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 $pp \rightarrow pp\pi ^0$ and $pn \rightarrow d\pi^0$ 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 $pp \to pp\pi^0$. 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 $pn \rightarrow d\pi^0$. 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.