Nuclear energy density functional from chiral pion-nucleon dynamics

Abstract

We calculate the nuclear energy density functional relevant for N=Z even-even nuclei in the systematic framework of chiral perturbation theory. The calculation includes the one-pion exchange Fock diagram and the iterated one-pion exchange Hartree and Fock diagrams. From these few leading order contributions in the small momentum expansion one obtains already a very good equation of state of isospin symmetric nuclear matter. We find that in the region below nuclear matter saturation density the effective nucleon mass $\widetilde M^*(\rho)$ deviates by at most 15% from its free space value $M$, with $0.89M<\widetilde M^*(\rho)<M$ for $\rho < 0.11 {\rm fm}^{-3}$ and $\widetilde M^*(\rho)>M$ for higher densities. The parameterfree strength of the $(\vec\nabla \rho)^2$-term, $F_\nabla(k_f)$, is at saturation density comparable to that of phenomenological Skyrme forces. The magnitude of $F_J(k_f)$ accompanying the squared spin-orbit density $\vec J ^2$ comes out somewhat larger. The strength of the nuclear spin-orbit interaction, $F_{so}(k_f)$, as given by iterated one-pion exchange is about half as large as the corresponding empirical value, however, with the wrong negative sign. The novel density dependencies of $\widetilde M^*(\rho)$ and $F_{\nabla,so,J}(k_f)$ as predicted by our parameterfree calculation should be examined in nuclear structure calculations (after introducing an additional short range spin-orbit contribution constant in density).