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HK: Fachverband Hadronen und Kerne
HK 46: Hadron Structure and Spectroscopy I
HK 46.4: Vortrag
Mittwoch, 18. März 2009, 15:00–15:15, H-ZO 20
First Results on the ABC Effect from Kinematically Complete Measurements of the Double-Pionic Fusion to 4He* — •Annette Pricking — Physikalisches Institut der Universität Tübingen
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 to the few-body systems d, 3He and 4He. Its explanation has been a puzzle for 50 years.
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, 3He and 4He with WASA at COSY. Here we report on the measurements of the double-pionic fusion reactions dd → 4He π0π0 and dd → 4He π+π−, which have been carried out at nine beam energy settings in the range Tp = 0.8 - 1.4 GeV covering thus the full energy region, where the ABC effect has been observed previously in inclusive reactions.
As a first result we find a strong low-mass enhancement in the ππ-invariant mass in agreement with previous results. However, we do not observe the high-mass enhancement suggested by the inclusive data and predicted in conventional t-channel ΔΔ calculations.But this finding is in support of a dibaryonic s-channel resonance as it is observed for the basic pn → d π0π0 reaction. Further results from the analysis of these reactions will be discussed, in particular also with respect to the energy dependence of the observed ABC effect. * supported by BMBF, COSY-FFE, DFG (Eur. Graduate School) and Wallenberg Foundation