Suppression of fast neutrino flavor conversions occurring at large distances in core-collapse supernovae

Abstract

Neutrinos propagating in dense neutrino mediasuch as core-collapse supernovaeand neutron star merger remnantscan experience the so-called fast flavorconversions on scalesmuch shorter than those expected in vacuum. A very generic class of fast flavor instabilities is the ones which are produced by the backward scatteringof neutrinos off the nuclei at relatively large distances from the supernova core.In this study we demonstrate that despite their ubiquity, such fast instabilities are unlikelyto cause significant flavor conversionsif the population of neutrinos in the backward direction is not large enough.Indeed, the scattering-induced instabilities can mostly impact theneutrinos traveling in the backward direction, which represent only a small fraction of neutrinos at large radii.We show that this can be explained by the shapeof the unstable flavor eigenstates, which can be extremely peaked at the backward angles.