ππ PRODUCTION IN PROTON-PROTON COLLISIONS.

At CELSIUS-WASA the two-pion production in proton-proton collisisons has been measured exclusively from threshold up to the energy regime, where both of the collision partners are expected to be excited to the Δ state. The measurements constitute the first kinematically complete data samples of solid statistics in this energy range. Most of the data have been obtained for the π⁺π^- and π⁰π⁰channels. Whereas at near-threshold energies the differential distributions can be succesfully explained by chiral dynamics and Roper excitation, respectively, the data for the π⁺π^- channel in the ΔΔ region can be described only, if the special configuration (ΔΔ)_0⁺ is assumed. The data for the π⁰π⁰ channel moreover exhibit a low-mass enhancement in the π⁰π⁰ invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei. [ABSTRACT FROM AUTHOR]