Electroproduction of single positive pions above the resonance region

In this letter we report results of measurements of the reaction e § above the resonance region for k ~ (four-momentum transfer squared to the proton) equal to 0.7 (GeV/c) ~ and a 7:+n invariant mass s 89 equal to 1.9 GeV/c ~. These results are part of a preliminary programme of coincidence electroproduction measurements to establish the contribution of different hadronie states to the deep continuum crosssection. The results are fitted with fixed-t dispersion relation calculations of DEVENISH and LYTH (1) which have shown the cross-sections to be very sensitive to the electromagnetic charge form factor of the pion. From these fits a value is deduced for the pion form factor at this value of k 2. ENGELS et al. (3) have shown that similar fixed-t dispersion relation calculations fit measurements of y q p + n + r : + at our value of s, with their calculation demonstrating the importance of the Born terms, in particular the pionpole term. The importance of the pion-pole term in eleetroproduction has also been remarked on by BERENDS (3) and by MANWEILER and SCHMII)T (4). The experinient was performed at the Daresbury electron synchrotron using a 4.5 GeV extracted beam with a typical intensity of 2-1012 electrons/s and a duty ratio between 5 and 10%. The electron beam was focused into a 7.5 cm liquid-hydrogen target with a spot size typically {0.2x0.1)em 2, and divergence (2• 2)mrad% The beam intensity was continuously monitored with a 17-layer secondary emission monitor, which had been calibrated relative to a Faraday cup. An identical dummy target cell was used to determine the empty-target contribution to the scattering. The scattered electron and produce4 =+-meson were detected in two similar magnetic spectrometers, each consisting of two half-quadrupoles and a bending magnet. Scintillation counter hodoscopes were used to measure the momentum and production angles of the produced particles. The electron spectrometer had a momentum acceptance of =~ 5% and a