Determining the gluon distributions in the proton and photon from two-jet production at HERA

Two-jet production from the direct photon contribution at Hadron-Elektron-Ring Anlage (HERA) is a sensitive measure of the small-x gluon in the proton. We propose measurements of ratios of the jet cross-sections which will clearly distinguish between gluons with or without singular behaviour at small x. Furthermore, we show that analogous ratio measurements for the resolved photon contribution provide a sensitive way of determining the gluon distribution in the photon. (author)