Chiral approach to nuclear matter: role of two-pion exchange with virtual delta-isobar excitation

We extend a recent three-loop calculation of nuclear matter by including the effects from two-pion exchange with single and double virtual Delta(1232)-isobar excitation. Regularization dependent short-range contributions from pion-loops are encoded in a few NN-contact coupling constants. The empirical saturation point of isospin-symmetric nuclear matter, (E) over bar (0) = -16 MeV, rho(0) = 0.16 fm(-3), can be well reproduced by adjusting the strength of a two-body term linear in density (and tuning an emerging three-body term quadratic in density). The nuclear matter compressibility comes out as K = 304 MeV. The real single-particle potential U(p, k (f0)) is substantially improved by the inclusion of the chiral pi N pi Delta-dynamics: it grows now monotonically with the nucleon momentum p. The effective nucleon mass at the Fermi surface takes on a realistic value of M*(k (f0)) = 0.88M. As a consequence of these features, the critical temperature of the liquid-gas phase transition gets lowered to the value T-c similar or equal to 15 MeV. In this work we continue the complex-valued single-particle potential U(p, k (f)) + iW(p, k (f)) into the region above the Fermi surface p > k (f). The effects of 2 pi-exchange with virtual Delta-excitation on the nuclear energy density functional are also investigated. The effective nucleon mass associated with the kinetic energy density is M*(rho(0)) = 0.64M. Furthermore, we find that the isospin properties of nuclear matter get significantly improved by including the chiral pi N Delta-dynamics. Instead of bending downward above rho(0) as in previous calculations, the energy per particle of pure neutron matter A(k (f)) and the asymmetry energy A(kj,) now grow monotonically with density. In the density regime rho = 2 rho(n) < 0.2 fm(-3) relevant for conventional nuclear physics our results agree well with sophisticated many-body calculations and (semi)-empirical values. For the definition of the quantities gamma(min), s, sigma, t and R and the anti symmetrization prescription A(y) we refer to Section 4.