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Exclusive measurements of the quasi-free np → npπ 0 π 0 reaction have been performed by means of dp collisions at T d = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region √ s = (2.35-2.46) GeV, which includes the region of the ABC effect and its associated d * (2380) resonance. Adding the d * resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Fäldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the π 0 π 0 -invariant mass spectrum, is found to be very modest -if present at all, which might pose a problem to some of its interpretations.

New data on quasifree polarized neutron-proton scattering, in the region of the recently observed d * resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, 2 checks of the quasifree character of the scattering process, on all obtained Ay angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the 3 D3-3 G3 coupled partial waves at (2380±10−i40±5) MeV -in accordance with the d * dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at T d = 2.27 GeV is discussed.

We calculate the charge distributions of d * (2380). Two different interpretations of the d * are considered for a comparison. One is a compact explanation with coupled ∆∆ + CC two-channel approximation in the chiral constituent quark model. Another is a resonance state of D12π. The remarkable differences of the charge distributions in the two pictures are shown and it is expected that the future experiments may provide a clear test for the different theoretical interpretations.