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.
Highlights
Neutrino dispersion in matter strongly modifies the flavor evolution caused by their masses and mixing parameters [1,2,3]
Axial symmetry breaking of supernova neutrinos leads to a dipole variation in the transverse plane, which on the crudest level of approximation can be seen as two azimuth bins, corresponding to a beam of left-moving and a beam of right-moving neutrinos
The topic of collective neutrino flavor conversion has undergone a shift of paradigm over the past couple of years
Summary
Neutrino dispersion in matter strongly modifies the flavor evolution caused by their masses and mixing parameters [1,2,3]. Self-induced flavor conversion can be “fast” in the s√ense that the evolution speed is of the order of the neutrino-neutrino interaction energy μ = 2GFnν instead of the much smaller vacuum oscillation frequency ω = ∆m2/2E [9, 32, 33]. While this phenomenon had been noted a long time ago [9], its significance had eluded much of the community.
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