Spin-asymmetry energy of nuclear matter

We calculate the density-dependent spin-asymmetry energy S(k(f)) of isospin-symmetric nuclear matter in the three-loop approximation of chiral perturbation theory. The interaction contributions to S(k(f)) originate from one-pion exchange, iterated one-pion exchange, and (irreducible) two-pion exchange with no, single, and double virtual Delta-isobar excitation. We find that the truncation to 1pi-exchange and iterated 1pi-exchange terms (which leads already to a good nuclear matter equation of state) is spin unstable, since S(k(f0)) < 0. The inclusion of the chiral piNDelta dynamics guarantees the spin stability of nuclear matter. The corresponding spin-asymmetry energy S(k(f)) stays positive within a wide range of an undetermined short-range parameter S-5 (which we also estimate from realistic NN potentials). Our results reemphasize the important role played by two-pion exchange with virtual Delta-isobar excitation for the nuclear matter many-body problem. Its explicit inclusion is essential in order to obtain good bulk and single-particle properties.