Two-pion production in proton-proton collisions near threshold
Two-pion production reactions in proton-proton collisions have been studied using the PROMICE/WASA detector and an internal cluster gas-jet target at the CELSIUS storage ring in Uppsala. The total cross sections for the pp → ppπ+π-, pp → ppπ0π 0 and the pp → pnπ0π- reactions are presented at beam energies ranging from 650 to 775 MeV. An isospin analysis of the amplitudes involved in these three reactions indicates that the excitation of the N* (1440) followed by direct two pion emission can be the most important amplitude when allowed. However, large contributions from other production mechanisms must also be prevalent to explain the large pp → pnπ0π+ cross section. The data are compared to an extensive model including non-resonant as well as resonant production via N* and δ excitation.