The ABC effect - a puzzling low-mass enhancement in the $pipi$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measur
ements of the most basic double pionic fusion reaction $pn to d pi^0pi^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(pipi)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $DeltaDelta$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.
