Emergent chirality and superfluidity of parity-doubled baryons in neutron stars

TL;DR

This paper explores novel superfluid phases in neutron stars induced by parity-doubled baryons, revealing emergent chiral symmetries, spontaneous symmetry breaking, and anisotropic fermionic excitations, with implications for neutron star physics.

Contribution

It introduces a new class of superfluids in neutron stars driven by parity-doubled baryons and emergent chiral symmetries, extending traditional models with novel symmetry breaking mechanisms.

Findings

Identification of vector-type condensates breaking emergent chiral symmetries

Discovery of massless Nambu-Goldstone bosons including emergent pions, phonons, and magnons

Observation of spatial anisotropy in fermionic excitation modes at the Dirac cone

Abstract

We propose novel superfluids induced by the parity-doubled baryons. The parity-doubled baryons, i.e., a nucleon $N(940)$ with spin-parity $J^{P}=1/2^{+}$ and an excited nucleon $N^{\ast}(1535)$ with $J^{P}=1/2^{-}$ in vacuum, become degenerate at sufficiently high density where the chiral symmetry is restored. In this study, we extend the conventional $\mathrm{U}(1)$ chiral symmetry to the higher dimensional symmetries, dubbed emergent chiral symmetries, including the naive and mirror assignments as their subgroups. Starting with the Lagrangian up to four-point interactions among the neutron $n$ and its chiral partner $n^{\ast}$, neutral components in $N$ and $N^{\ast}$, in pure neutron matter, we investigate the properties of the ground state with a pairing gap generated by the $n$ and $n^{\ast}$ in the mean-field approximation. We find vector-type condensates that induce the dynamical breaking of a new class of internal symmetries, emergent chiral symmetries, as well as the baryon number and the rotational symmetries of the real space, indicating the appearance of massless Nambu-Goldstone bosons consisting of six quarks: emergent pions, superfluid phonons, and magnons, respectively. We also study the fermionic excitation modes at low-energy scales, and show that they exhibit a spatial anisotropy of the propagation at the Dirac cone in momentum space. Some phenomenological implications are advocated, shedding new light on the properties of neutron stars.