Enhancement or damping of fast neutrino flavor conversions due to collisions

Abstract: Fast neutrino flavor conversion can occur in core-collapse supernovae or compact binary merger remnants when non-forward collisions are also at play, and neutrinos are not fully decoupled from matter. This work aims to shed light on the conditions under which fast flavor conversion is enhanced or suppressed by collisions. By relying on a neutrino toy model with three angular bins in the absence of spatial inhomogeneities, we consider two angular configurations: The first one with angular distributions of $\nu_e$ and $\bar\nu_e$ that are almost isotropic as expected before complete neutrino decoupling and showing little flavor conversion when collisions are absent. The second one with angular distributions of $\nu_e$ and $\bar\nu_e$ that are forward peaked as expected in the free-streaming regime and showing significant flavor conversion in the absence of collisions. By including angle-independent, direction-changing collisions, we find that collisions are responsible for an overall enhancement (damping) of flavor conversion in the former (latter) angular configuration. These opposite outcomes are due to the non-trivial interplay between collisions, flavor conversion, and the initial angular distributions of the electron type neutrinos. The enhancement in neutrino flavor conversion is found to be anticorrelated with the magnitude of flavor conversions in the absence of collisions.