Exclusive Photoproduction of Charged Pions in Hydrogen and Deuterium from 1 to 6 GeV

Abstract

The study of the transition region in the description of exclusive processes and hadron structure, from the nucleon-meson degrees of freedom in meson-exchange models at low energy to the quark-gluon degrees of freedom in pQCD at high energy, is essential for us to understand the strong interaction. The differential cross section measurements for exclusive reactions at fixed center-of-mass angles enable us to investigate the constituent counting rule, which explicitly connects the quark-gluon degrees of freedom to the energy dependence of differential cross sections. JLab Experiment E94-104 was carried out in Hall A with two high resolution spectrometers. It included the coincidence cross section measurement for the [gamma]n --> pi{sup -}[p] process with a deuterium target and the singles measurement for the [gamma]p --> pi{sup +}[n] process with a hydrogen target. The untagged real photons were generated by the electron beam impinging on a copper radiator. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to the center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles were fixed at 50 deg, 70 deg, 90 deg, and also 100 deg, 110 deg at a few energies. The JLab E94-104 data presented in this thesis contain four interesting features. Themore » data exhibit a global scaling behavior for both [pi]{sup -} and [pi]{sup +} photoproduction at high energies and high transverse momenta, consistent with the constituent counting rule and the existing [pi]{sup +} photoproduction data. This implies that the quark-gluon degrees of freedom start to play a role at this energy scale. The data suggests possible substructure of the scaling behavior, which might be oscillations around the scaling value. There are several possible mechanisms that can cause oscillations, for example the one associated with the generalized constituent counting rule involving quark orbital angular momentum. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV, where baryon resonances are not as well known as those at low energies. The differential cross section ratios for exclusive [gamma]n --> pi{sup -}[p] to [gamma]p --> pi{sup +}[n] process at [theta]{sub cm} = 90 deg start to show consistency with the prediction based on one-hard-gluon-exchange diagrams at high energies.« less