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

摘要

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 pnπ~+π~0 ppπ~+π~- ppπ~0π0 and also nnπ~+π~+. For intermediate incident energies T_p > 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of ΔΔ excitation, the total ppπ~0π~0 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.