# Neutrino Flavor Evolution in Neutron Star Mergers

## TL;DR

This paper investigates how neutrino flavor changes occur in neutron star merger remnants, revealing that initial neutrino conditions significantly influence the dominant flavor transformation mechanisms, with potential implications for nucleosynthesis.

## Contribution

It provides the first detailed analysis of neutrino flavor evolution in neutron star merger disks, highlighting the dependence on initial conditions and identifying different flavor transformation regimes.

## Key findings

- Matter-Neutrino Resonance (MNR) dominates in antineutrino-rich conditions.
- Bipolar spectral swap occurs in neutrino-rich conditions.
- High-energy electron neutrino tail may influence r-process nucleosynthesis.

## Abstract

We examine the flavor evolution of neutrinos emitted from the disk-like remnant (hereafter called \lq\lq neutrino disk\rq\rq) of a binary neutron star (BNS) merger. We specifically follow the neutrinos emitted from the center of the disk, along the polar axis perpendicular to the equatorial plane. We carried out two-flavor simulations using a variety of different possible initial neutrino luminosities and energy spectra, and for comparison, three-flavor simulations in specific cases. In all simulations, the normal neutrino mass hierarchy was used. The flavor evolution was found to be highly dependent on the initial neutrino luminosities and energy spectra; in particular, we found two broad classes of results depending on the sign of the initial net electron neutrino lepton number (i.e., the number of neutrinos minus the number of antineutrinos). In the antineutrino dominated case, we found that the Matter-Neutrino Resonance (MNR) effect dominates, consistent with previous results, whereas in the neutrino dominated case, a bipolar spectral swap develops. The neutrino dominated conditions required for this latter result have been realized, e.g, in a BNS merger simulation that employs the \lq\lq DD2\rq\rq\ equation of state for neutron star matter[Phys. Rev. D 93, 044019 (2016)]. For this case, in addition to the swap at low energies, a collective Mikheyev-Smirnov-Wolfenstein (MSW) mechanism generates a high-energy electron neutrino tail. The enhanced population of high-energy electron neutrinos in this scenario could have implications for the prospects of $r$-process nucleosynthesis in the material ejected outside the plane of the neutrino disk.