THE ABC EFFECT AND ITS ENERGY DEPENDENCE IN THE DOUBLE-PIONIC FUSION TO 4He

The double-pionic fusion reactions dd → 4 He π0π0 and dd → 4 He π+π- were measured exclusively and kinematically complete with the WASA detector at COSY. The measurements were performed in the energy range Tp = 0.8 - 1.4 GeV covering thus the full energy region, where the so-called ABC effect – a peculiar low-mass enhancement in the ππ-invariant mass spectrum – was observed previously in inclusive measurements. Our findings for the double-pionic fusion to 4 He are in accordance with the observations for the basic pn → dπ0π0 reaction pointing to the same fundamental mechanism. In both cases we observe that the ABC effect is associated with a peak in the energy dependence of the total cross section. In the 4 He case the peak structure in the total cross section is broadened in comparison to the deuteron case. This is consistent with both Fermi motion of the basic pn system within 4 He and with collision broadening as it is known, e.g., from excitations of the Δ resonance in nuclei.