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Exclusive and kinematically complete high-statistics measurements of thedouble pionic fusion reaction $dd \to ^4$He$\pi^0\pi^0$ have been performed inthe energy range 0.8 - 1.4 GeV covering thus the region of the ABC effect,which denotes a pronounced low-mass enhancement in the $\pi\pi$-invariant massspectrum. The experiments were carried out with the WASA detector setup atCOSY. Similar to the observation in the basic $pn \to d \pi^0\pi^0$ reaction,the data reveal a correlation between the ABC effect and a resonance-likeenergy dependence in the total cross section. The maximum occurs at m=2.37 GeV+ 2$m_N$, i.e. at the same position as in the basic reaction. The observedresonance width $\Gamma \approx$ 160 MeV can be understood from broadening dueto Fermi motion of the nucleons in initial and final nuclei together withcollision damping. Differential cross sections are described equally well bythe hypothesis of a $pn$ resonance formation during the reaction process.

ABC Resonance Structure in the Double-Pionic Fusion to 4He