ABC effect and resonance structure in the double-pionic fusion to 3He

Exclusive and kinematically complete measurements of the double pionic fusion to ³He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the π π -invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY (the cooler synchrotron at Forschungszentrum Jülich). Similar to the observations in the basic p n →d π⁰π⁰ reaction and in the d d →⁴He π⁰π⁰ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of d^* resonance formation during the reaction process in addition to the conventional t -channel Δ Δ mechanism. The deduced d^* resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.