Interaction of $\eta$ mesons with a three-nucleon system
  A study of the $\eta$-3N interaction in the energy region near the $\eta$-^3H elastic scattering threshold is presented. The calculational scheme is based on the four-body scattering formalism. A manageable form of the integral equations is achieved by using a separable ansatz for the driving two-body $\eta N$- and NN-forces as well as for the subamplitudes appearing in the (1+3) and (2+2) partitions of the $\eta$-3N system. Results presented for the $\eta$-^3H scattering length point to the existence of a virtual (antibound) s-wave $\eta$-^3H state. As a consequence, a large enhancement of the cross section for $\eta$-production from three-body nuclei is predicted. 
A. Fix and H. Arenhoevel