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Theory of neutrino fast flavor evolution. Part II. Solutions at the edge of instability

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arxiv 2409.17232 v3 pith:4G6MCWU7 submitted 2024-09-25 hep-ph astro-ph.HE

classification hep-phastro-ph.HE
keywords unstabledistributionsexpressionsgrowthmodesneutrinoconversionsexplicit
verification ladder T0 review T1 audit T2 compute T3 formal
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In dense neutrino environments, such as provided by core-collapse supernovae or neutron-star mergers, neutrino angular distributions may be unstable to collective flavor conversions, whose outcome remains to be fully understood. These conversions are much faster than hydrodynamical scales, suggesting that self-consistent configurations may never be strongly unstable. With this motivation in mind, we study weakly unstable modes, i.e., those with small growth rates. We show that our newly developed dispersion relation (Paper~I of this series) allows for an expansion in powers of the small growth rate. For weakly unstable distributions, we show that the unstable modes must either move with subluminal phase velocity, or very close to the speed of light. The instability is fed from neutrinos moving resonantly with the waves, allowing us to derive explicit expressions for the growth rate. For axisymmetric distributions, often assumed in the literature, numerical examples show the accuracy of these expressions. We also note that for the often-studied one-dimensional systems one should not forget the axial-symmetry-breaking modes, and we provide explicit expressions for the range of wavenumbers that exhibit instabilities.

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Cited by 6 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Flavomons in Matter Gradients: Ray Tracing and Amplitude Evolution

    hep-ph 2026-04 unverdicted novelty 7.0 of 10

    Matter gradients slow but do not suppress neutrino-mass-induced flavor instabilities, so flavomon ray tracing is required instead of local stability analysis alone.

  2. Predicting the outcome of collisional neutrino flavor conversion

    hep-ph 2025-05 conditional novelty 7.0 of 10

    Collisional neutrino flavor instabilities settle into a state at the edge of instability with nonzero flavor coherence, and explicit formulas predict this final state.

  3. Solar-System Abundances of $p$-Nuclides Probe Collective Neutrino Oscillations in Supernovae

    hep-ph 2026-07 conditional novelty 6.0 of 10

    Nearby neutrino flavor conversion in a supernova can boost νp-process yields of p-nuclides like 92Mo and 92Nb by up to two orders of magnitude, matching solar abundances when conversion starts within ~10 km of the pro...

  4. Single-wave solutions of the neutrino fast flavor system. Part II. Weak instabilities and their resonant behavior

    hep-ph 2026-01 conditional novelty 6.0 of 10

    For shallow angular crossings, the nonlinear evolution of a single-wave fast flavor instability is a flavor pendulum whose amplitude and period are set by the linear growth rate.

  5. Single-wave solutions of the neutrino fast flavor system. Part I. Mechanical properties

    hep-ph 2026-01 conditional novelty 6.0 of 10

    Single-wave neutrino flavor solutions form a non-integrable spin system without Gaudin invariants, so an exact flavor pendulum exists only for two beams and does not extend to continuous angle distributions.

  6. Comparative Testing of Subgrid Models for Fast Neutrino Flavor Conversions in Core-collapse Supernova Simulations

    astro-ph.HE 2025-06 conditional novelty 6.0 of 10

    A 1D supernova simulation with four-species BGK subgrid modeling shows that three-species assumptions overestimate flavor conversion and that semi-implicit time integration is the most reliable.

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