Electromagnetic Properties of Few-Body Systems Within a Point-Form   Approach
  We use a Poincare-invariant coupled-channel approach based on point-form relativistic quantum mechanics to investigate the electromagnetic properties of two-body bound systems with spin 0 and spin 1. Elastic scattering of an electron by the bound state is treated as a two-channel problem for a Bakamjian-Thomas-type mass operator. In this way retardation effects in the photon-exchange interaction are fully taken into account. The electromagnetic bound-state current is extracted from the one-photon-exchange optical potential. Wrong cluster properties, inherent in the Bakamjian-Thomas framework for more than 2 particles, are seen to cause spurious (unphysical) contributions in the current, which are associated with a dependence on the sum of the electron momenta. The Lorentz structure of our current resembles the one obtained from an explicitly covariant light-front approach, where spurious contributions also show up and are associated with a four-vector describing the orientation of the light front. For spin-0 systems, like a charged pion, the spurious contributions can be eliminated by choosing the total invariant mass of the electron-bound-state system large enough. In this case equivalence with the usual front-form expression for the form factor, resulting from a spectator current in the q^+=0 reference frame, is established. For spin-1 systems, like a charged rho meson or the deuteron, some spurious contributions cannot be completely eliminated by solely choosing an infinitely large invariant mass. Nevertheless, there is an unambiguous way how to separate them from the physical contributions such that one is left with a physical bound-state current with the required properties. 
Elmar P. Biernat
nucl-th