Pseudo Nambu-Goldstone modes in neutron stars

TL;DR

This paper investigates Nambu-Goldstone modes in neutron stars using a high-density quark model, revealing that strong coupling effects increase their mass and prevent certain condensations, impacting gravitational wave signals.

Contribution

It introduces a schematic quark model constrained by neutron star observations to study NG modes and demonstrates the significant impact of strong coupling on their properties.

Findings

NG modes are more massive at strong coupling

Charged kaons do not condense at high density

Implications for gravitational wave signals from neutron star mergers

Abstract

If quarks and gluons are either gapped or confined in neutron stars (NSs), the most relevant light modes are Nambu-Goldstone (NG) modes. We study NG modes within a schematic quark model whose parameters at high density are constrained by the two-solar mass constraint. Our model has the color-flavor-locked phase at high density, with the effective couplings as strong as in hadron physics. We find that strong coupling effects make NG modes more massive than in weak coupling predictions, and would erase several phenomena caused by the stressed pairings in mismatched Fermi surfaces. For instance, we found that charged kaons, which are dominated by diquark and anti-diquark components, are not light enough to condense at strong coupling. Implications for gravitational wave signals for NS-NS mergers are also briefly discussed.