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arXiv · 1906.08794

Fast Neutrino Flavor Conversion: Collective Motion vs. Decoherence

Francesco Capozzi·Georg Raffelt·Tobias Stirner
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Abstract

In an interacting neutrino gas, flavor coherence becomes dynamical and can propagate as a collective mode. In particular, tachyonic instabilities can appear, leading to "fast flavor conversion" that is independent of neutrino masses and mixing angles. On the other hand, without neutrino-neutrino interaction, a prepared wave packet of flavor coherence simply dissipates by kinematical decoherence of infinitely many non-collective modes. We reexamine the dispersion relation for fast flavor modes and show that for any wavenumber,there exists a continuum of non-collective modes besides a few discrete collective ones. So for any initial wave packet, both decoherence and collective motion occurs, although the latter typically dominates for a sufficiently dense gas. We derive explicit eigenfunctions for both collective and non-collective modes. If the angular mode distribution of electron-lepton number crosses between positive and negative values, two non-collective modes can merge to become a tachyonic collective mode. We explicitly calculate the interaction strength for this critical point. As a corollary we find that a single crossing always leads to a tachyonic instability. For an even number of crossings, no instability needs to occur.

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Francesco Capozzi, Georg Raffelt, Tobias Stirner. 2019-09-03. Fast Neutrino Flavor Conversion: Collective Motion vs. Decoherence. https://doi.org/10.1088/1475-7516%2F2019%2F09%2F002

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