Abstract
The flux of papers from electron positron colliders containing data on the photon structure function ended naturally around 2005. It is thus timely to review the theoretical basis and confront the predictions with a summary of the experimental results. The discussion will focus on the increase of the structure function with x (for x away from the boundaries) and its rise with log Q**2, both characteristics beeing dramatically different from hadronic structure functions. The agreement of the experimental observations with the theoretical calculations of the real and virtual photon structure is a striking success of QCD. It also allows a new determination of the QCD coupling constant which very well agrees with the value quoted in the literature.
Highlights
The notion that hadron production in inelastic electron photon scattering can be described in terms of structure functions like in electron nucleon scattering is on first sight surprising because photons are pointlike particles whereas nucleons have a radius of roughly 1 fm
Which is identical to the vector meson dominance (VMD) ansatz describing the hadronic nature of the photon γ = απ ρ + απ ω + απ φ γρ γω γφ if the photon vector meson couplings γV are taken from the quark model neglecting mass effects
Measurements of the photon structure function F2γ taken at e+e− colliders were confronted with theoretical models
Summary
The notion that hadron production in inelastic electron photon scattering can be described in terms of structure functions like in electron nucleon scattering is on first sight surprising because photons are pointlike particles whereas nucleons have a radius of roughly 1 fm. C. Berger calculated the markedly different x dependence of the structure function in QCD but demonstrated that the QCD parameter Λ could in principle be determined by measuring an absolute cross section quite in contrast to lepton nucleon scattering, where small scale breaking effects in the Q2 evolution of the structure function have to be studied. Berger calculated the markedly different x dependence of the structure function in QCD but demonstrated that the QCD parameter Λ could in principle be determined by measuring an absolute cross section quite in contrast to lepton nucleon scattering, where small scale breaking effects in the Q2 evolution of the structure function have to be studied This “remarkable result” [5] initiated intensive discussions between theorists and experimentalists and passed the first experimental test [6] with flying colors. The concern of the 1980’s is still valid, albeit at a higher order in the perturbative series
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