Effective nucleon mass in relativistic mean field theory

In the $\sigma$-$\omega$-$\rho$ model of the relativistic mean field theory with nonlinear $\sigma$-meson self-interaction, the effective nucleon mass $M^*$ is discussed with relation to the symmetry incompressibility $K_s$ of nuclear matter, based on the model parameters fitted to nuclear matter properties. It is shown that $M^*$ is larger than $0.73M$, if $K_s$ is assumed to be negative and the nuclear matter incompressibility $K_0$ is kept less than $300 MeV$. Furthermore, the field system is shown to be stable, as the $\sigma$-meson self-interaction energy is lower bounded in this parameter region.