Hard photo-disintegration of proton pairs in 3He nuclei

Abstract

Extensive studies of high-energy deuteron photodisintegration over the past two decades have probed the limits of meson-baryon descriptions of nuclei and nuclear reactions. At high energies, photodisintegration cross sections have been shown to scale as a power law in s (the total cm energy squared), which suggests that quarks are the relevant degrees of freedom. In an attempt to more clearly identify the underlying dynamics at play, JLab/Hall A experiment 03-101 measured the hard photodisintegration of {sup 3}He into p-p and p-d pairs at θ{sub c.m.} = 90◦ and E{sub {gamma}} = 0.8 - 4.7 GeV. The basic idea is that the measurement should be able to test theoretical predictions for the relative size of pp versus pn disintegrations. This document presents data for the energy dependence of the high energy 90◦ c.m. photodisintegration of {sup3]He: dσ/dt(γ + {sup3}He → p + p + n{sub spectator}), and dσ/dt(γ + {sup 3}He → p + d). The cross sections were observed to scale as a function of s{sup −n} where n was found to be 11.1±0.1 and 17.4±0.5 for the two reactions respectively. The degree of scaling found for d#27;{sigma}/dt (γ + {sup 3}He → p + d) is the highest degree of scaling ever observed in a nuclear process. The onset of the observed scaling are at photon energy of 2.2 GeV for the pp breakup and 0.7 GeV for the pd breakup. The magnitude of the invariant cross section for pp pair breakup was found to be dramatically lower than for the breakup of pn pairs and theoretical predictions. At energies below the scaling region, the scaled cross section was found to present a strong energy-dependent structure not observed in the pn breakup. The data indicate a transition from three-nucleon hadronic photodisintegration processes at low energies to two-nucleon quark-dominated photodisintegration processes at high energies.