π+Photoproduction from Hydrogen at Laboratory Energies from 589 to 1269 MeV

Abstract

The differential cross section for the reaction $\gamma +p\to {\pi }^{+}+n$ was measured at 32 laboratory photon energies between 589 and 1269 MeV at the Caltech synchrotron. At each energy, data have been obtained at typically $15{\pi }^{+}$ angles between 6° and 90° in the center-of-mass (c.m.) system. A magnetic spectrometer was used to detect the ${\pi }^{+}$ photoproduced in a liquid-hydrogen target. Two Cerenkov counters were used to reject background of positrons and protons. The data clearly show the presence of a pole in the production amplitude due to one-pion exchange. Moravcsik fits to the angular distributions, including data from another experiment carried out by Thiessen, are presented. Extrapolation of these fits to the pole gives a value for the pion-nucleon coupling constant of 14.2±1.7, which is consistent with the accepted value. The "second" and "third" pion-nucleon resonances are evident as peaks in the total cross section and as changes in the shape of the angular distributions. At the third resonance, there is evidence for both a $D_{\frac{5}{2}}$ and an $F_{\frac{5}{2}}$ amplitude. The absence of large variations with energy in the 0° and 180° cross sections implies that the second and third resonances are mostly produced from an initial state with helicity $\frac{3}{2}$.