Two-pion production in proton-proton collisions - experimental total cross sections and their isospin decomposition

The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to T-P = 1.36 GeV. Total and differential Cross sections have been obtained for the channels pn pi(+)pi(0), pp pi(+)pi(-), pp pi(0)pi(0) and also nn pi(+)pi(+). For intermediate incident energies T-P > 1GeV, i.e. in the region which is beyond the Roper excitation but at the onset of Delta Delta excitation, the total pp pi(0)pi(0) cross section falls behind theoretical predictions by as much as all order of magnitude near 1.2 GeV, whereas the nn pi(+)pi(+) cross section is a factor of five larger than predicted. An isospin decomposition of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the Delta(1232). As possible candidates the Delta(1600) and the Delta(1700) are discussed. (C) 2009 Elsevier B.V. All rights reserved.