"Two distinct theoretical attempts have been made to describe the neutron distribution for the process of [pi]+ production in proton-proton collisions at 970 Mev. The OPE and OBE Models both are one particle exchange theories. Trying to extend the former, using its production vertex technique, leads to divergent integrals. However, using in part the perturbation methods of the OBE model, a finite two pion exchange term is realized. It is found, contrary to the assumption made in the OBE model, that the two pion term gives a contribution greater than the one pion term. (In calculating the two pion term the [pi]-[pi] interaction in the crossed channel is neglected. This has been taken into account in the OBE model by the exchange of multipion resonances considered as particles in the usual sense.) This result invalidates the claim that the OBE model completely accounts for the dominant contributions to the production process under consideration here. In the light of this fact it is suggested that the fundamental tenets of the OBE model be subjected to critical scrutiny. Further, because of the close relationship between the OPE model and the one pion term of the OBE model, it is inferred that the former is also in need of reevaluation. [...]" --

The two reactions np .-->. nn.pi. and np .-->. pp.pi.− were studied at 790 MeV (incident neutron energy). Pion spectra were measured at 10 different angles with a multiwire proportional chamber spectrometer. Approximately 100,000 events were analyzed. The angular distribution of pions in the np center of momentum system (d sigma/d.cap omega.*) was given by ((123.1 +- 2.7) + (88.3 +- 4.9)cos2(theta*))(.mu.b/sr). The cross section sigma(np .-->. NN.pi./sup +-/) was determined to be 1.92 +- 20 mb by integrating (d sigma/d.cap omega.*) over all angles. The partial cross section for pion production from T = 0 np interactions (sigma01) was found to be .1/sub -.1//sup +.5/ mb by using the relation sigma01 = 2sigma(np .-->. NN.pi./sup +- )--sigma(pp .-->. pp.pi.°). Stronger indications of nonresonant pion production were given by the presence of asymmetries between the positive and negative pion spectra and a comparison of the data with an isobar model calculation.