Fast neutrino flavor conversion: roles of dense matter and spectrum crossing

TL;DR

This paper investigates how dense matter and spectrum crossings influence rapid neutrino flavor conversions in astrophysical environments, revealing conditions under which fast conversions occur, especially in supernovae.

Contribution

It provides analytical models clarifying the roles of dense matter and ELN spectrum crossing in enabling fast neutrino flavor conversions.

Findings

Dense matter can induce fast flavor conversions in supernovae.

ELN spectrum crossing is crucial for fast conversions.

Fast conversions occur on scales much shorter than traditional oscillation times.

Abstract

The flavor conversion of a neutrino usually occurs at densities $\lesssim G_F^{-1} \omega$, whether in the ordinary matter or the neutrino medium, and on time/distance scales of order $\omega^{-1}$, where $G_F$ is the Fermi weak coupling constant and $\omega$ is the vacuum oscillation frequency of the neutrino. In 2005 Sawyer and more recently both he and other groups have shown that neutrino flavor conversions can occur on scales much shorter than $\omega^{-1}$ in a very dense, anisotropic neutrino gas such as that in a core-collapse supernova or a binary neutron star merger. It has also been suggested that the fast neutrino flavor conversion require a crossing in the electron lepton number (ELN) angular distribution or spectrum. We study a few simple analytical models with which we elucidate the roles of the dense matter and the ELN spectrum crossing in neutrino fast flavor conversions. We show that a large matter density can induce fast neutrino flavor conversions in certain astrophysical scenarios such as at the early epoch of a core-collapse supernova.