σ-channel threshold enhancement in double-pionic fusion

The double-pionic fusion process has been measured at CELSIUS-WASA for the first time exclusively and kinematically completely for the basic reaction pn → dπ0π0 at Tp = 1.04 GeV as well as for dp → 3He π0π0 and 3He π+π- at Tp = 0.9 GeV. In all cases we observe a huge low-mass enhancement in the σ-channel of the ππ invariant mass distribution. This enhancement at the ππ threshold is much larger than anticipated from previous inclusive measurements, accompanied with the excitation of a ΔΔ intermediate system, however, not accompanied with a high-mass enhancement as speculated previously. It points to a strong attractive interaction in the ΔΔ system, may be even to the formation of a bound state. Such an assumption leads to a quantitative description of the inclusive data on dd → 4He X, too. Exclusive data on the latter reaction will be provided by the analysis of the last WASA run at CELSIUS as well as by measurements at WASA rate at COSY. (orig.)