Introduction

The ZEUS and H1 experiments at HERA have published a variety of studies on hard processes in quasi-real photoproduction. In lowest order (LO) QCD, an incoming photon (or parton from the photon) interacts with a parton from the proton, and two outgoing high p_T partons emerge in the final state, which can then hadronise to give rise to two observable jets. Two major classes of LO process are defined: direct, in which the entire photon interacts in the QCD subprocess, and resolved, in which the photon is a source of partons one of which interacts. A major objective is to determine the parton densities in the photon and the proton, the latter complementing the many measurements which have been made in DIS processes.

A problem concerning measurements of this type concerns the effects of final state QCD radiation, or higher order QCD effects in general. These are reduced if we can measure processes in which the emerging particles from the QCD subprocess are not themselves subject to QCD effects. Two classes of process which have been used in this way in hadron collisions are those in which high-p_T photons are produced, i.e. so-called ``prompt" photons, and those in which quark-antiquark annihilation gives rise a pair of leptons, known as Drell-Yan processes. Fig. 1 illustrates the different types of process. In ``dijet" processes, both final state particles in the basic diagram are quarks or gluons, while the others involve a photon or a pair of leptons.

In $p\bar p$ collisions at Fermilab, prompt photon processes provide a way to study the gluon content of the proton [1,2]. At HERA, the accessible kinematic range restricts the main sensitivity to the quark content of the photon [3-6], together with the quark and gluon contents of the proton. The particular virtue of prompt photon processes is that the observed final-state photon emerges from the QCD process directly, without the intermediate hadronisation that accompanies the observation of a quark or gluon through the means of a final state jet. This, together with the availability of NLO calculations [7,5], makes such processes attractive in providing the prospect of a relatively clean technique for studying QCD. On the other hand, the cross sections are substantially lower than those of dijet processes.

Drell-Yan (DY) processes in photoproduction are by definition resolved at LO. There is no exactly parallel class of direct events, but a background comes from Bethe-Heitler (BH, ``photon-photon") processes, as illustrated in Fig. 1. (A further BH diagram, not shown, has an inelastic excitation of the proton at the lower vertex.) Both BH photons interact in a direct way in producing the lepton pair. Direct-resolved and resolved-resolved photon photon interactions can also occur and give hadronic final states. A higher order ``direct" DY diagram can also be drawn in which the photon remnant is replaced by a high-p_T $\bar q$ or q [8].

 

Figure 1: Main LO diagrams for direct dijet and prompt photon processes in hard photoproduction, and examples for resolved processes. Diagrams for Bethe-Heitler and LO Drell-Yan processes are also shown. Broad arrows represent photon or photon remnants.

The study of lepton pair production through DY processes is of interest because it can provide a further alternative way to measure the parton densities of the photon and the proton, testing perturbative QCD and the determination of the running coupling constant. It is also an important background process for other production mechanisms of lepton pairs, such as $J/\Psi$ and $\Upsilon$ decays. The twist-two chirality violating proton structure function h₁(x) can be measured in the DY reaction when both beam and target are transversely polarized. In the framework of the nuclear program at HERA the DY reaction will allow investigation of the violation of charge symmetry in the valence quark distributions of the nucleon at large x, and tests of SU(2)-flavour symmetry breaking.

In 1971 Jaffe [9] suggested that the photon structure function could be determined through the DY process in photoproduction experiments. Since then, several other authors have studied lepton pair production theoretically using the QCD formalism at kinematic conditions at fixed target experiments [10], as well as ep colliders such as HERA [11]. Much attention was paid to find proper kinematic variables and to determine a kinematic region where the major contribution from the DY leptons would not be dominated by the background processes. All of them, however, reached the not very comforting conclusion that, because of the overwhelming Bethe-Heitler background, photoproduction of lepton pairs from a proton target is not an adequate way of measuring the photon structure function through the DY effect.

The planned HERA upgrade, however, will increase the integrated luminosity by two orders of magnitude and allow stricter selection criteria to separate the processes of interest. The conclusion of [11] for HERA was based on a comparison of inclusive spectra (p_T and rapidity), calculated for $4\pi$ geometry without taking into account the acceptances of the H1 or ZEUS detectors. One should therefore make an effort to analyse the Drell-Yan and background processes using a more precise approach to the experimental conditions. Results presented below show that it is possible to find kinematic criteria under which the Bethe-Heitler background is totally suppressed.