Determination of Proton-Form Factors Derived from Electron-Proton- Scattering

Abstract

Following a series of outstanding successes of QED, in other words the interaction of the electromagnetic field with the field of electrons and positrons, achieved during the last years of the forties, scientists began to describe the electromagnetic qualities of strong-interacting particles in connection with this new formalism. Especially striking and demonstrative was the calculation of the scattering of electrons on protons. This process, in analogy to the Miller-scattering, was attributed to the interaction of the charge and the magnetic moments, both of which are assumed as a point, with the external electromagnetic field A _µ ^(e) (x) (the so-called Møller-potential) of the scattered electrons. In the course of the experiments it was found that calculated cross sections for momentum transfers of several Fermi⁻¹ (1f ⁻¹ = 200 MeV) were decidedly greater than previous experimental values. The cause of such a discrepancy is to be found in the fact that in the matrix element

$$\int {{d^4}} x{J^\mu }\left( X \right)A_\mu ^{\left( e \right)}\left( X \right)$$

we have substituted the QED expression i.e. the expression for a point particle for the proton-current J ^µ (x). Thus it was essential to consider both the anomalous magnetic moment of the proton and an apparent spatial extension of charge and moment. It was possible to give a clear and instructive interpretation of these corrections in the following explanations: It has been established that resulting from the strong coupling of nucleons with other strong interacting particles the nucleon is surrounded by a cloud of such virtual particles which are usually identified with known mesons.