Bonn 2010 – wissenschaftliches Programm
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 dd → 4Heππ 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 dd → 4He π0π0 and dd → 4He 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 pn → d 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)