The ABC Effect in the Double-Pionic Fusion to ⁴He.

Physikalisches Institut der Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen

^a e-mail: pricking@pit.physik.uni-tuebingen.de

Abstract

The double-pionic fusion to ⁴He was measured exclusively and kinematically complete with the WASA 4π detector at COSY for the reactions dd →⁴Heπ⁰π⁰ and dd →⁴Heπ⁺π⁻ . The measurements were performed over the full energy region, where the so-called ABC effect was observed previously in inclusive measurements. Whereas the latter exhibit not only a low-mass enhancement – the ABC-effect – in the ⁴He missing mass spectrum but also a prominent high-mass enhancement, we observe only a very strong low-mass enhancement and no pronounced high mass enhancement. This is in contrast to theoretical predictions based on t-channel ∆∆ excitation. Also the energy dependence of the observed total cross section does not fit to such predictions. Both features, however, agree very well with the observations in the most basic double-pionic fusion process in the isoscalar channel pn → dπ⁰π⁰.