Collisions and collective flavor conversion: Integrating out the fast dynamics

TL;DR

This paper develops a simplified framework to analyze how collisions influence collective neutrino flavor conversions in dense astrophysical environments by focusing on slow dynamics and deriving new equations.

Contribution

It introduces a novel approach that integrates out fast scales to focus on slow collision-driven flavor evolution, simplifying the analysis of complex neutrino interactions.

Findings

New slow-dynamics equations for flavor conversion

Qualitative understanding of final states after instabilities

Application to both collisional and fast instabilities

Abstract

In dense astrophysical environments, notably core-collapse supernovae and neutron star mergers, neutrino-neutrino forward scattering can spawn flavor conversion on very short scales. Scattering with the background medium can impact collective flavor conversion in various ways, either damping oscillations or possibly setting off novel collisional flavor instabilities (CFIs). A key feature in this process is the slowness of collisions compared to the much faster dynamics of neutrino-neutrino refraction. Assuming spatial homogeneity, we leverage this hierarchy of scales to simplify the description accounting only for the slow dynamics driven by collisions. We illustrate our new approach both in the case of CFIs and in the case of fast instabilities damped by collisions. In both cases, our strategy provides new equations, the slow-dynamics equations, that simplify the description of flavor conversion and allow us to qualitatively understand the final state of the system after the instability, either collisional or fast, has saturated.