Constraining the nuclear equation of state from terrestrial experiments and neutron star observations using relativistic mean-field models

We investigate the nuclear equation of state (EoS) for isospin-asymmetric matter using a new set of RMF interactions with the $\sigma$-$\delta$ and $\omega$-$\rho$ mixing, referred to as the OMEG family. These interactions are optimized so as to reproduce both terrestrial nuclear measurements and astrophysical constraints extracted from NICER and GW170817. The $\sigma$-$\delta$ mixing softens the nuclear symmetry energy and pressure around twice the saturation density, which enables relatively small neutron-star radii and tidal deformabilities while keeping the nuclear EoS sufficiently stiff at high densities to support $2M_{\odot }$ neutron stars. We find that the curvature parameter, $K_{\text{sym}}$, plays an important role in realizing the soft-to-hard behavior of the nuclear EoS, and the astrophysical data favor small or even negative values of $K_{\text{sym}}$.