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: G(C)(Q2): G(M)(Q2) : G(Q)(Q2) = (1-2/3-eta) : 2 : -1. 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 LAMBDA(QCD)/Q and LAMBDA(QCD)/M(rho,d). They axe 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\ >> LAMBDA(QCD)M(d), but do not require the kinematical ratio eta = Q2/4M(d)2 to be large. These results provide new all-angle predictions for the leading power behavior of the tensor polarization T20(Q2,theta) 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 mu = e/M and Q = -e/M2 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.