ΔΔ excitation in proton-proton induced π0π0 production

Exclusive measurements of the pp→ppπ0π0 reaction have been performed at CELSIUS/WASA at energies from threshold up to Tp=1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies Tp⩾1 GeV, where the ΔΔ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional t-channel ΔΔ excitation. This holds also for the double-pionic fusion to the quasibound 2He. The data are compared to model predictions, which are based on both π- and ρ-exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the ρ-exchange. A phenomenological modification allowing only a small ρ-exchange contribution leads to a quantitative description of the data.