Parity-Doubled Nucleons Can Rapidly Cool Neutron Stars

In confined hadronic matter, the spontaneous breaking and restoration of chiral symmetry can be described by considering nucleons, N_{+}(939), and excited states of opposite parity, N_{-}(1535). In a cold, dense hadronic phase where chiral symmetry remains spontaneously broken, direct Urca decay processes involving the N_{-} are possible, e.g., N_{-}→N_{+}+e^{-}+ν[over ¯]_{e}. We show that at low temperature and moderate densities, because the N_{-} are much heavier than the N_{+}, such cooling dominates over standard N_{+} direct Urca processes. This provides a strong astrophysical signature of the pattern of chiral symmetry restoration in neutron stars.