Gluon content of the nucleon probed with real and virtual photons

A common fit is performed to lepton-hadron deep-inelastic scattering and large-$p_T$ direct-photon cross sections, using complete beyond-leading-logarithm QCD expressions. For direct-photon cross sections, theoretical uncertainties coming from the renormalization and factorization schemes are removed by applying the principle of minimal sensitivity. We find that the direct-photon data, especially from the recent fixed-target experiments, strongly constrain the gluon distribution function, whereas the deep-inelastic data determine the quark distribution functions and constrain the value of ${\Lambda }_{\mathrm{MS}\mathrm{¯}}$ (MS¯ denotes the modified minimal-subtraction scheme). Our analysis is a successful test of perturbative QCD, which provides a coherent and quantitative description of two very different reactions.