D.R. Botterill

Threshold π+ electroproduction at high-momentum transfer: A determination of the nucleon axial vector form factor

Abstract Threshold π+ electroproduction has been measured at momentum transfers ‖k2‖ of 0.45, 0.58 and 0.88 (GeV/c)2, extending the range of earlier measurements. Using PCAC and current algebra-based models we have deduced the axial vector form factor of the nucleon GA(k2), and find that the dipole parametrisation is favoured over the monopole. The value of MA in the dipole parametrisation is, in a weak PCAC model, 0.96 ± 0.03 GeV, in excellent agreement with neutrino measurements. The threshold cross section is also in excellent agreement with the predictions of a fixed-t dispersion relation model.

The total cross section for π+ electroproduction on hydrogen near threshold

Abstract The total cross section for the reaction ep → enπ+ at momentum transfers |k2| between 0.08 and 0.32 ( GeV c )2 and for centre of mass energies between threshold, at W = 1.078 GeV, and W = 1.106 GeV has been measured by detecting the electron and neutron in coincidence. Comparisons are made with both current algebra and dispersion relation theoretical models.

A measurement of the azimuthal angle dependence of the process ep → enπ+ near threshold☆

Abstract The azimuthal angle dependence of the process ep → en π + has been measured by detecting the electron and neutron in coincidence. Data are presented for momentum transfers (− k 2 ) between 0.08 and 0.32 ( GeV c ) 2 and at energies close to threshold. Fixed- t dispersion relation models give predictionsin good qualitative agreement with the data.

Elastic electron-proton scattering between 0.05 and 0.30 (GeV/c)2

Abstract Elastic electron proton scattering has been used to check the validity of the dipole fit of the proton form factors at momentum transfer between 0.05 and 0.30 (GeV/ c ) 2 . The general behaviour of the cross sections is in agreement with previous measurements and is close to the dipole predictions but there is the suggestion of some small amplitude deviations. It is speculated that these deviations may be related to similar effects in the proton formfactor derived from the ISR pp elastic scattering data via a Chou-Yang model.

The pion form factor at a timelike momentum transfer of approximately 1 fm−2 from π+ electroproduction at threshold

Abstract From an extrapolation of threshold π+ electroproduction data combined with a kinematical constraint at an unphysical point, we derive the value of the pion form factor at a time-like value of the momentum transfer k2π ≅ 1 fm−2. The result is Fπ(k2π) = 1.19 ± 0.1. From this value, using a twice subtracted dispersion relation, we obtain r π ⪆ 0.98 ± 0.24 fm .

Threshold π0 electroproduction on hydrogen at −k2 = 0.88 (GeV/c)2

Abstract We have measured pπ0 electroproduction at −k2 = 0.88 (GeV/c)2 near threshold. Comparison with models shows that a ‘weak PCAC’ model, a fixed-t dispersion relation model and a pseudo-vector-Born approximation all adequately describe the small threshold cross section. Away from threshold the cross section dependence suggests that the pseudo-vector-Born approximation is not adequate.