The region of validity of perturbative QCD for the photon structure function F2γ(x,Q2)

We analyse the region of validity of perturbative QCD (in Q2) for the photon structure function F2γ(x, Q2), from the standpoint of establishing the value of Q2 = Q02 at which it is possible to start a perturbative evolution of F2γ(x, Q2). We find that we can only reproduce the high Q2 data if we start our evolution at a value of Q02 at which the point-like component predicted by perturbative QCD has already developed significantly. In practice this turns out to be in the region Q2 ≅ 4-5 GeV2, and at this value of Q02 it is necessary to represent the input distributions as a sum of hadronic and point-like components. We also investigate related questions such as sensitivity of the evolved structure function to the input distributions, in particular the poorly determined gluon distribution, and the QCD mass scale Λ. (author)