Data on single pion photoproduction below 1.5 GeV have been collected and are presented in the form of tables and graphs. For the most part, the compilation is non-critical; the numbers are those contained in the original papers, and data from different experiments have not been normalized so as to make them agree with each other as well as possible. However, in making polynomial fits to some of the angular distributions, certain data have been arbitrarily omitted. These omissions are listed in the introductory pages at the beginning of each section of graphs.
Photoproduction of pions from complex nuclei has become an investigative tool for (1) the detailed form of the elementary photopion amplitude, (2) the pion-nucleus optical potential, (3) nuclear structure, and (4) off-shell and medium effects on the elementary amplitude in nuclear processes. In this book, all these aspects are considered in detail. With improved experimental accuracy and beam tech- nology the study of nuclear pion photoproduction will break new ground and become an even more powerful investigative tool. This monograph is intended as an introductory guide as well as a reference manual for grad- uate students and researchers working in this important area of physics.
Measurements of the differential cross-section for the reaction gamma + p to pion ( - ) + pion (+) + p have been made at several angles for photon energies in the range 550-1000 MeV, using the Stanford Mark III linear accelerator. The pion( - ) were detected and momentum analyzed using a 90 degree magnetic spectrometer. It was found that the reaction was dominated by the pseudo-2-body photoproduction gamma + p to pion ( - ) + N* (1238) especially near the threshold for this process. At low momenta, the pion( - ) were identified by range. At high momenta (>250 MeV/c), the contaminating electrons were eliminated by using a lead and scintillation-counter sandwich system. The yields of both pion ( - ) and pion (+) off hydrogen were measured and the normalization was obtained by comparing with the known cross-sections for gamma + p to pion (+) + n. Computer calculations of the shapes of the yield curves expected from 2-body and 3-body production enabled the data to be separated into the two possible states gamma + p to pion ( - ) + N* (1238) and gamma + p to pion ( - ) + pion (+) + p. Angular distributions and total cross-sections are presented. (Author).
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 ?n --> ?-[p] process with a deuterium target and the singles measurement for the ?p --> ?+[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. The data exhibit a global scaling behavior for both ?- and ?+ photoproduction at high energies and high transverse momenta, consistent with the constituent counting rule and the existing ?+ 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 ?n --> ?-[p] to ?p --> ?+[n] process at ?cm = 90 deg start to show consistency with the prediction based on one-hard-gluon-exchange diagrams at high energies.
