The properties of nuclear matter and the structures of neutron stars are analyzed with a baryonic extended linear sigma model in mean-field approximation, where the masses of baryons and mesons are generated via the spontaneous chiral symmetry breaking. The couplings between the iso-scalar scalar meson and nucleons, $g_{ \sigma NN}$, the iso-vector scalar meson and nucleons, $g_{ a_0 NN}$, and the four-vector meson couplings play an important role in the properties of nuclear matter and neutron stars. The introduction of the $\delta$ meson leads to a plateau structure of the symmetry energy, $E_{ \rm sym}(n)$, at intermediate densities, which is crucial to the consistency of neutron skin thickness of $^{ 208}$Pb and the tidal deformability of a canonical neutron star. The explicit chiral symmetry breaking term is then introduced with a constant background field, $\xi$, which can be related to the current quark mass and thus the pion-nucleon sigma term, $\sigma_{ \pi N}$. A negative $\sigma_{ \pi N}$ leads to a stiffer EOS of neutron star matter and thus a larger maximum mass of neutron stars, but the value of $\sigma_{ \pi N}$ needed to satisfy the astrophysical constraints is negative, not positive as the vacuum value. The study may provide insights into the running behaviors of the parameters in the low-energy effective model to give the density-dependent description for the EOS of neutron star matter.
Categories: nucl-th