Bonn 2010 – wissenschaftliches Programm

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HK: Fachverband Physik der Hadronen und Kerne

HK 25: Hadronenstruktur und -spektroskopie V

HK 25.5: Vortrag

Dienstag, 16. März 2010, 17:30–17:45, HG III

The ABC Effect in the Double-Pionic Fusion to 4He from exclusive dd4Heππ measurements* — •Annette Pricking — Physikalisches Institut, Univ. Tübingen and HISKP, Bonn

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 Td = 0.8 - 1.4 GeV. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions.

As a result we find a huge low-mass enhancement in the ππ-invariant mass in agreement with previous measurements. However, we do not observe a pronounced high-mass enhancement, which is seen in the inclusive data and predicted in conventional ΔΔ calculations. This finding is in accordance with the observations in the basic pnd π0π0 reaction. In both reactions we observe a resonant energy dependence of the total cross section, tentatively attributed to a "ABC resonance", which apparently is robust enough to survive even in nuclei.

* supported by BMBF, COSY-FFE, DFG (Eur. Graduate School)

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