On the treatment of NN interaction effects in meson production in NN collisions

We clarify under what circumstances the nucleon--nucleon final state interaction fixes the energy dependence of the total cross--section for the reaction $NN \to NNx$ close to production threshold, where $x$ can be any meson whose interaction with the nucleon is not too strong. We strongly criticize the procedure used recently by several authors to include the final state interaction in the reactions under discussion. In addition, we give a formula that allows one to estimate the effect of the initial state interaction for the production of heavy mesons.