Pion production mechanism in nucleon-nucleon collisions

The pion production pn --> pp pi(-) and pp --> pp pi(0) are discussed. The theoretical analysis has been performed by using the models of heavy-meson exchange and pion-rescattering including the effect of nucleon resonances. The experimental data of the pi(-) production support the relative sign bet ween the s-wave and p-wave pion-production amplitudes which is given by bot h of the two models. The d-wave pion production amplitude is discussed abou t the angular distribution of pi(0) production. The spin correlation observ ables give information about the partial wave amplitudes.