Collective neutrino flavor conversion: Recent developments

TL;DR

This paper reviews recent advances in understanding collective neutrino flavor conversion, highlighting new phenomena like spontaneous symmetry breaking and fast flavor conversions driven by neutrino-neutrino interactions in astrophysical environments.

Contribution

It summarizes recent theoretical developments and identifies novel phenomena in collective neutrino oscillations, including symmetry breaking and rapid flavor conversions, using simple models.

Findings

Self-induced flavor conversion can occur without neutrino masses.

Unstable modes grow at energies comparable to neutrino-neutrino interactions.

Spontaneous symmetry breaking influences flavor evolution in dense neutrino gases.

Abstract

Neutrino flavor evolution in core-collapse supernovae, neutron-star mergers, or the early universe is dominated by neutrino-neutrino refraction, often spawning "self-induced flavor conversion", i.e., shuffling of flavor among momentum modes. This effect is driven by collective run-away modes of the coupled "flavor oscillators" and can spontaneously break the initial symmetries such as axial symmetry, homogeneity, isotropy, and even stationarity. Moreover, the growth rates of unstable modes can be of the order of the neutrino-neutrino interaction energy instead of the much smaller vacuum oscillation frequency: self-induced flavor conversion does not always require neutrino masses. We illustrate these newly found phenomena in terms of simple toy models. What happens in realistic astrophysical settings is up to speculation at present.