Threshold meson production in nucleon-nucleon collisions

A brief overview on recent model calculations for near threshold pion production in nucleon-nucleon collisions is given. Results from our own investigations of the reactions pp --> pp pi(0) and pn --> d pi(0) are presented. Direct production, heavy meson exchange and pion rescattering are taken into account. For the latter a T-matrix obtained from a microscopic model of the pi N interaction is employed. We obtain a significant contribution from rescattering, but not enough to describe the data for pp --> pp pi(0). The missing production rate can be provided by heavy meson exchanges. For the first time the effect of off-shell rescattering is investigated for the reaction pn --> d pi(0). Isoscalar rescattering in combination with isovector rescattering is able to describe the s-wave Production data. We confirm that heavy meson exchanges are negligible in this reaction.