The δ expansion approximation and the Zimanyi-Moszkowski model

In this work, we test once more the optimized δ expansion method in the Zimanyi-Moszkowski model, developed to solve some of the drawbacks of the Walecka model, as the too small effective nucleon mass obtained at high densities and its too large incompressibility. Zimanyi and Moszkowski introduced alternative couplings among the mesons and the nucleon. The inclusion of these coupling renders the model non-renormalizable. In this work we are interested in applying the optimized δ-expansion method and we have chosen to implement it in the simplest possible way. For this reason we are just considering medium contributions and the fact that ZM model is nor renormalizable does not pose a problem. Notice that, in contrast to the non-linear Walecka model, where non-linear terms were introduced to the original Walecka Lagrangian and more reasonable results for the compression modulus and the effective mass of the saturation density were obtained, the Zimanyi-Moszkowski model does not introduce extra free parameters. (author)