Bulk viscosity in 2SC and CFL quark matter

TL;DR

This paper calculates and compares the bulk viscosities of CFL and 2SC color-superconducting quark matter phases, highlighting their potential impact on damping stellar oscillations and indicating the presence of such phases in compact stars.

Contribution

It provides the first detailed comparison of bulk viscosities in CFL and 2SC phases, considering their unique fermionic and bosonic contributions.

Findings

Bulk viscosity can exceed that of unpaired quark matter at certain temperatures.

Different dominant processes contribute to bulk viscosity in CFL and 2SC phases.

Bulk viscosity influences r-mode damping in compact stars.

Abstract

The bulk viscosities of two color-superconducting phases, the color-flavor locked (CFL) phase and the 2SC phase, are computed and compared to the result for unpaired quark matter. In the case of the CFL phase, processes involving kaons and the superfluid mode give the largest contribution to the bulk viscosity since all fermionic modes are gapped. In the case of the 2SC phase, ungapped fermionic modes are present and the process u+d <-> u+s provides the dominant contribution. In both cases, the bulk viscosity can become larger than that of the unpaired phase for sufficiently large temperatures (T >~ 1 MeV for CFL, T >~ 0.1 MeV for 2SC). Bulk viscosity (as well as shear viscosity) is important for the damping of r-modes in compact stars and thus can potentially be used as an indirect signal for the presence or absence of color-superconducting quark matter.