Reexamining constraints on neutron star properties from perturbative QCD

TL;DR

This paper reexamines how perturbative QCD calculations constrain neutron star properties, showing that current uncertainties limit their impact and suggesting potential implications for the neutron star core composition and color superconductivity.

Contribution

It provides a detailed analysis of the constraints from pQCD on neutron star equations of state, highlighting the limitations due to uncertainties and exploring implications for core physics.

Findings

pQCD constraints mainly affect the most massive neutron stars near the TOV limit

Current pQCD uncertainties prevent definitive bounds on neutron star EOS

Refined low-pressure pQCD predictions could indicate color superconductivity in neutron star cores

Abstract

The implications of perturbative QCD (pQCD) calculations on neutron stars are carefully examined. While pQCD calculations above baryon chemical potentials $\mu_B\simeq2.4$ GeV demonstrate the potential of ruling out a wide range of neutron star equations of state (EOSs), such constraints only affect the most massive neutron stars in the vicinity of the Tolman-Oppenheimer-Volkoff (TOV) limit, resulting in constraints that are orthogonal to current or expected astrophysical bounds. In the most constraining scenario, pQCD considerations favor low values of the squared speed sound $C_s$ at high $\mu_B$ relevant for the most massive neutron stars, but leave predictions of the radii and tidal deformabilities almost unchanged. Such considerations become irrelevant if the maximum speed of sound squared inside neutron stars does not exceed about $C_{s,\mathrm{max}}\simeq0.5$, or if pQCD breaks down below $\mu_B\simeq2.9$ GeV. Furthermore, the large pQCD uncertainties preclude any meaningful bounds on the neutron star EOS at the moment. Interestingly, if pQCD predictions for the pressure at around $\mu_B\simeq2.5$ GeV are refined and found to be low ($\lesssim 1.5$ GeV/fm$^3$), evidence for a soft neutron star inner core EOS in combination with the existence of two-solar-mass pulsars would indicate the presence of color superconductivity beyond neutron star densities. We point out that two-solar-mass pulsars place robust upper bounds on this non-perturbative effect and require the pairing gap to be less than $\Delta_\mathrm{CFL}\lesssim500$ MeV at $\mu_B\simeq2.5$ GeV.