Universal properties of the electromagnetic interactions of spin-one systems

The dominance of helicity-conserving amplitudes in gauge theory is shown to imply universal ratios for the charge, magnetic, and quadrupole form factors of spin-one bound states: GC(Q2):GM(Q2):GQ(Q2)=(1-23):2:-1. These ratios hold at large spacelike or timelike momentum transfer in the case of composite systems such as the or deuteron in QCD with corrections of order QCDQ and QCDM,d. They are also the ratios predicted for the electromagnetic couplings of the W for all Q2 in the standard model at the tree level. In the case of the deuteron, the leading-twist perturbative QCD predictions are valid at Q2=|q2|QCDMd, but do not require the kinematical ratio =Q24Md2 to be large. These results provide new all-angle predictions for the leading power behavior of the tensor polarization T20(Q2) and the invariant ratio B(Q2)A(Q2). We also use a generalization of the Drell-Hearn-Gerasimov sum rule to show that the magnetic and quadrupole moments of any composite spin-one system take on the canonical values =eM and Q=-eM2 in the strong binding limit of the zero bound-state radius or infinite excitation energy. This allows new empirical constraints on the possible internal structure of the Z0 and W vector bosons. Simple gauge-invariant and Lorentz-covariant models and null zone theory are used to illustrate these results. Complications that arise when the Breit frame is used for form-factor analyses are also pointed out.