ISSN - 00319007, EISSN - 10797114, Physical Review Letters, 2009, vol. 102, p. -
Double-pionic fusion of nuclear systems and the "aBC" effect: Approaching a puzzle by exclusive and kinematically complete measurements
The ABC effect-a puzzling low-mass enhancement in the ππ invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn→dπ0π0 at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (ππ)I=L=0 channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a ΔΔ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data. © 2009 The American Physical Society.