Two-pion production in nucleon-nucleon collisions-Back to dibaryons?

The two-pion production in nucleon-nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all pp-induced isovector production channels in the energy range from threshold up to Tp = 1.4 GeV can be well understood by t-channel Roper, ΔΔ and possibly Δ(1600) excitation. In contrast to that the purely isoscalar reaction channel pn→dπ0π0 exhibits a narrow resonance-like structure with m=2.37GeV and Γ=70MeV in the total cross section, which is correlated with an intriguing low-mass enhancement in the ππ invariant mass spectrum (ABC effect) as well as with the formation of a ΔΔ system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in I(JP)=0(3+). The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured pn resonance is obviously robust enough to survive in nuclei.