Quasi-linear theory of fast flavor instabilities in homogeneous environments

TL;DR

This paper develops a quasi-linear theory for fast flavor instabilities in dense neutrino plasmas, enabling analysis of nonlinear effects and backreaction, validated against quantum kinetic simulations.

Contribution

It introduces the first quasi-linear framework for neutrino flavor instabilities, incorporating backreaction and nonresonant interactions, bridging plasma physics and neutrino physics.

Findings

Quasi-linear theory closely matches quantum kinetic simulations in homogeneous models.

Supports using QLT to analyze nonlinear flavor evolution without full simulations.

Validates QLT as a practical tool for studying neutrino flavor instabilities.

Abstract

Dense neutrino plasmas can develop instabilities that drive collisionless flavor exchange, equivalent to the emission of flavomons, the quanta of flavor waves. We treat these waves, for the first time, as independent linear degrees of freedom and develop a quasi-linear theory (QLT), including backreaction on the neutrino distribution and nonresonant neutrino--flavomon interactions, while neglecting wave--wave processes. In a homogeneous, axisymmetric model, the saturated neutrino and flavomon distributions agree closely with periodic-box solutions of the original quantum kinetic equation. These results support the use of QLT, well established in plasma physics, to bypass nonlinear small-scale effects that challenge direct simulations.