Mapping the proton unintegrated gluon distribution in dijets correlations in real and virtual photoproduction at HERA

We discuss how the dijet azimuthal correlations in DIS and real photoproduc tion at HERA probe the differential (unintegrated) gluon distribution in th e proton. We find a strong dependence of the azimuthal correlation pattern on Bjorken-x, photon virtuality and the cut on the jet transverse momenta. A rise of the azimuthal decorrelations is observed with decreasing Bjorken- x due to the interplay of perturbative and nonperturbative effects. We pred ict a strong rise of the same-side jet rate with photon energy for real pho toproduction. We discuss conditions for the correlation function to be domi nated by hard perturbative gluons and ways of constraining the size of the nonperturbative soft component. We make some predictions for the THERA ener gy range. The analysis of the energy dependence of the isolated jet and two -jet cross sections in photoproduction would be a new way to study the not yet well constrained unintegrated gluon distributions and to explore the on set of the pQCD regime. (C) 2001 Published by Elsevier Science B.V.