Abstract
A short overview over the ABC phenomenon is given, which is known since 50 years without any satisfactory explanation. On the basis of new exclusive and kinematically complete data obtained with the WASA detector at COSY we show that this phenomenon is linked with a resonant energy dependence in the total cross section of isoscalar double-pionic fusion processes. This resonance structure is connected with the baryon-baryon system, in particular with the pn and ΔΔ systems, however, has a mass, which is 90 MeV below the nominal ΔΔ threshold and a width of only 70 MeV, i.e., five times smaller than expected from a conventional t-channel ΔΔ scenario.
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