Probing the parton densities of polarized photons at a linear e+e− collider

The present theoretical status of spin-dependent parton densities ∆f(x, Q) of circularly polarized photons is briefly reviewed. It is then demonstrated that measurements of the deep-inelastic spin asymmetry Aγ1 ≃ gγ 1 /F γ 1 and of di-jet rapidity distributions at a future linear e e collider appear to be particularly suited for a determination the spin-dependent photonic quark and gluon densities, respectively. Special emphasis is devoted to a comparison of the different sources of polarized photons at a linear collider: the equivalent photon approximation and backscattered laser (Compton) photons. It is shown that backscattered laser photons are highly favorable, even indispensable, for decent measurements of the ∆f(x,Q).