Pion Production in Nucleon-Nucleon Collisions at Energies near Threshold

A calculation using the Chew theory indicates that nucleon-nucleon and pion-nucleon final-state interactions both play a role in contributing to the pion production cross sections in nucleon-nucleon collisions at energies near threshold. In particular, it is found that the cross section will be enhanced if (1) tensor forces lead to a $D$-wave nucleon-nucleon final state interaction or if (2) the pion and one of the nucleons is in a state of isotopic spin $\frac{3}{2}$ and angular momentum $\frac{3}{2}$. Both of these effects are present if the isotopic spin of the initial two nucleons is unity and the isotopic spin of the final nucleons is zero. In the case of deuteron formation, it is found that the two effects are of the same order of magnitude, but depend differently on energy.