Experimental and theoretical study of pions electroproduction near threshold. Axial and pseudo-scalar form factors measurement

The cross sections of the positive pion electroproduction near the threshold have been measured with the final goal of determining the axial and pseudoscalar form factors of the nucleon. The electron beams of 300 to 680 MeV of the Saclay Linear Accelerator (ALS) have been bombarded onto a liquid hydrogen target. With this energy range of the electron beams, three values of the squared 4 momentum transfer, 0.070, 0.136 and 0.195 GeV squared have been explored. The scattered electron and the produced pion have been detected in coincidence with the Spectrometer 600 and the Pion Spectrometer of experimental hall 1 of ALS. The coincidence pion production has been identified using the time of flight and the missing mass. The effective solid angle of the detector System was determined with a Monte Carlo simulation including the radiative corrections and the detector efficiencies. Finally, we got cross sections with maximum statistical error of about 5 per cent. These cross sections have been interpreted using the low energy theorem to determine the form factors. The corrections necessary in this interpretation have been calculated using a bag model for the nucleon. After usual dipole parameterization for the axial form factor, we get axial mass 1.15 GeV. The pseudoscalar form factor, measured for the first time for these momentum transfers, has been found compatible with the PCAC prediction. (author)