Introduction

  Deep inelastic lepton-nucleon scattering [1] has played an important role for probing the structure of the proton and understanding the fundamental electromagnetic, weak and strong interactions at the level of quarks and leptons. With the order of magnitude increase in the centre-of-mass energy now available through ep collisions in HERA, this line of research will continue to be at the forefront.

The basic lepton-quark scattering processes have well-defined cross section formulae within the electroweak standard model [2]. With the inclusion of parton density distributions and perturbative QCD corrections the problem of practical evaluations become quite complex and analytical calculations are only possible in simplified cases or through approximations. Normal numerical methods are in many cases possible, but Monte Carlo simulation is often preferable because of its generality and applicability to complex problems. In the case of the multiparticle hadronic final state, the only viable alternative is, in fact, the Monte Carlo method.

The present program, LEPTO, is a general and flexible Monte Carlo (MC) to simulate complete lepton-nucleon scattering events and integrate cross sections. It is based on the leading order electroweak cross sections for the underlying parton level scattering processes. Thus, electroweak radiative corrections are not included, but can be simulated using the interface DJANGO [3] between LEPTO and HERACLES [4]. The main emphasis of LEPTO is rather on the hadronic part of the event. QCD corrections are therefore included using exact first order matrix elements and higher orders in the leading parton cascade approach. The fragmentation of produced partons into observable hadrons is performed with the Lund string hadronization model [5]. An arbitrary configuration of a lepton and a nucleon can be defined with constraints on the scattering kinematics and the generated events can be transformed to different frames. The present version is the latest development in a series of earlier versions [6, 7] that have developed in stages and profitted greatly by feed-back from extensive comparisons with experimental data on the hadronic final state in leptoproduction. This has given valuable insights concerning the QCD processes of gluon radiation and boson-gluon fusion as well as the confinement induced hadronization of colour charged partons. The generally good agreement between data, in particular from the European Muon Collaboration and the neutrino experiments WA21 and WA25 at CERN, and the Monte Carlo shows the validity of the models and procedures used in the program. This can be illustrated by the results on longitudinal momentum spectra of different identified hadrons [8], transverse momentum properties, energy flows and jet structure [9]. The much higher energies available in ep collisions at HERA provides both qualitative and quantitative new information on QCD effects, such that detailed tests of MC models are now being performed [10, 11].

In the following a comprehensive description is given of the theoretical framework built into the program (section 2) as well as the various program components (section 3) and their usage (section 4).