The ABC Effect in the Double-Pionic Fusion to 4He.
Physikalisches Institut der Universität Tübingen,
Auf der Morgenstelle
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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.
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