Relativistic quasipotential equations with u-channel exchange interactions

Various quasipotential two-body scattering equations are studied at the one-loop level for the case of tand u-channel exchange potentials. We find that the quasipotential equations devised to satisfy the one-body limit for the t-channel exchange potential can be in large disagreement with the fieldtheoretical prediction in the case of u-channel exchange interactions. Within the spectator model, the description of the u-channel case improves if another choice of the spectator particle is made. Since the appropriate choice of the spectator depends strongly on the type of interaction used, one faces a problem when both types of interaction are contained in the potential. Equaltime formulations are presented, which, in the light-heavy particle system corresponding to the mass situation of the πN system, approximate in a reasonable way the field-theoretical result for both types of interactions. Typeset using REVTEX