PHOTOPRODUCTION OF LARGE-MASS LEPTON PAIRS AT HERA AS A PROBE OF THE SMALL X-STRUCTURE OF THE PROTON

The photoproduction of Drell-Yan lepton pairs at HERA is studied. We s how 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 d istinguish singular xq is similar to x-1/2 behaviour from standard Reg ge xq is similar to x0 behaviour. The effect on the event rate of expe rimental acceptance cuts is studied with the help of a Monte Carlo eve nt generator.