Cold quark matter

We perform an O (ₒ^2) perturbative calculation of the equation of state of cold but dense QCD matter with two massless and one massive quark flavor, finding that perturbation theory converges reasonably well for quark chemical potentials above 1 GeV. Using a running coupling constant and strange quark mass, and allowing for further nonperturbative effects, our results point to a narrow range where absolutely stable strange quark matter may exist. Absent stable strange quark matter, our findings suggest that quark matter in (slowly rotating) compact star cores becomes confined to hadrons only slightly above the density of atomic nuclei. Finally, we show that equations of state including quark matter lead to hybrid star masses up to M2M_, in agreement with current observations. For strange stars, we find maximal masses of M2. 75M_ and conclude that confirmed observations of compact stars with M>2M_ would strongly favor the existence of stable strange quark matter.