Neutrino Mass Hierarchy and Stepwise Spectral Swapping of Supernova Neutrino Flavors

TL;DR

This paper investigates the spectral swapping phenomenon in supernova neutrinos, showing how the energy threshold depends on the neutrino mass hierarchy and mixing angles, with implications for determining the hierarchy from supernova signals.

Contribution

The study provides large-scale numerical simulations revealing the dependence of spectral swap energy on neutrino mass hierarchy and mixing angles, aiding hierarchy determination from supernova neutrino observations.

Findings

In normal hierarchy, the swap energy decreases with smaller mixing angles.

In inverted hierarchy, the swap energy is independent of the mixing angle.

Results can help determine neutrino mass hierarchy from future supernova signals.

Abstract

We examine a phenomenon recently predicted by numerical simulations of supernova neutrino flavor evolution: the swapping of supernova $\nu_e$ and $\nu_{\mu,\tau}$ energy spectra below (above) energy $\EC$ for the normal (inverted) neutrino mass hierarchy. We present the results of large-scale numerical calculations which show that in the normal neutrino mass hierarchy case, $\EC$ decreases as the assumed $\nu_e\rightleftharpoons\nu_{\mu,\tau}$ effective $2\times 2$ vacuum mixing angle ($\simeq \theta_{1 3}$) is decreased. However, these calculations also indicate that $\EC$ is essentially independent of the vacuum mixing angle in the inverted neutrino mass hierarchy case. With a good neutrino signal from a future Galactic supernova, the above results could be used to determine the neutrino mass hierarchy even if $\theta_{13}$ is too small to be detected in terrestrial neutrino oscillation experiments.