Relativistic Mean-Field Models with Effective Hadron Masses and Coupling Constants, and rho^- Condensation

Abstract

We study relativistic mean-field models with hadron masses and coupling constants depending self-consistently on a scalar meson field. We demonstrate that by the field redefinition some models can be equivalently transformed into each other. Thereby the large variety of scaling functions for masses and couplings can be reduced to a restricted set of functions with a constrained dependence on a scalar field. We show how by choosing properly the latter scaling functions one may stiffen or soften the equation of state at high densities and simultaneously increase the threshold density for the direct Urca process without any change of the description of nuclear matter close to the saturation density. The stiffening of the equation of state might be motivated by recent neutron star mass measurements, whereas the increase of the threshold density for the direct Urca process is required by the analysis of neutron star cooling data. We demonstrate that if a rho meson is included in a mean-field model as a non-Abelian gauge boson, then there is a possibility for a charged rho-meson condensation in dense nuclear matter. We show that such a novel phase can be realized in neutron star interiors already for sufficiently low densities, typically $\sim 3÷4 n_0$, where $n_0$ is the nuclear saturation density. In the framework of the relativistic mean field model the new phase arises in a second-order phase transition. The appearance of a $ρ^-$ condensate significantly alters the proton fraction in a neutron star but changes moderately the equation of state. The neutrino emissivity of the processes involving a $ρ^-$ meson condensate is estimated.