Collisional flavor instability in dense neutrino gases*

Although the charged-current neutrino processes tend to destroy the flavor coherence among the weak-interaction states of a single neutrino and thus damp its flavor oscillation, they can lead for flavor instability in a dense neutrino gas such as that inside a core-collapse supernova or the black hole accretion disk formed in a compact binary merger. There exist two types of collisional flavor instability in a homogeneous and isotropic neutrino gas which can be identified by the dependence of their real frequencies on the neutrino density. The instability transitions from one type to the other and exhibits a resonance-like behavior in the region where the net electron lepton number of the neutrino gas is negligible. We show that the neutrino gas in the black hole accretion disk is susceptible to the collision-induced flavor conversion where the neutrino densities are the highest. As a result, large amounts of heavy-lepton flavor neutrinos can be produced through flavor conversion, which can have important ramifications in the subsequent evolution of the remnant.