From CELSIUS to COSY: on the observation of a dibaryon resonance

Using a high-quality beam of storage rings in combination with a pellet target and a hermetic WASA detector covering practically the full solid angle, two-pion production in nucleon-nucleon collisions has been systematically studied by exclusive and kinematically complete measurements-first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect, which denotes an unusual low-mass enhancement in the pp-invariant mass spectrum, in double-pionic fusion reactions led the trace to the observation of a narrow dibaryon resonance with I(J(P)) = 0(3(+)) about 80 MeV below the nominal mass of the conventional Delta Delta system. New neutron-proton scattering data, taken with a polarized beam at COSY, produced a pole in the coupled D-3(3)-(3)G(3) partial waves at (2380 +/- 10 - i40 +/- 5) MeV, establishing thus the first observation of a genuine s-channel dibaryon resonance.