Spin-Dependent Structure Functions of Real and Virtual Photons

The implications of the positivity constraint, |g γ(P 2) 1 (x,Q 2)| ≤ F γ(P 2) 1 (x,Q 2), on the presently unknown spin–dependent structure function g γ(P 2) 1 (x,Q 2) of real and virtual photons are studied at scales Q2 ≫ P 2 where longitudinally polarized photons dominate physically relevant cross sections. In particular it is shown how to implement the physical constraints of positivity and continuity at P 2 = 0 in NLO calculations which afford a nontrivial choice of suitable (DIS) factorization schemes related to g 1 and F γ 1 and appropriate boundary conditions for the polarized parton distributions of real and virtual photons. The predictions of two extreme ‘maximal’ and ‘minimal’ saturation scenarios are presented and compared with results obtained within the framework of a simple quark ‘box’ calculation expected to yield reasonable estimates in the not too small regions of x and P 2.