Chiral symmetry restoration by parity doubling and the structure of neutron stars

We investigate the equation of state for the recently developed hybrid quark-meson-nucleon model under neutron-star conditions of β-equilibrium and charge neutrality. The model has the characteristic feature that, at increasing baryon density, the chiral symmetry is restored within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. Most important for this study are the nucleon (neutron, proton) and N(1535) states. We present different sets for two free parameters, which result in compact star mass-radius relations in accordance with modern constraints on the mass from PSR J0348+0432 and on the compactness from GW170817. We also consider the threshold for the direct Urca process for which a new relationship is given, and suggest as an additional constraint on the parameter choice of the model that this process becomes operative at best for stars with masses above the range for binary radio pulsars, M>1.4 M⊙.