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The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ invariantmass spectrum - is well-known from inclusive measurements of two-pionproduction in nuclear fusion reactions. Here we report on first exclusive andkinematically complete measurements of the most basic double pionic fusionreaction $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, whichhave been carried out at CELSIUS-WASA, reveal the ABC effect to be a$(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-likeenergy dependence in the integral cross section and the formation of a$\Delta\Delta$ system in the intermediate state. A corresponding simples-channel resonance ansatz provides a surprisingly good description of thedata.

Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements