PARTON CONTENT OF VIRTUAL PHOTONS

The parton content of virtual transverse photons γ(P2) is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components. The resulting LO and NLO parton distributions fγ(P2)(x,Q2) are smooth in P2. They apply uniformly to all P20 whenever γ(P2) is probed at a scale Q2P2 where the transverse photons furthermore also dominate physically relevant cross sections. Predictions are given for F2γ(P2)(x,Q2) and μγ(P2)(x,Q2), gγ(P2)(x,Q2) relevant for future CERN LEP and present DESY HERA measurements, respectively. Except for certain kinematic regions, these virtual structure functions and parton distributions at Λ2P2Q2 are not solely described by purely perturbative contributions, in contrast with ''naive'' expectations; these remaining nonperturbative components, although being based on VMD-inspired simplicity, represent the largest uncertainty in our quantitative results and eventually have to be tested by future experiments. © 1995 The American Physical Society.