The production of mesons in electron-nucleon collisions is treated by using the method of dispersion relations. The result is analogous to that obtained by Chew, Low, Goldberger, and Nambu for photoproduction using the same method. Apart from the appearance of longitudinal and scalar components of the virtual photon polarization, the important difference is that charge and magnetic moments have to be multiplied by the appropriate nucleon electromagnetic form factors. Two formulas are given for the scattering amplitude: one is a static approximation which is valid if the electron invariant momentum transfer X is less than 500 Mev; the other takes account of the recoil effects, and may be trusted within 30% even up to A^4 Bev. It is suggested that the measurement of the energy and angular distribution of the electron can offer an alternative method for the determination of the nucleon form factors.

The processpp → e − e + is considered in the general case of polarized initial particles. A relation between the difference of the phases of the electromagnetic form factors G M and G E in the time-like region and measurable asymmetries is derived. It is shown that the moduli of the form factors can be determined from measurements of the total unpolarized cross section and of the integral asymmetry for longitudinally polarized (or transversely polarized) p and p. The behaviour of the proton form factors at high q 2 in the time-like region is also discussed. From the Phragmén-Lindelöf's theorem it follows that the asymptotical behaviour of the form factors in the space-like and time-like regions must be the same. An analysis of experimental data in both regions based on perturbative QCD is presented.⋆ Bitnet: GIUNTI@TORINO.INFN.IT

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