ISSN - 01466410, Progress in Particle and Nuclear Physics, 2008, vol. 61, p. 276-282
Single- and double-pion production in nucleon collisions on the nucleon and on nuclei-the ABC effect and its origin in a dibaryonic resonance
The ABC effect-an intriguing low-mass enhancement in the π π 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 3He reveal this effect to be a σ-channel phenomenon associated with the formation of a Δ Δ 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 p n and N N π π 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 N Δ resonances is reexamined. © 2007 Elsevier B.V. All rights reserved.