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)=(1-2/3η):2:-1. These ratios hold at large spacelike or timelike momentum transfer in the case of composite systems such as the ρ or deuteron in QCD with corrections of order ΛQCDQ and ΛQCDM_ρ,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|ΛQCDMd, but do not require the kinematical ratio η=Q²4Md² to be large. These results provide new all-angle predictions for the leading power behavior of the tensor polarization T₂₀(Q²,θ) 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 μ=e/M 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.