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Two-pion production in proton-proton collisions near threshold

Calén H., Fransson K., Gustafsson L., Häggström S., Höistad B., Johansson A., Johanson J., Johansson T., Khoukaz A., Kullander S., Ruber R., Zlomańczuk J., Ekström C., Bilger R., Brodowski W., Clement H., Wagner G. J., Oelert W., Sefzick T., Greiff J., Koch I., Scobel W., Kupsc A., Marciniewski P., Stepaniak J., Morosov B., Povtorejko A., Sukhanov A., Zabierowski J., Shwartz B.

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.