EPJ

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EPJ C - Particles and Fields
Particles and Fields


Eur. Phys. J. C 20, 271-281
DOI: 10.1007/s100520100677

Spin-dependent structure functions of real and virtual photons

M. Glück, E. Reya and C. Sieg

Universität Dortmund, Institut für Physik, 44221 Dortmund, Germany

(Received: 16 March 2001 / Published online: 18 May 2001 -© Springer-Verlag / Società Italiana di Fisica 2001)

Abstract
The implications of the positivity constraint, $\vert g_1^{\gamma(P^2)}(x,Q^2)\vert \leq F_1^{\gamma(P^2)}(x,Q^2)$, 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 P2=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 P2.



© Società Italiana di Fisica, Springer-Verlag 2001

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