Properties of few-body systems in relativistic quantum mechanics and constraints from transformations under Poincaré space-time translations

5 pages, to be published in the proceedings of the 18th International Conference on Few-Body Problems in Physics (Santos, Sao Paulo, Brasil)International audienceDifferent approaches have been applied to the calculation of form factors of various hadronic systems within relativistic quantum mechanics. In a one-body current approximation, they can lead to results evidencing large discrepancies. Looking for an explanation of this spreading, it is shown that, for the largest part, these discrepancies can be related to a violation of Poincaré space-time translation invariance. Beyond energy-momentum conservation, which is generally assumed, fulfilling this symmetry implies specific relations that are generally ignored. Their relevance within the present context is discussed in detail both to explain the differences between predictions and to remove them