  The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ 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 measurements of the most basic double pionic fusion
reaction $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, which
have been carried out at CELSIUS-WASA, reveal the ABC effect to be a
$(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-like
energy dependence in the integral cross section and the formation of a
$\Delta\Delta$ system in the intermediate state. A corresponding simple
s-channel resonance ansatz provides a surprisingly good description of the
data.

Nuclear Experiment