Spin-Dependent Structure Functions of Real and Virtual Photons
Abstract
The implications of the positivity constraint, |g_1^{\gamma(P^2)}(x,Q^2)| \leq F_1^{\gamma(P^2)}(x,Q^2), on the presently unknown spin--dependent structure function g_1^{\gamma(P^2)}(x,Q^2) of real and virtual photons are studied at scales Q^2\gg 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^{\gamma} and F_1^{\gamma} 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.