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. $N_{-}\to N_{+}+e^{-}+{\bar{\nu }}_e$. We show that at low temperature and moderate densities, because the $N_{-}$ is 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.