THE pp → dπ⁺π⁰ REACTION — A CASE OF ΔΔ EXCITATION WITHOUT ABC-EFFECT

The ABC effect, a low-mass enhancement in the invariant ππ mass, is observed in double-pionic fusion reactions leading to a bound nuclear system in the final state. From previous measurements there have been indications that this phenomenon is resctricted to the σ channel of the ππ system. With exclusive measurements of the pp → dπ+π0 reaction at Tp = 1.1 GeV we demonstrate that, indeed, the ABC effect does not occur in the vector-isovector ππ channel (ρ channel) despite the fact that the ΔΔ excitation is oberved to be the dominant reaction process. We also show that this reaction is well described by a t -channel ΔΔ excitation with the subsequent decay into the vector-isovector ππ channel given by the ρ channel operator.