Electroproduction of neutral pions on the proton

Abstract

The double-differential coincidence cross section for the reaction 1H(e,e′π 0)p has been measured at Q 2 = 0.1 (GeV/ c) 2 for a range in the invariant energy W of 1 to 14 MeV above threshold in order to determine the s-wave multipoles E 0+ and L 0+. The experiment was performed with a 525 MeV electron beam of the Amsterdam Pulse Stretcher and a cryogenic hydrogen target. The scattered electron and the recoiling proton were detected in two high-resolution magnetic spectrometers. By using a recent LET prediction for the relevant combinations of p-wave multipoles, values for E 0+ and L 0+ were extracted from the cos ϑ π ∗ and cos φ π ∗ dependent terms in the cross section. The data are compared to calculations from Chiral Perturbation Theory and from several Lagrangian models. All models give a fair to good description of L 0+. ChPT gives also a good description of E 0+, but the Lagrangian models tend to underestimate its value. The data for the angle-independent part of the cross section indicate that the value for the 2 M 1+ + M 1− combination of p-wave multipoles is overestimated by a recent ChPT prediction.