Covariant evolution operator in composite models of quantum field theory

An approach to describing the interaction of composites is proposed in which the key role is played by the covariant Schrödinger equation and by the state evolution operator between two arbitrary space-like surfaces following from this equation. A representation appropriate for developing perturbation theory is found for the operator of the state momentum transformation. The form factor is used to show how the scattering amplitude of composites can be expressed through the wave functions of a bound state at rest. The simplicity of the proposed covariant approach in applications is demonstrated by calculating the asymptotic behaviour of the form factor taking into account logarithmic corrections.