Medium modification of the charged current neutrino opacity and its implications

TL;DR

This paper investigates how including nucleon potential energies and collisional broadening in neutrino interaction models significantly impacts neutrino energies and the neutron richness of the neutrino-driven wind in proto-neutron stars, affecting nucleosynthesis.

Contribution

It demonstrates that mean field effects and collisional broadening alter neutrino decoupling and energies, challenging previous assumptions of proton-rich winds and enabling neutron-rich conditions.

Findings

Electron neutrinos decouple at lower temperatures with lower energies.

Neutrino-driven wind can be neutron rich for extended periods.

Nuclear symmetry energy influences electron fraction in the wind.

Abstract

Previous work on neutrino emission from proto-neutron stars which employed full solutions of the Boltzmann equation showed that the average energies of emitted electron neutrinos and antineutrinos are closer to one another than predicted by older, more approximate work. This in turn implied that the neutrino driven wind is proton rich during its entire life, precluding $r$-process nucleosynthesis and the synthesis of Sr, Y, and Zr. This work relied on charged current neutrino interaction rates that are appropriate for a free nucleon gas. Here, it is shown in detail that the inclusion of the nucleon potential energies and collisional broadening of the response significantly alters this conclusion. Iso-vector interactions, which give rise to the nuclear symmetry energy, produce a difference between the neutron and proton single-particle energies $\Delta U=U_n-U_p$ and alter the kinematics of the charged current reactions. In neutron-rich matter, and for a given neutrino/antineutrino energy, the rate for $\nu_e+n\rightarrow e^-+p$ is enhanced while $ \bar{\nu}_e+p\rightarrow n+e^+$ is suppressed because the $Q$ value for these reactions is altered by $\pm\Delta U$, respectively. In the neutrino decoupling region, collisional broadening acts to enhance both $\nu_e$ and $\bar{\nu}_e$ cross-sections and RPA corrections decrease the $\nu_e$ cross-section and increase the $\bar \nu_e$ cross-section, but mean field shifts have a larger effect. Therefore, electron neutrinos decouple at lower temperature than when the nucleons are assumed to be free and have lower average energies. The change is large enough to allow for a reasonable period of time when the neutrino driven wind is predicted to be neutron rich. It is also shown that the electron fraction in the wind is influenced by the nuclear symmetry energy.