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: $G_C\left(Q^2\right):G_M\left(Q^2\right):G_Q\left(Q^2\right)=(1-\frac{2}{3}\eta ):2:-1\mathrm{}$. These ratios hold at large spacelike or timelike momentum transfer in the case of composite systems such as the $\rho$ or deuteron in QCD with corrections of order $\frac{{\Lambda }_{\mathrm{QCD}}}{Q}$ and $\frac{{\Lambda }_{\mathrm{QCD}}}{M_{\rho ,d}}$. They are also the ratios predicted for the electromagnetic couplings of the $W^{\pm }$ for all $Q^2$ in the standard model at the tree level. In the case of the deuteron, the leading-twist perturbative QCD predictions are valid at $Q^2=\left|q^2\right|\gg {\Lambda }_{\mathrm{QCD}}{M}_d$, but do not require the kinematical ratio $\eta =\frac{Q^2}{4M_d^2}$ to be large. These results provide new all-angle predictions for the leading power behavior of the tensor polarization $T_{20}(Q^2,\theta )$ and the invariant ratio $\frac{B\left(Q^2\right)}{A\left(Q^2\right)}$. 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 $\mu =\frac{e}{M}$ and $Q=\frac{-e}{M^2}$ 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 $Z^0$ and $W^{\pm }$ 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.

• Received 17 March 1992

DOI:https://doi.org/10.1103/PhysRevD.46.2141