Abashian-Booth-Crowe effect in basic double-pionic fusion: a new resonance?

We report on an exclusive and kinematically complete high-statistics measurement of the basic double-pionic fusion reaction pn→dπ(0)π(0) over the full energy region of the ABC effect, a pronounced low-mass enhancement in the ππ-invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ΔΔ process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance with m=2.37 GeV, Γ≈70 MeV and I(J(P))=0(3(+)) in both pn and ΔΔ systems. Necessary further tests of the resonance interpretation are discussed.

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 pipi 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-->dpi;{0}pi;{0} 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 (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike 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.