Hard photodisintegration of 3 He into p - p , p - n , and p - d pairs

Extensive studies of high-energy deuteron photodisintegration over the past two decades have probed the limits of meson-baryon descriptions of nuclei and reactions. At high energies, photodisintegration cross sections have been shown to follow the constituent counting rules, which suggests that quarks are the relevant degrees of freedom. In an attempt to more clearly identify the underlying dynamics at play in high-energy nuclear photodisintegration, E03-101 measured, in 2007, the hard photodisintegration of two protons, using He. The basic idea is that theoretical models should be able to predict the relative size of pp versus pn disintegrations. The results from E03-101 clearly indicate the onset of scaling; but due to the cross section being smaller than expected, the statistics were insufficient to determine the underlying mechanism that produces high transverse momentum proton pairs. Preliminary data on a two body hard breakup of He to p-d pairs are also available from that experiment over a small s-range. We propose here a second generation measurement of hard photodisintegration of He into np, pp, and pd pairs. The experiment is aimed at addressing issues the previous measurements and current theory are insufficient to resolve: 1. A measurement of both γ He → pp(n) and γ He → pn(p) will address the issue of the small ratio of γ He → pp(n) to γ d → pn. 2. High quality data over a large s-range on the two body break-up of γ He → pd will confirm the scaling of the invariant cross section as a function of s and determine the power with a small uncertainty. 3. Higher statistics for γ He → pp(n) will provide a conclusive check to determine if hard re-scattering is the correct dynamical treatment to explain the production of high PT nucleons as proposed. The experimental hall beam time request is for 19 days. No new equipment and no special setup or development time are required. This measurement does not need the 12 GeV upgrade but JLab is the only facility world wide at which such a measurement can be done.