Pion electroproduction in the low-energy region

Abstract

We saw that electroproduction experiments in the resonance region can, in principle, give information on (γ N N*)-transition form factors, the pion form factor F _π, the neutron electric form factor G _e ⁿ and on the axial nucleon form factor G _A. At present, electroproduction is the only practical source of information on Fπ in the spacelike region and on the transition form factors. However, several difficulties are involved:

Most easy is the determination of the pion form factor and of the dominant (γNδ)-form factor C ₃. The form factors for the non-dominant transitions to the δ(1236), C ₄ and C ₅, are already difficult to define because these transitions have a quite unsymmetrical shape of their energy dependence. For the determination of G _e ⁿ electroproduction does not seem to be much superior to electron-deuteron scattering because unknown dispersion effects make the extraction of G _e ⁿ from electroproduction data ambiguous.

Of fundamental importance is the clear-cut time-reversal test in electroproduction which was described in Sec. II.7.

The theoretical models which aim to predict the λ²-dependence of the (γNδ)-transition form factors showed that it is crucial to include assumptions on the short-range forces in order to get the observed λ²-dependence of M ₁₊ ^3/2.

The detailed prediction of the angular distributions from fixed-t-dispersion relations agrees reasonably well with the data. When more experimental data from coincidence experiments (possibly for different values of ɛ) become available it will be interesting to see whether the λ²-dependences of the different multipoles are predicted correctly.

The author is grateful to Drs. F. Brasse, J. Drees, F. Gutbrod, D. Schwela, C. Verzegnassi and H. Wessel for interesting discussions. Thanks are also due to H. Wessel for calculating the angular distributions shown in Fig. 12.