Near-threshold photo- and electroproduction of neutral pions on protons

The state of the art of the problem on threshold neutral-pion production on protons by photons and electrons is examined. New experimental data on the threshold of pion production are analyzed within the phenomenological model based on the gauge invariance, the hypothesis of partial conservation of the axial-vector current, and allowance for explicit chiral-symmetry breaking. The amplitudes of the p-wave threshold multipoles and the photon asymmetry are derived for the pion photoproduction on protons with a photon energy of 159.5 MeV. The differential cross sections are calculated as functions of the pion emission angle and amplitudes of the s- and p-wave multipoles for the threshold pion electroproduction on protons at the squared momentum transfer k2 = 0.05 GeV2/c2. The total reaction cross sections are also obtained as functions of k2. The results obtained are compared with new experimental data and available theoretical predictions. © 2002 Plenum Publishing Corporation.