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

We report on a high-statistics measurement of the basic double pionic fusion reaction $pn\to d{\pi }^0{\pi }^0$ over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in the $\pi \pi$-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 $\mathrm{\Delta }\mathrm{\Delta }$ systems. Necessary further tests of the resonance interpretation are discussed.