The Transparency of 12C for Protons

TL;DR

This paper analyzes 12C(e,e'p) reaction data across various kinematic conditions, confirming earlier measurements, and investigates nuclear transparency and spectroscopic factors at high momentum transfer, revealing unexpected results and potential breakdowns of theoretical models.

Contribution

It provides a comprehensive analysis of 12C(e,e'p) data, confirming normalization, and offers new insights into nuclear transparency and spectroscopic factors at high Q^2, challenging existing theories.

Findings

Nuclear transparency for 12C is higher than previous estimates.

Spectroscopic factors depend on momentum transfer, indicating complex reaction mechanisms.

High Q^2 data suggest possible breakdown of quasi-particle concept.

Abstract

Existing cross-section data for 1p-shell knockout in the reaction 12C(e,e'p) - 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) 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) data taken at a four-momentum transfer squared 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 Q^2 data were used instead for an independent determination of the spectroscopic strength for 1p + 1s knockout. Combining these results with the low Q^2 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 quasi-particle concept at high Q^2 are discussed as well.