Encounters with di-baryons - from the ABC effect to a new resonance?

The ABC effect, an intriguing low-mass enhancement in the $\pi\pi$ invariant mass spectrum, is known from inclusive measurements of two-pion production in nuclear fusion reactions to the few-body systems d, $^{3}$He and $^{4}$He. It was first observed 1960 by Abashian, Booth and Crowe in the inclusive pd $\to$ $^{3}$He X reaction. Its explanation has been a puzzle since then. In an effort to solve this long-standing problem by exclusive and kinematically complete high-statistics experiments, we have measured the fusion reactions to d, $^{3}$He and $^{4}$He with WASA-at-COSY. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions. In a recent kinematically complete measurement of the $pn \to d\pi ^{0}\pi ^{0}$ reaction we have shown that the ABC effect in this basic double-pionic fusion reaction is correlated with a narrow structure in the total cross section with quantum numbers I(J$^{P}$) = 0(3$^{+}$), a mass of 2.37 GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 times the mass of $\Delta$, the mass of a $\Delta\Delta$ system, and the width is three times narrower than expected from a conventional t-channel $\Delta\Delta$ process. In the double-pionic fusion reaction to the helium isotope $dd \to ^{4}$He$\pi ^{0}\pi ^{0}$ again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From a previous exclusive experiment at CELSIUS-WASA it is known that the double-pionic fusion to $^{3}$He also exhibits a pronunced ABC effect. New data from COSY on the $pd \to ^{3}$He$\pi ^{0}\pi ^{0}$ reaction scanning the full ABC region are presented as well as the status of measurements in other reaction channels, where the new resonance might contribute