THE REGION OF VALIDITY OF PERTURBATIVE QCD FOR THE PHOTON STRUCTURE-FUNCTION F2-GAMMA(X, Q(2))

We analyse the region of validity of perturbative QCD (in Q2) for the photon structure function F2gamma(x, Q2), from the standpoint of establishing the value of Q2 = Q0(2) at which it is possible to start a perturbative evolution of F2gamma (x, Q2). We find that we can only reproduce the high Q2 data if we start our evolution at a value of Q0(2) 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 congruent-to 4-5 GeV2, and at this value of Q0(2) it is necessary to represent the input distributions as a sum of hadronic and point-like components. We also investigate related questions such as the sensitivity of the evolved structure function to the input distributions, in particular the poorly determined gluon distribution, and to the QCD mass scale LAMBDA.