[Paper Review] Nonlinear evolution of fast neutrino flavor conversion in the preshock region of core-collapse supernovae

Masamichi Zaizen, Taiki Morinaga|arXiv (Cornell University)|Apr 21, 2021

Neutrino Physics ResearchPhysics and Astronomy57 references44 citations

TL;DR

This study investigates nonlinear fast neutrino flavor conversion in the preshock region of core-collapse supernovae, using a three-flavor model with realistic neutrino angular distributions. It demonstrates that tiny crossings in the neutrino flavor lepton number (NFLN) distribution—induced by coherent scattering and muon production—trigger nonlinear cascades, enabling fast flavor conversion even in high-density environments where linear theory predicts suppression.

ABSTRACT

In environments with high dense neutrino gases, such as in core-collapse supernovae, the neutrinos can experience collective neutrino oscillation due to their self-interactions. In particular, fast flavor conversion driven by the crossings in the neutrino angular distribution can affect explosion mechanism, nucleosynthesis, and neutrino observation. We perform the numerical computation of nonlinear flavor evolution on the neutrino angular distribution with tiny crossings expected to be generated in the preshock region. We demonstrate that the fast instability is triggered and a cascade develops under a realistic three-flavor model considering muon production and weak magnetism in the SN dynamics. The tiny crossing excites specific spatial modes, and then the flavor instability propagates into other modes which otherwise remain stable due to the nonlinear effects. Our results indicate that fast flavor conversion can rise in the preshock region and have a sufficient impact on the flavor contents.

Motivation & Objective

• To investigate whether fast neutrino flavor conversion can occur in the preshock region of core-collapse supernovae despite high matter density.
• To examine the role of nonlinear effects in amplifying instability from tiny NFLN crossings.
• To assess the impact of three-flavor dynamics, including muon and tauon contributions, on flavor evolution.
• To determine whether fast flavor conversion can significantly alter neutrino flavor content in regions previously considered stable.

Proposed method

• Numerical solution of the neutrino kinetic equation in Fourier space, decomposing polarization vectors into spatial modes K along the radial direction.
• Use of a three-flavor framework with monochromatic energy spectra and normal mass ordering.
• Incorporation of vacuum, matter, and self-interaction Hamiltonians via the EoM for density matrices.
• Application of a dealiasing approach with K modes up to nK = 300 and initial perturbations of O(10−12).
• Inclusion of muon and tauon lepton number contributions to model realistic SN dynamics.
• Linear stability analysis to identify initial growth conditions, followed by nonlinear evolution to study cascade development.

Experimental results

Research questions

• RQ1 Can fast flavor conversion be triggered by tiny NFLN crossings in the preshock region under realistic three-flavor dynamics?
• RQ2 How do nonlinear effects propagate instability to modes that are stable under linear analysis?
• RQ3 What is the role of muon and tauon production in modifying the flavor instability landscape?
• RQ4 Does the inclusion of vacuum oscillation terms alter the growth dynamics compared to the pure self-interaction case?
• RQ5 Can nonlinear cascades lead to significant flavor content changes in the preshock region?

Key findings

• Fast flavor conversion is triggered by tiny NFLN crossings in the preshock region, even under high matter density where linear theory predicts suppression.
• Nonlinear effects cause a cascade in Fourier space, enabling instability to grow in modes that are stable under linear analysis.
• The e−τ sector shows the earliest and strongest instability growth, followed by coupling to e−µ and µ−τ sectors.
• Inclusion of vacuum terms leads to broader instability growth across all flavor sectors, unlike the isolated e−τ dominance in the no-vacuum case.
• The presence of muon production and weak magnetism in SN dynamics enables the development of nonlinear cascades from small initial crossings.
• The results indicate that fast flavor conversion can significantly alter neutrino flavor content in the preshock region, with implications for explosion mechanisms and nucleosynthesis.