Delineating the properties of matter in cold, dense QCD

Abstract

The properties of dense QCD matter are delineated through the construction of equations of state which should be consistent with the low and high density limits of QCD, nuclear laboratory experiments, and the neutron star observations. These constraints, together with the causality condition of the sound velocity, are used to develop the picture of hadron-quark continuity in which hadronic matter continuously transforms into quark matter (modulo small 1st order phase transitions). The resultant unified equation of state at zero temperature and β -equilibrium, which we call Quark-Hadron-Crossover (QHC19), is consistent with the measured properties of neutron stars as well as the microphysics known for the hadron phenomenology. In particular to ∼ 10n 0 (n 0 : saturation density) the gluons remain as non-perturbative as in vacuum and the strangeness can be as abundant as up-and down-quarks at the core of two-solar mass neutron stars. Within our modeling the maximum mass is found less than 2.35 times solar mass and the baryon density at the core ranges in ∼ 5-8n 0 .

Impact of Multiple Phase Transitions in Dense QCD on Compact Stars

“…The first-order phase transition leaves a marked imprint on the macroscopic properties of compact stars because the EoS contains a density jump, which may give rise to new stable branches of compact stars (i.e., their third family) separated from nucleonic stars by a region of instability [34][35][36][37]. Smooth crossover without changes in the values of the order parameter or the wave function of the three-quark states would be a less dramatic change in the slope of the EoS, best visualized in terms of the speed of sound [10,22,38]. As mentioned above, two sequential first-order phase transitions can lead to the appearance of a new branch of compact stars-fourth family-separated from the third family by an instability region [15,33,39,40], assuming the classical stability criterion dM/dρ c > 0 is valid.…”

QCD equations of state and speed of sound in neutron stars

“…More microscopic arguments were proposed in [110] based on the quarkyonic matter hypothesis [111] in which high-density matter has the quark Fermi sea but baryonic Fermi surface. Baryons on top of the Fermi sea may be regarded as relativistic baryons with three quarks collectively moving in the same direction, unlike baryons at low density where the moving directions of three quarks are opposite one another, leaving a small baryon momentum (and pressure) but a large mass density [112]. Changes from the non-relativistic to relativistic regime may be a microscopic origin of phenomenological repulsion used in hadronic models.…”

Chiral condensates for neutron stars in hadron-quark crossover; from a parity doublet nucleon model to an NJL quark model

“…One of the key observables in neutron star physics is mass-radius sequence of neutron stars (M -R relation), which has a one-to-one correspondence with the underlying equation of state (EOS). The shape of the M -R curve is correlated with the stiffness of EOS at several fiducial densities [6]. The low density part (n B 2n 0 ) is largely correlated with the overall radii of neutron stars while the high density part (n B 3-5n 0 ) determines the maximum mass.…”