Introduction

Dilepton production in the electroweak theory is important as a background for various physics analyses like, for example, exclusive or production and new physics searches. So far only the generator LPAIR [1] has been used in experimental analyses to estimate the dilepton background [3]. LPAIR is based on the calculation of the diagrams of the two-photon (2-) process [2], the so called Bethe-Heitler (BH) process as seen in Fig.1-(a) without Z contribution. The 2- BH process is dominant in most of the phase space. It is, however, expected that in the region of low invariant masses of the dilepton system, internal photon conversion (CO) diagrams as seen in Fig.1-(b) and Fig.2-(b) become dominant. In the high mass region, there is an additional interesting process, i.e. Z production which is implemented in the Monte Carlo generator EPVEC [4]. EPVEC, however, does not include 2- BH nor CO diagrams. In the di-electron channel, interference effects of the final state or are not included, neither in LPAIR nor in EPVEC.

Here a new Monte Carlo generator for dilepton production based on the electroweak matrix elements in electron/positron-proton (ep) collisions is presented. The Feynman amplitudes in this program were generated by GRACE [7] which is an automatic calculation system. The name of GRAPE means GRAce-based generator for Proton-Electron collisions.

This generator has the following features.

• All diagrams in the electroweak theory at tree level except for proton-Z couplings and lepton pairs generated by photon radiation from the (quasi-)elastic proton are included in the matrix element calculation. In the di-electron channel, interference effects of the final state or are also included. It is possible to select any sub-set of diagrams in the calculation.
• All fermion masses in this calculation are kept non-zero both in the matrix elements and in the kinematics, which makes it possible to use this program with arbitrary small scattering angles of / and/or small invariant masses of dilepton down to the kinematical limits.
• 3 types of proton vertices are available, which are elastic, quasi-elastic and DIS (Deep Inelastic eq Scattering).
• The effect of Initial State Radiation (ISR) can be included in the cross-section calculation based on the Structure Function method [5].
• Final State Radiation (FSR) and hadronization (in DIS) are performed by PYTHIA [6].

In this version, exclusive production of vector mesons (, , etc.) decaying to dileptons is not included.