Two and three pion production in pp collisions at 6.92 GeV/c

Cross sections are given for the main processes contributing to the final states ppπ⁺π^-, ppπ⁺π^-π⁰ and pnπ⁺π⁺π^-. In all reactions N^*(1238) is observed to be strongly produced. For these three reactions the baryons are strongly forward/backward peaked in the c.m. The reaction pp → N^*++(1238)pπ^- has been examined in some detail, and compared with the predictions of various peripheral models. As is already well known, the Ferrari-Selleri model is inadequate, but when some of the effects of absorption are included, the agreement with the data is considerably improved. The alternative of replacing the propagator corresponding to elementary pion exchange by that for a Regge exchange also gives good agreement with experiment. Finally we have tested a double-Regge-exchange model, and found that its prediction for one of the Treiman-Yang angles is unsatisfactory. A discussion is also given of a classification in which events are divide into categories depending on which particles are emitted in the same hemisphere. The two groupings p(pπ⁺π^-) and (pπ⁺)(pπ^-) each accounts for approximately half of the ppπ⁺π^- final state. The final states ppω, ppη and possibly ppρ{variant}⁰ are observed, and cross sections are compared with values obtained at other momenta. An attempt to deduce the reaction mechanism from the longitudinal momentum plot is discussed in an appendix.