Fast neutrino flavor instability and neutrino flavor lepton number crossings

Abstract

In this paper, we show the equivalency between the existence of fast neutrino flavor instability and that of neutrino flavor lepton number (NFLN) crossings, which indicates that an NFLN angular distribution takes both signs. The veracity of this proposition has been uncertain and sometimes controversial despite its indispensability in the flavor evolutions of dense neutrinos. This study clarifies that the occurrence of an NFLN crossing is both necessary and sufficient for fast instability.