Universal properties of the electromagnetic interactions of spin-one systems.

Abstract

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: {ital G}{sub {ital C}}({ital Q}{sup 2}):{ital G}{sub {ital M}}({ital Q}{sup 2}):{ital G}{sub {ital Q}}({ital Q}{sup 2})=(1{minus}2/3{eta}):2:{minus}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. They are also the ratios predicted for the electromagnetic couplings of the {ital W}{sup {plus minus}} for all {ital Q}{sup 2} in the standard model at the tree level. In the case of the deuteron, the leading-twist perturbative QCD predictions are valid at {ital Q}{sup 2}={vert bar}{ital q}{sup 2}1{much gt}{Lambda}{sub QCD}{ital M{ital d}}, but do not require the kinematical ratio {eta}={ital Q}{sup 2}/4{ital M}{sub {ital d}}{sup 2} to be large. These results provide new all-angle predictions for the leading power behavior of the tensor polarization {ital T}{sub 20}({ital Q}{sup 2},{theta}) and the invariant ratio {ital B}({ital Q}{sup 2})/{ital A}({ital Q}{sup 2}). 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}={ital e}/{ital M} and {italmore » Q}={minus}{ital e}/{ital M}{sup 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 {ital Z}{sup 0} and {ital W}{sup {plus minus}} vector bosons. Simple gauge-invariant and -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.« less