The role of the axial anomaly in measuring spin-dependent parton distributions

It is shown that forward matrix elements of j5μ, the flavor singlet axial vector current, do not merely measure the helicity carried by quarks and antiquarks but also include a spin-dependent gluonic component due to the anomaly. In perturbation theory the exact value of the gluonic component depends on the infrared regulator, and we argue that computing off-shell matrix elements with zero-mass quarks gives the proper method of regulation to exhibit the chiral properties of the theory. We suggest measuring two-jet production in deep inelastic scattering off polarized targets as a means of determining the spin-dependent gluonic component of j5μ. (orig.)