GAUGE-INDEPENDENT CONTRIBUTIONS TO THE AMPLITUDE OF ELASTIC ELECTRON-SCATTERING BY NUCLEI (SYSTEMS OF BOUND PARTICLES)

The problem of ensuring fulfillment of the requirement that the amplit ude of elastic electron scattering hv nuclei (bound systems) be indepe ndent of the choice of the gauge of the electromagnetic field is studi ed. On the basis of Ward-Takahashi identities and their extension, ope rator conditions are formulated for breaking down the amplitude into t wo gauge-invariant parts: a one-particle contribution that is calculat ed in the impulse approximation and multiparticle contributions that a re due to meson-exchange currents (interaction currents). The Bethe-Sa lpeter formalism is used to show that, in the case of ed and en scatte ring, these conditions are satisfied for fermion-fermion interactions of the ladder and separable types. The pion (<q(q)over bar> pair) char ge form factor is considered in a certain extension of the Nambu-Iona- Lasinio model.