Alpha-cluster knockout in the 16O(e,e′α)12C reaction

The knockout of α-clusters induced by electrons has been used to study four-nucleon correlations in 16O. For the transition to the ground state of 12C the (e,e′α) cross section has been measured for three values of the momentum transfer and four values of the missing momentum. The momentum-transfer dependence gives evidence for a quasi-elastic α-knockout mechanism. The shape of the measured missing-momentum distribution is reasonably well described by shell-model and cluster-model calculations, but the theoretical curves overshoot the data by a factor three to four.