Parton content of virtual photons

The parton content of virtual transverse photons γ(${\mathit{P}}^2$) is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components. The resulting LO and NLO parton distributions ${\mathit{f}}^{\gamma (\mathit{P}2}$)(x,${\mathit{Q}}^2$) are smooth in ${\mathit{P}}^2$. They apply uniformly to all ${\mathit{P}}^2$≥0 whenever γ(${\mathit{P}}^2$) is probed at a scale ${\mathit{Q}}^2$≫${\mathit{P}}^2$ where the transverse photons furthermore also dominate physically relevant cross sections. Predictions are given for ${\mathit{F}}_2^{\gamma (\mathit{P}2}$)(x,${\mathit{Q}}^2$) and ${\mathrm{\mu }}^{\gamma (\mathit{P}2}$)(x,${\mathit{Q}}^2$), ${\mathit{g}}^{\gamma (\mathit{P}2}$)(x,${\mathit{Q}}^2$) relevant for future CERN LEP and present DESY HERA measurements, respectively. Except for certain kinematic regions, these virtual structure functions and parton distributions at ${\mathrm{\Lambda }}^2$≪${\mathit{P}}^2$≪${\mathit{Q}}^2$ 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.