Photoproduction of large-mass lepton pairs at HERA as a probe of the small $x$ structure of the proton

Abstract

The photoproduction of Drell-Yan lepton pairs at HERA is studied. We show that in the backward rapidity region the cross section is strongly sensitive to the small-$x$ behaviour of the quark distributions in the proton, and that with sufficient luminosity it should be possible to distinguish singular $xq \sim x^{-1/2}$ behaviour from standard Regge $xq \sim x^0$ behaviour. The effect on the event rate of experimental acceptance cuts is studied with the help of a Monte Carlo event generator.