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

Abstract

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 ππ-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 pn→dπ0π0 reaction and in the dd→4Heπ0π0 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 ΔΔ mechanism. The deduced d∗ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei. © 2015 American Physical Society.