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Quasilinear Theory of Fast Flavor Instabilities

Damiano F. G. Fiorillo1,2 and Georg G. Raffelt3

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Phys. Rev. Lett. 137, 171001 – Published 21 October, 2026

DOI: https://doi.org/10.1103/f1dk-yk6g

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 as independent linear degrees of freedom and develop a quasilinear theory, including backreaction on the neutrino distribution and nonresonant neutrino-flavomon interactions, while neglecting wave-wave processes. In a statistically homogeneous, axisymmetric model, the saturated neutrino and flavomon distributions agree closely with periodic-box solutions of the original quantum kinetic equation. Our results provide the first theoretical interpretation for the numerical observations. They also show that flavomon phases have little influence on the final state, supporting the quasilinear theory as a framework for capturing flavor evolution without resolving nonlinear small-scale dynamics.

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Outline

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Phys. Rev. Lett. 137, 171001– Published 21 October, 2026
  • Received 9 April 2026
  • Revised 14 August 2026
  • Accepted 24 September 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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