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The ABC effect -- an intriguing low-mass enhancement in the $\pi\pi$invariant mass spectrum -- is known from inclusive measurements of two-pionproduction in nuclear fusion reactions. First exclusive measurements carriedout at CELSIUS-WASA for the fusion reactions leading to d or $^3$He reveal thiseffect to be a $\sigma$ channel phenomenon associated with the formation of a$\Delta\Delta$ system in the intermediate state and combined with aresonance-like behavior in the total cross section. Together with theobservation that the differential distributions do not change in shape over theresonance region the features fulfill the criteria of an isoscalar s-channelresonance in $pn$ and $NN\pi\pi$ systems, if the two emitted nucleons arebound. It obviously is robust enough to survive in nuclei as a dibaryonicresonance configuration. In this context also the phenomenon of $N\Delta$resonances is reexamined.

progress in particle and nuclear physics

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