Weak transitions in the quasi-elastic reaction 12C(e, e′p) 11B

Abstract

In a high-resolution quasi-elastic 12C(e, e'p) 11B experiment several weak transitions have been observed to excited final states with spin and parity characteristic of direct knockout from orbitals above the 1p shell. The momentum distributions, which have been measured in parallel kinematics at an outgoing-proton energy of 70 MeV in the range of missing momentum −170 ⩽ p m ⩽ 210 MeV c, show the shape expected for a single-step knockout process. It is demonstrated that the interference between a direct-knockout process and a two-step process leading to the same final state in the (e, e'p) reaction may cause important modifications of the deduced spectroscopic factors. Explicit coupled-channels (CC) calculations show that the spectroscopic factor for the transition to the 7 2 − state at 6.743 MeV is reduced by a factor of 6, whereas the spectroscopic factors of the other weak transitions observed in the present experiment are uncertain by a factor of 2 due to CC-effects. Since the strength of these transitions is larger than can be explained by a pure two-step process, we interpret the observation of these transitions as direct evidence for the existence of ground-state correlations in 12C. The total spectroscopic strength in the E x region between 6 and 12 MeV amounts to 0.1, or 4.1 % of the observed strength for 1p knockout in the low E x region. Two peaks have been identified in the missing-energy spectrum that hitherto have not been reported: a narrow peak at E x = 9.82 (3) MeV with an l = 0 character and a broad structure centered at about 11.5 MeV with an l = 1 character. The missing-energy spectrum between E x = 12 and 24 MeV corresponding to 1 s 1 2 knockout has also been analyzed. The deduced momentum distribution shows evidence for the onset of a two-nucleon mechanism beyond the two-particle emission threshold.