Flavor Equilibration of Supernova Neutrinos: Exploring the Dynamics of Slow Modes

TL;DR

This paper investigates the dynamics of slow flavor conversion in supernova neutrinos, revealing how neutrino gases evolve toward flavor equilibrium constrained by lepton number, with implications for supernova modeling.

Contribution

It provides a systematic analysis of kinematic decoherence in dense neutrino gases undergoing slow flavor conversion, highlighting a simple ratio that describes the quasi-steady state.

Findings

Neutrino gases evolve toward flavor equilibrium constrained by lepton number.

Equilibration occurs within a few inverse vacuum oscillation frequencies.

The neutrino-antineutrino ratio $n_{ar{ u}_e}/n_{ u_e}$ characterizes the steady state.

Abstract

Neutrinos experience collective flavor conversion in extreme astrophysical environments such as core-collapse supernovae (CCSNe). One manifestation of collective conversion is slow flavor conversion (SFC), which has recently attracted renewed interest owing to its ubiquity across different regions of the supernova environment. In this study, we systematically examine the evolution of kinematic decoherence in a dense neutrino gas undergoing SFC, considering lepton number asymmetries as large as $30\%$. Our findings show that the neutrino gas asymptotically evolves toward a generic state of coarse-grained flavor equilibration which is constrained by approximate lepton number conservation. The equilibration occurs within a few factors of the inverse vacuum oscillation frequency, $\omega^{-1}$, which corresponds to (anti)neutrinos reaching near flavor equipartition after a few kilometers for typical supernova neutrino energies. Notably, the quasi-steady state of the neutrino number densities can be quantitatively described by the neutrino-antineutrino number density ratio $n_{\bar{\nu}_e}/n_{\nu_e}$ alone. Such a simple estimation opens new opportunities for incorporating SFC into CCSN simulations, particularly in regions where SFC develops on scales much shorter than those of collisions.