Spectral diversity in collisional neutrino-flavor conversion: flavor equipartition or swap

TL;DR

This paper explores the nonlinear dynamics of collisional neutrino-flavor conversion in astrophysical environments, revealing how different unstable modes lead to distinct spectral behaviors like flavor equipartition or swap.

Contribution

It provides a detailed numerical analysis of collisional neutrino-flavor conversion, highlighting the dependence of asymptotic states on unstable modes and spectral behaviors.

Findings

High-energy neutrinos reach flavor equipartition in one mode.

Low-energy neutrinos undergo flavor swap in another mode.

Spectral behaviors depend on stability analysis and flavor pendulum.

Abstract

Quantum kinetics of neutrinos are known to potentially change the classical neutrino radiation field in high-energy astrophysical sources such as core-collapse supernovae and binary neutron-star mergers. However, the mixing phenomena still have open issues in the nonlinear dynamics and the asymptotic states, particularly for recently discovered collision-induced flavor conversion. In this paper, we investigate linear and nonlinear dynamics of collisional neutrino-flavor conversion (CFC) with multi-energy neutrino gases through numerical simulations, demonstrating that the asymptotic states dramatically change depending on unstable modes dominating the system. In one unstable mode, high-energy neutrinos reach a flavor equipartition, but low-energy neutrinos return back to almost their initial states. In contrast, in the other one, rather low-energy neutrinos achieve a full flavor swap, but high-energy neutrinos undergo less flavor conversion. We clarify the distinct spectral behaviors in two different ways based on stability analysis and flavor pendulum. Our result suggests that CFC with flavor swap can become crucial at deeper radii with low electron fraction and requires more detailed theoretical modeling of neutrino quantum kinetics.