Narrow Results

The pp→ppγγ reaction has been measured at CELSIUS using the WASA 4π-detector with hydrogen pellet target. At the current status of analyses of measurements regarding beam energies between T_p=1.20 and 1.36 GeV a narrow structure is observed around the ππ threshold in the invariant γγ mass spectrum M_γγ. The angular dependence of this structure is compatible with a radiative decay originating from the σ channel.

After examination of the correlated two-pion exchange in the NN interaction in terms of chiral pion loops in the t-channel, we consider loops in the s-channel, which get lifted on mass shell in the two-pion production process. We show that the thus obtained dynamical correlations in the σ channel lead to a quantitative description of recent exclusive measuerements near threshold.

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA@COSY for the fusion reactions leading to d, ³He and ⁴He. They all reveal this effect to be a σ channel phenomenon combined with a resonance-like behavior in the total cross section. The latter points to the formation of an isoscalar dibaryonic resonance, which couples to the pn channel as well as to a ΔΔ intermediate state.