Chiral dynamics of Σ hyperons in the nuclear medium -: art. no. 068201

Using SU(3) chiral perturbation theory, we calculate the density-dependent complex mean field U-Sigma(k(f))+i W-Sigma(k(f)) of a Sigma hyperon in isospin-symmetric nuclear matter. The leading long-range Sigma N interaction arises from one-kaon exchange and from two-pion exchange with a Sigma or a Lambda hyperon in the intermediate state. We find from the Sigma N ->Lambda N conversion process at nuclear matter saturation density rho(0)=0.16 fm(-3) an imaginary single-particle potential of W-Sigma(k(f0))=-21.5 MeV, in fair agreement with existing empirical determinations. The genuine long-range contributions from iterated (second order) one-pion exchange with an intermediate Sigma or Lambda hyperon sum to a moderately repulsive real single-particle potential of U-Sigma(k(f0))=59 MeV. Recently measured (pi(-),K+) inclusive spectra related to Sigma(-) formation in heavy nuclei give evidence for a Sigma-nucleus repulsion of similar size. Our results suggest that the net effect of the short-range Sigma N interaction on the Sigma nuclear mean field could be small.