A Systematic Study on the Resonance in Collisional Neutrino Flavor Instability

TL;DR

This paper systematically investigates the resonance phenomena in collisional neutrino flavor instability, revealing its occurrence in both isotropic and anisotropic modes and its significant impact on the growth rate, with implications for neutrino physics.

Contribution

It provides a comprehensive analysis of resonance in CFI, including criteria, effects on nonlinear evolution, and interactions with fast flavor conversion, expanding understanding beyond previous isotropy-preserving focus.

Findings

Resonance occurs in both isotropic and anisotropic modes.

Resonance significantly enhances the growth rate of CFI.

Resonance coexists with fast flavor conversion phenomena.

Abstract

Investigations on the resonance in the collisional flavor instability (CFI) of neutrinos, which were reported recently, are reported. We show that the resonance occurs not only for the isotropy-preserving modes as pointed out in the previous work but also for the isotropy-breaking modes and that it enhances the growth rate of CFI by orders of magnitude. Employing the linear analysis and nonlinear numerical simulations in the two-flavor scheme and under the relaxation approximation for the collision term, we discuss the criterion for the resonance, its effect on the nonlinear evolution as well as the influences of homogeneity-breaking (k \ne 0) perturbations as well as of anisotropy in the background on the resonance. We will also touch the cohabitation of the resonance with the fast flavor conversion (FFC).