The ABC Effect in Double-Pionic Nuclear Fusion and a pn Resonance as its Possible Origin

The ABC effect – a long-standing puzzle in double-pionic fusion – has been reexamined by the first exclusive and kinematically complete measurements of solid statistics for the fusion reactions $pn\to d{\pi }^0{\pi }^0$, $pd \to ^3$He$\pi\pi$and$dd \to ^4$He$\pi\pi$ using the WASA detector, first at CELSIUS and recently at COSY – the latter with a statistics increased by another two orders of magnitude. In all cases we observe a huge low-mass enhancement in the $\pi \pi$-invariant mass accompanied by a pronounced $\mathrm{\Delta }\mathrm{\Delta }$ excitation. For the most basic fusion reaction, the $pn \to d\pi^0\pi^0$ reaction, we observe in addition a very pronounced resonance-like energy dependence in the total cross section with a maximum 90 MeV below the $\mathrm{\Delta }\mathrm{\Delta }$ mass and a width of only 50 MeV, which is five times smaller than expected from a conventional $t$-channel $\mathrm{\Delta }\mathrm{\Delta }$ excitation. This reveals the ABC effect to be the consequence of a s-channel resonance with the formfactor of this dibaryonic state being reflected in the low-mass enhancement of the $\pi \pi$-invariant mass. From the fusion reactions to ${}^3$He and ${}^4$He we learn that this resonance is robust enough to survive even in nuclei.