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 Tp=1.36 GeV. Total and differential cross sections have been obtained for the channels pnπ+π0, ppπ+π−, ppπ0π0 and also nnπ+π+. For intermediate incident energies Tp>1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of ΔΔ excitation, the total ppπ0π0 cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the nnπ+π+ 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 Δ(1232). As possible candidates the Δ(1600) and the Δ(1700) are discussed.