Double pionic fusion to ⁴He. Kinematically complete measurements over the energy region of the ABC effect

The double-pionic fusion to light nuclei slightly above the threshold is governed by the so-called ABC effect, a huge enhancement in the invariant mass spectrum of the pion pair at low masses. Now 50 years have passed since its discovery, but still this phenomenon is lacking a satisfying explanation. One reason is that all the early experiments were inclusive, measuring only the heavy ejectile. Thus not all kinematic variables could be obtained. For this reason exclusive, kinematically complete measurements of the double-pion production were performed with the WASA detector. This work is about the fusion to ⁴He. At nine beam energies over the whole energy region of the ABC effect the dd→⁴Heπ⁰π⁰ reaction was measured. For the first time the energy dependence of the ABC effect was investigated in exclusive measurements. For these experiments new detector components for the WASA detector system were developed, which was also part of this thesis. The results open a completely new view on the ABC effect. Apparently the additional information gained by the measurement of all kinematic variables is essential for the understanding of the phenomenon. Previous models developed at hand of inclusive measurements cannot reproduce the new results. The data identify the enhancement at low ππ invariant masses as a very strong and narrow peak accompanied with a strong energy dependence of the total cross section. A comparison with the recent results in the basic reaction of the ABC effect, pn→dππ, confirms the assumption that both reactions, the fusion to deuteron and the fusion to ⁴He, are governed by the same underlying process. The results indicate that the ABC effect is driven by a resonance in the pn and ΔΔ systems, which is strong enough to survive in a nuclear medium. And in fact the recently developed ABC resonance model describes both reactions with amazing accuracy.