Fast oscillations, collisionless relaxation, and spurious evolution of supernova neutrino flavor

TL;DR

This paper investigates how fast neutrino flavor conversions in supernovae accelerate flavor relaxation and cause computational challenges due to cascade effects, impacting the accuracy of oscillation simulations.

Contribution

It reveals the cascade process in neutrino flavor fields, highlighting its role in relaxation and the computational issues it causes, and suggests new methods to prevent spurious evolution.

Findings

FFC enhances flavor relaxation via cascade effects.

Small-scale power accumulation leads to errors in simulations.

Addressing cascade-related errors is crucial for realistic modeling.

Abstract

Mounting evidence indicates that neutrinos likely undergo fast flavor conversion (FFC) in at least some core-collapse supernovae. Outcomes of FFC, however, remain highly uncertain. Here we study the cascade of flavor-field power from large angular scales in momentum space down to small ones, showing that FFC enhances this process and thereby hastens relaxation. Cascade also poses a computational challenge, which is present even if the flavor field is stable: When power reaches the smallest angular scale of the calculation, error from truncating the angular-moment expansion propagates back to larger scales, to disastrous effect on the overall evolution. Essentially the same issue has prompted extensive work in the context of plasma kinetics. This link suggests new approaches to averting spurious evolution, a problem that presently puts severe limitations on the feasibility of realistic oscillation calculations.