Two-Pion-Exchange Contribution to theΛ-Nucleon Scattering Matrix

The two-pion-exchange (TPE) contribution to the $\Lambda$-nucleon scattering matrix has been calculated with the Dyson $S$-matrix formalism under the assumption of even $\Lambda -\Sigma$ parity and a universal pion-baryon interaction. This calculation leads to a TPE contribution to the zero-energy $\Lambda$-nucleon scattering length which is spin-independent. For scattering at finite energies, the scattering matrix is spin-dependent and contains an anti-symmetric spin-orbit term, which does not appear in the charge-symmetric nucleon-nucleon scattering matrix. The results suggest the use of a semiphenomenological $\Lambda$-nucleon potential which, for large separations, is predominantly spin-independent.