Nuclear matter in relativistic mean field theory with isovector scalar meson

Relativistic mean field (RMF) theory of nuclear matter with the isovector scalar mean field corresponding to the δ-meson [a0(980)] is studied. While the δ-meson mean field vanishes in symmetric nuclear matter, it can influence properties of asymmetric nuclear matter in neutron stars. The RMF contribution due to δ-field to the nuclear symmetry energy is negative. To fit the empirical value, Es ≈ 30 MeV, a stronger ϱ-meson coupling is required than in the absence of the δ-field. The energy per particle of neutron matter is then larger at high densities than the one with no δ-field included. Also, the proton fraction of β-stable matter increases. Splitting of proton and neutron effective masses due to the δ-field can affect transport properties of neutron star matter.