Production of Pions in Nucleon-Nucleon Collisions at Cyclotron Energies

All available data on the production of pions in nucleon-nucleon collisions at cyclotron energies are compiled and are compared with the phenomenological theory of Watson and Brueckner. The principle of conservation of isotopic spin allows all these cross sections to be written in terms of only three independent cross sections, whose excitation functions are predicted by the phenomenological model. The theory represents satisfactorily the excitation functions and the angular distributions that are known experimentally; however the experimental ratio $\frac{\sigma (P+P\to {\pi }^{+}+N+P)}{\sigma (P+P\to {\pi }^{+}+D)}$ appears to be about a factor 2 larger than predicted.