Polarized virtual photon structure function g₂^γ and twist-3 effects in QCD

We investigate twist-3 effects in the polarized virtual photon structure. The structure functions g₁^γ and g₂^γ of a polarized photon could be experimentally studied in future polarized ep or e⁺e^- colliders. The leading contributions to g₁^γ are twist-2 effects, while another structure function g₂^γ, which exists only for the virtual photon target, receives not only twist-2 but also twist-3 contributions. We first show that twist-3 effects actually exist in the box-diagram contributions and we extract the twist-3 part, which can also be reproduced by the pure QED operator product expansion. We then calculate the nontrivial lowest moment (n=3) of the twist-3 contribution to g₂^γ in QCD. For large N_c (the number of colors), the QCD analysis of twist-3 effects in the flavor nonsinglet part of g₂^γ becomes tractable and we can obtain its moments in a compact form for all n.