BGK subgrid model for neutrino quantum kinetics

TL;DR

This paper introduces a BGK-based subgrid model for neutrino quantum kinetics, enabling efficient incorporation of collective neutrino oscillation effects into supernova and merger simulations, with promising accuracy demonstrated against full quantum models.

Contribution

The paper develops a novel BGK relaxation-time subgrid model for neutrino quantum kinetics, compatible with classical transport schemes, and validates it against quantum simulations.

Findings

Reproduces quantum kinetic properties with ~20% error in neutrino survival probabilities.

Provides explicit equations for multi-angle and moment-based neutrino transport.

Shows the model's capability through comparison with full quantum kinetic simulations.

Abstract

We present a new subgrid model for neutrino quantum kinetics, which is primarily designed to incorporate effects of collective neutrino oscillations into neutrino-radiation-hydrodynamic simulations for core-collapse supernovae and mergers of compact objects. We approximate the neutrino oscillation term in quantum kinetic equation by Bhatnagar-Gross-Krook (BGK) relaxation-time prescription, and the transport equation is directly applicable for classical neutrino transport schemes. The BGK model is motivated by recent theoretical indications that non-linear phases of collective neutrino oscillations settle into quasi-steady structures. We explicitly provide basic equations of the BGK subgrid model for both multi-angle and moment-based neutrino transport to facilitate the implementation of the subgrid model in the existing neutrino transport schemes. We also show the capability of our BGK subgrid model by comparing to fully quantum kinetic simulations for fast neutrino-flavor conversion. We find that the overall properties can be well reproduced in the subgrid model; the error of angular-averaged survival probability of neutrinos is within $\sim 20 \%$. By identifying the source of error, we also discuss perspectives to improve the accuracy of the subgrid model.