Münster 2011 – wissenschaftliches Programm
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HK: Fachverband Physik der Hadronen und Kerne
HK 24: Hadronenstruktur und -spektroskopie VI
HK 24.1: Gruppenbericht
Dienstag, 22. März 2011, 16:30–17:00, C-2
The ABC Effect in the Double-Pionic Fusion to Light Nuclei - Do We Observe a Resonance Phenomenon?* — •Annette Pricking — Physikalisches Institut, Univ. 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 now 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. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions.
As a result we find in all cases 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.
Most intriguing is our experimental finding of a resonance-like energy dependence of the total cross section in the ABC region. From the angular distributions for the basic fusion reaction to the deuteron we infer I(JP) = 0(3+) for this structure with a width of only 70 MeV. From the fusion reactions to the He-isotopes we infer that this structure is apparently robust enough to survive even in nuclei. Model calculations for this scenario are in agreement with the data.
* supported by BMBF, COSY-FFE, DFG (Eur. Graduate School)