ABC Effect in Basic Double-Pionic Fusion --- Observation of a new resonance?

Abstract

We report on a high-statistics measurement of the basic double pionic fusion reaction $pn\to d\ddot{I}{€}^0\ddot{I}{€}^0$ over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in the $\ddot{I}€\ddot{I}€$-invariant mass spectrum. The measurements were performed with the WASA detector setup at COSY. The data reveal the ABC effect to be associated with a Lorentzian shaped energy dependence in the integral cross section. The observables are consistent with a resonance with $I(J^P)=0(3^{+})$ in both $pn$ and $\hat{I}”\hat{I}”$ systems. Necessary further tests of the resonance interpretation are discussed.