Article révisé par les pairs
Résumé : The double-differential inclusive di-jet cross section in photoproduction processes is measured with the H1 detector at HERA. The cross section is determined as a function of the average transverse jet energy ETjets for ranges of the fractional energy xγjets of the parton from the photon side. An effective leading order parton distribution in the photon is determined at large parton fractional energies for scales between 80 < pT2 < 1250 GeV2. The measurement is compatible with the logarithmic scale dependence that is predicted by perturbative QCD.