing supernova bounds on light dark particles can be generally eluded by a similar self-trapping mechanism. This also implies that nonstandard strongly self-interacting neutrino is not consistent with the SN1987a observation. Same effects can also take place for other known stellar bounds on dark sector particles.