Chiral pasta: Mixed phases at the chiral phase transition

Abstract

Interiors of neutron stars are ultradense and may contain a core of deconfined quark matter. Such a core connects to the outer layers smoothly or through a sharp microscopic interface or through an intermediate macroscopic layer of inhomogeneous mixed phases, which is globally neutral but locally contains electrically charged domains. Here I employ a nucleon-meson model under neutron star conditions that shows a first-order chiral phase transition at large densities. In the vicinity of this chiral transition I calculate the free energies of various mixed phases-different "pasta structures"-in the Wigner-Seitz approximation. Crucially, chirally broken nuclear matter and the approximately chirally symmetric phase (loosely interpreted as quark matter) are treated on the same footing. This allows me to compute the interface profiles of bubbles, rods, and slabs fully consistently, taking into account electromagnetic screening effects and without needing the surface tension as an input. I find that the full numerical results tend to disfavor mixed phases compared to a simple steplike approximation used frequently in the literature and that the predominantly favored pasta structure consists of slabs with a surface tension Σ ≃ 6 MeV=fm 2 .

Holographic quarkyonic matter

“…In our model we could construct such a mixed phase by assigning different volume fractions (to be determined dynamically) to the pure quark phase and the pure baryonic phase. It is conceivable that such a phase is preferred in a certain region of the phase diagram, in particular close to a first-order phase transition between the two pure phases, as discussed routinely in the context of dense matter in neutron stars [55][56][57]. In this work we ignore such a mixed phase, but emphasize that the quarkyonic phase is qualitatively different.…”

Building a realistic neutron star from holography

“…The construction we use here is often employed at the transition between nuclear matter and quark matter in the core, see for instance Refs. [56,[62][63][64]. The resulting structure of the crust is somewhat simplistic.…”

“…For a given volume fraction χ, the size of the Wigner-Seitz cell is then determined by the competition between surface tension (preferably large unit cells) and electrostatic Coulomb energy (preferably small unit cells). In principle, these effects can be included fully dynamically if the interface profiles are calculated, which also includes screening effects automatically [64,69]. Here we proceed with a simple approximation that assumes the interfaces to be sharp surfaces, with spatially uniform charge density in either phase, as often used for the quark-hadron mixed phase [56].…”

“…A brief summary and derivation of this approximation can be found in Ref. [64]; here we simply quote the relevant results.…”

A Bayesian analysis of the properties of hybrid stars with the NJL model

“…In the core of a neutron star, the transition between hadronic and deconfined matter might well occur through an inhomogeneous phase consisting of coexisting individually charged domains of (less dense) hadronic and (more dense) quark matter [57][58][59][60][61]. However, because of the divergence of the Coulomb energy density at the thermodynamic limit if the net electric charge is not zero, those domains must be mesoscopic and the interface energy not negligible, thus preventing a phase coexistence ruled by a standard Gibbs construction.…”