Parity Doubling and the Dense-Matter Phase Diagram under Constraints from Multi-Messenger Astronomy

We extend the recently developed hybrid quark-meson-nucleon model by augmenting a six-point scalar interaction and investigate the consequences for neutron-star sequences in the mass-radius diagram. One of the characteristic features of the model is that the chiral symmetry is restored within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. At low temperature and finite baryon density, the model predicts a first- or second-order chiral phase transition, or a crossover, depending on the expectation value of a scalar field, and a first-order deconfinement phase transition. We discuss two sets of free parameters, which result in compact-star mass-radius relations that are at tension with the combined constraints for maximum-mass (2M circle dot) and the compactness (GW170817). We find that the most preferable mass-radius relations result in isospin-symmetric phase diagram with rather low temperature for the critical point of the chiral phase transition.