Using SU(3) chiral perturbation theory we calculate the density-dependent
complex mean field UΣ(kf)+iWΣ(kf) of a Σ-hyperon in
isospin-symmetric nuclear matter. The leading long-range ΣN-interaction arises from one-kaon exchange and from two-pion exchange with a
Σ- or a Λ-hyperon in the intermediate state. We find from the
ΣN→ΛN conversion process at nuclear matter saturation density
ρ0=0.16fm−3 an imaginary single-particle potential of
WΣ(kf0)=−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 Σ- or Λ-hyperon
sum up to a moderately repulsive real single-particle potential of
UΣ(kf0)=59 MeV. Recently measured (π−,K+) inclusive spectra
related to Σ−-formation in heavy nuclei give evidence for a
Σ-nucleus repulsion of similar size. Our results suggest that the net
effect of the short-range ΣN-interaction on the Σ-nuclear mean
field could be small.Comment: 7 pages, 2 figures, published in: Phys. Rev. C 71, 068201 (2005