On the Possibility of a Strong First-order Phase Transition in Neutron Stars

Abstract Whether cold dense quantum chromodynamics (QCD) matter undergoes a strong first-order phase transition remains an open question. In nature, neutron stars provide the most direct probe of cold dense QCD matter. Theoretically, chiral effective field theory constrains the equation of state of dense matter near nuclear saturation density, while perturbative QCD calculations constrain it at densities well beyond stable neutron-star interiors. We perform Bayesian inference with nonparametric Gaussian-process equation of state for β -equilibrated neutron-star matter under the assumption with and without a strong first-order phase transition, using the tidal deformability from GW170817; the NICER mass–radius measurements of PSR J0740+6620, PSR J0030+0451, PSR J0437−4715, and PSR J0614−3329; chiral effective field theory; and perturbative QCD. Within this Bayesian inference framework, the model comparison mildly to moderately favors a strong first-order phase transition, with its onset most likely lying above the central density of the most massive neutron star. Such an onset reconciles the stiffness required to support massive neutron stars with the softening favored by perturbative QCD from asymptotically high density.