EXPLICITLY COVARIANT LIGHT-FRONT DYNAMICS AND RELATIVISTIC FEW-BODY SYSTEMS

The wave function of a composite system is defined in relativity on a space-time surface. In the explicitly covariant light-front dynamics, reviewed in the present article, the wave functions are defined on the plane omega . x = 0, where w is an arbitrary four-vector with omega(2 ) = 0. The standard non-covariant approach is recovered as a particula r case for omega = (1, 0, 0, - 1). Using the light-front plane is of c rucial importance, while the explicit covariance gives strong advantag es emphasized through all the review. The properties of the relativist ic few-body wave functions are discussed in detail and are illustrated by examples in a solvable model. The three-dimensional graph techniqu e for the calculation of amplitudes in the covariant light-front pertu rbation theory is presented. The structure of the electromagnetic ampl itudes is studied. We investigate the ambiguities which arise in any a pproximate light-front calculations, and which lead to a non-physical dependence of the electromagnetic amplitude on the orientation of the light-front plane. The elastic and transition form factors free from t hese ambiguities are found for spin 0, 1/2 and 1 systems. The formalis m is applied to the calculation of the relativistic wave functions of two-nucleon systems (deuteron and scattering state), with particular a ttention to the role of their new components in the deuteron elastic a nd electrodisintegration form factors and to their connection with mes on exchange currents. Straightforward applications to the pion and nuc leon form factors and the p - pi transition are also made. (C) 1998 El sevier Science B.V. All rights reserved.