Pion production mechanism in nucleon-nucleon collisions

The pion production pn → ppπ- and pp → ppπ0 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 π- production support the relative sign between the s-wave and p-wave pion-production amplitudes which is given by both of the two models. The d-wave pion production amplitude is discussed about the angular distribution of π0 production. The spin correlation observables give information about the partial wave amplitudes