  Charged-current neutrino processes such as $\nu_e + n \rightleftharpoons p +
e^-$ and $\bar\nu_e + p \rightleftharpoons n + e^+$ destroy the flavor
coherence among the weak-interaction states of a single neutrino and thus damp
its flavor oscillation. 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, however, these "collision" processes can trigger large flavor
conversion in cooperation with the strong neutrino-neutrino refraction. We show
that there exist two types of collisional flavor instability in a homogeneous
and isotropic neutrino gas which are identified by the dependence of their real
frequencies on the neutrino density $n_\nu$. 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. In the
transition region, the flavor instability grows exponentially at a rate
$\propto n_\nu^{1/2}$. We find 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 may be produced through flavor conversion, which
can potentially have important ramifications in the subsequent evolution of the
remnant.

High Energy Physics - Phenomenology;High Energy Astrophysical Phenomena;Nuclear Theory