Angular instabilities of a homogeneous neutrino gas from (pseudo)scalar nonstandard self-interactions

TL;DR

This paper investigates how (pseudo)scalar nonstandard interactions can induce angular flavor instabilities in a homogeneous neutrino gas, revealing complex late-time behaviors and potential observational signatures in supernova neutrinos.

Contribution

It demonstrates the existence of angular flavor instabilities caused by (pseudo)scalar interactions and analyzes their properties through linear stability and numerical simulations.

Findings

Identifies a neutrino-antineutrino instability triggered by (pseudo)scalar interactions.

Discovers three distinct late-time phases of neutrino behavior, including a chaotic, thermalized state.

Suggests supernova observations could constrain nonstandard neutrino interactions.

Abstract

We demonstrate that in the presence of (pseudo)scalar four-fermion nonstandard interactions, a homogeneous and isotropic gas of Majorana neutrinos placed in a strong magnetic field can exhibit an angular flavor instability mixing the two neutrino helicities (i.e., a `neutrino-antineutrino' instability). This instability is most pronounced for the inverted mass hierarchy and high neutrino number densities; at the same time, even a tiny transition magnetic moment of the order of $10^{-24}\mu_{\text{B}}$ is quite sufficient to trigger it. We study the properties of the neutrino-antineutrino mode using the linear stability analysis and then perform a numerical simulation to analyze the late-time properties of an oscillating neutrino gas with V-A and (pseudo)scalar interactions. The latter analysis reveals three phases of a chaotic, `thermalized' late-time state of neutrinos: along with a low-density phase with the SO(3) symmetry broken by V-A interactions and a symmetric high-density phase, another, (pseudo)scalar-induced phase with a broken rotational symmetry appears. The phases can be distinguished by the average late-time probabilities of (anti)neutrino flavors and/or by the neutrino-to-antineutrino ratio, the latter being a key quantity for the (pseudo)scalar phase. We thus show that observations of these quantities from a supernova could place new constraints on the (pseudo)scalar nonstandard neutrino interactions.