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. 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