Two-Pion Production in Proton-Proton Collisions -- Experimental Total Cross Sections and their Isospin Decomposition

Abstract

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 $pn\ddot{I}{€}^{+}\ddot{I}{€}^0$, $pp\ddot{I}{€}^{+}\ddot{I}{€}^{-}$, $pp\ddot{I}{€}^0\ddot{I}{€}^0$ and also $nn\ddot{I}{€}^{+}\ddot{I}{€}^{+}$. For intermediate incident energies $T_p>$ 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of $\hat{I}”\hat{I}”$ excitation, the total $pp\ddot{I}{€}^0\ddot{I}{€}^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nn\ddot{I}{€}^{+}\ddot{I}{€}^{+}$ cross section is a factor of five larger than predicted. An isospin decompostion 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 $\hat{I}”(1232)$. As possible candidates the $\hat{I}”(1600)$ and the $\hat{I}”(1700)$ are discussed.