Transparency of ¹²C for protons

Existing cross-section data for 1p-shell knockout in the reaction ¹²C(e,e′ p)¹¹B - as obtained under different kinematic conditions - are shown to be mutually consistent, apart from a recent measurement performed in Mainz. New data have been collected at the Amsterdam Pulse Stretcher that confirm the normalization of the older measurements. An analysis of the world's ¹²C(e,e′ p)¹¹B data has yielded precise values of the spectroscopic factor for 1p-shell and 1s-shell knockout from ¹²C. These values have been used to evaluate the transparency of the ¹²C nucleus for 1p-shell and 1s-shell protons separately on the basis of recent high-energy ¹²C(e,e′ p)¹¹B data taken at a four-momentum transfer squared Q² of 1.1 (GeV/c)². As the resulting average value of the nuclear transparency, 0.81±0.04, is considerably higher than the value obtained from previous analyses and theoretical estimates, the high Q² data were used instead for an independent determination of the spectroscopic strength for 1p+1s knockout. Combining these results with the low Q² data the spectroscopic factors appear to be momentum-transfer dependent. Possible explanations of these surprising results in terms of reaction-mechanism effects or a possible breakdown of the quasiparticle concept at high Q² are discussed as well.