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Bochum 2009 – scientific programme

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HK: Fachverband Hadronen und Kerne

HK 46: Hadron Structure and Spectroscopy I

HK 46.4: Talk

Wednesday, March 18, 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 dd4He π0π0 and dd4He 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 pnd 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

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