Bulk viscosity in kaon-condensed color-flavor locked quark matter

TL;DR

This paper calculates the bulk viscosity of kaon-condensed color-flavor locked quark matter, revealing it varies as T^7 and is smaller than in uncondensed CFL phase, impacting neutron star r-mode damping.

Contribution

It provides the first detailed computation of bulk viscosity in the CFL-K^0 phase, highlighting its temperature dependence and comparison with other phases.

Findings

Bulk viscosity scales as T^7 in CFL-K^0 phase.

Bulk viscosity is smaller than in uncondensed CFL phase.

Implications for r-mode damping in compact stars.

Abstract

Color-flavor locked (CFL) quark matter at high densities is a color superconductor, which spontaneously breaks baryon number and chiral symmetry. Its low-energy thermodynamic and transport properties are therefore dominated by the H (superfluid) boson, and the octet of pseudoscalar pseudo-Goldstone bosons of which the neutral kaon is the lightest. We study the CFL-K^0 phase, in which the stress induced by the strange quark mass causes the kaons to condense, and there is an additional ultra-light "K^0" Goldstone boson arising from the spontaneous breaking of isospin. We compute the bulk viscosity of matter in the CFL-K^0 phase, which arises from the beta-equilibration processes K^0<->H+H and K^0+H<->H. We find that the bulk viscosity varies as T^7, unlike the CFL phase where it is exponentially Boltzmann-suppressed by the kaon's energy gap. However, in the temperature range of relevance for r-mode damping in compact stars, the bulk viscosity in the CFL-K^0 phase turns out to be even smaller than in the uncondensed CFL phase, which already has a bulk viscosity much smaller than all other known color-superconducting quark phases.