Fractionized Skyrmions in Dense Compact-Star Matter

TL;DR

This paper explores a novel phase of dense hadronic matter where skyrmions fractionize into half-skyrmions, leading to a phase with restored chiral symmetry but confined quarks, impacting the equation of state in neutron stars.

Contribution

It introduces the concept of a half-skyrmion phase in dense matter, characterized by specific symmetry properties and its role in the transition of the equation of state in compact stars.

Findings

Identification of a half-skyrmion phase at density n_{1/2}

Chiral symmetry is globally restored with confined quarks

Implication for a stiffer equation of state in neutron stars

Abstract

The hadronic matter described as a skyrmion matter embedded in an FCC crystal is found to turn into a half-skyrmion matter with vanishing (in the chiral limit) quark condensate and {\it non-vanishing} pion decay constant {$f_\pi$} at a density $n_{1/2}$ lower than or near the critical density $n_c$ at which hadronic matter changes over to a chiral symmetry restored phase with possibly deconfined quarks. When hidden local gauge fields and a dilaton scalar of spontaneously broken scale symmetry with decay constant $f_\chi$ are incorporated, this half-skyrmion phase is characterized by $f_\chi\approx f_\pi\neq 0$ with the hidden gauge coupling $g\neq 0$ but $\ll 1$. While chiral symmetry is restored {\it globally} in this region in the sense that space-averaged, $\la\bar{q}q\ra$ vanishes, quarks are still confined in massive hadrons and massless pions. This phase is shown to play a crucial role in the model for a smooth transition from a soft EoS at low density to a stiffer EoS at high density, the changeover taking place at $n_{1/2} \sim 2n_0$. It resembles the "quarkyonic phase" predicted in large $N_c$ QCD and represents the "hadronic freedom" regime which figures as a doorway to chiral restoration. The fractionization of skyrmion matter into half-skyrmion matter has a tantalizing analogy to what appears to happen in condensed matter in (2+1) dimensions where half-skyrmions or "merons" enter as relevant degrees of freedom at the interface.