For many purposes it is necessary to have a better description for the interaction of two particles than the perturbation theory we have discussed so far. This is particularly so if we deal with bound states: the particles stay together infinitely long and they can therefore interact arbitrarily often as depicted in Fig. 6.1. It is clear that this situation cannot be described by the summation of a few Feynman diagrams. If one of the particles is much heavier than the other the problem can be considerably simplified: one solves the Dirac equation for the light particle with an additional external potential, which is produced by the heavier particle. Because of the large mass ratio the influence of the lighter particle on the heavier one can be neglected.
KeywordsDirac Equation Dirac Operator Momentum Space Ladder Approximation Interaction Kernel
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