Single- and Double-Pion Production in Nucleon Collisions on the Nucleon and on Nuclei -- the ABC Effect and its Possible Origin in a Dibaryonic Resonance

The ABC effect -- an intriguing low-mass enhancement in the $pipi$ invariant mass spectrum -- is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive measurements carried out at CELSIUS-WASA for the fusion reactions leading to d or $^3$He reveal this effect to be a $sigma$ channel phenomenon associated with the formation of a $DeltaDelta$ system in the intermediate state and combined with a resonance-like behavior in the total cross section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in $pn$ and $NNpipi$ systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of $NDelta$ resonances is reexamined.