Plasmon excitations in homogeneous neutron star matter

TL;DR

This paper investigates collective plasma excitations in neutron-star matter, revealing how Coulomb interactions influence various modes, including a high-energy plasmon with mixed electron-muon character, affecting neutron-star thermodynamics.

Contribution

It provides a detailed analysis of plasma modes in npeμ matter using RPA, highlighting the impact of Coulomb interactions and screening effects on excitation spectra in neutron-star conditions.

Findings

The plasmon mode is high-energy, mainly electron with muon contribution, and undamped at low momentum.

Coulomb interactions create multiple excitation branches with complex spectral structures.

Proton plasma modes are suppressed and transformed into sound-like modes due to screening effects.

Abstract

We study the possible collective plasma modes which can affect neutron-star thermodynamics and different elementary processes in the baryonic density range between nuclear saturation ($\rho_0$) and $3\rho_0$. In this region, the expected constituents of neutron-star matter are mainly neutrons, protons, electrons and muons ($npe\mu$ matter), under the constraint of beta equilibrium. The elementary plasma excitations of the $pe\mu$ three-fluid medium are studied in the RPA framework. We emphasize the relevance of the Coulomb interaction among the three species, in particular the interplay of the electron and muon screening in suppressing the possible proton plasma mode, which is converted into a sound-like mode. The Coulomb interaction alone is able to produce a variety of excitation branches and the full spectral function shows a rich structure at different energy. The genuine plasmon mode is pushed at high energy and it contains mainly an electron component with a substantial muon component, which increases with density. The plasmon is undamped for not too large momentum and is expected to be hardly affected by the nuclear interaction. All the other branches, which fall below the plasmon, are damped or over-damped.