Entanglement of Fock-space expansion and covariance in light-front Hamiltonian dynamics

We investigate in a model with scaler "nucleons" and mesons the contributio ns of higher Fock states that are neglected in the ladder approximation of the Lippmann-Schwinger equation. This leads to a breaking of covariance, bo th in light-front and in instant;form Hamiltonian dynamics. The lowest Fock sector neglected has two mesons in the intermediate state and corresponds to the stretched box. First we show in a simplified example that the contri butions of higher Fock states are much smaller on the light-front than in i nstant-form dynamics. Then we show for a scattering amplitude above thresho ld that the stretched boxes are small, however necessary to retain covarian ce. For an off-energy-shell amplitude covariance is not necessarily maintai ned and this is confirmed by our calculations. Again, the stretched boxes a re found to be small. [S0556-2813(98)06412-7].