Z-DIAGRAMS OF COMPOSITE OBJECTS

We examine the effect of particle compositeness on the importance of "Z diagrams", i.e., virtual particle-antiparticle states appearing in scattering processes. The examples of positronium in QED, and of the nucleon in the QCD-based quark model, are discussed in detail. Generally, if the composite particle consists of N constituents, its Z-diagram amplitude involves creation and annihilation of N constituent-anticonstituent pairs. This process (which we assume to be governed by Coulomb-type interactions with the "fine structure constant" alpha) must take place in a small volume approximately 1/M3, where M is the particle's mass; an additional suppression is due to the fact that the created system is electrically (or color) neutral. The composite particle's Z-diagram amplitude is then suppressed, compared to that for an elementary particle, by at least a factor f(z) approximately (q2/M2)[alpha-2/(M3R3)]N-1, where q is the momentum transfer to the particle, and R the composite particle's size. The decoupling of the composite Z diagrams at zero momentum transfer is consistent with low-energy theorems.