LEPTO 6.5 - A Monte Carlo Generator for Deep Inelastic Lepton-Nucleon Scattering

TL;DR

LEPTO 6.5 presents a comprehensive Monte Carlo generator for deep inelastic lepton-nucleon scattering that integrates LO electroweak cross sections, exact first-order QCD matrix elements, and leading-log parton showers with Lund string hadronization. It also introduces a soft colour interaction mechanism to generate rapidity gaps, supports multiple parton-density parametrizations (including intrinsic charm), and provides flexible kinematic handling and grid-based or per-event evaluation of cross sections. The package emphasizes modularity and backward compatibility, coupling with Jetset/Pythia for hadronization and offering extensive control via common blocks and subroutines to study QCD effects, hadronization, and gap phenomena in DIS. This tool enables detailed DIS phenomenology analyses and systematic comparisons with HERA and prior experiments, facilitating investigations into gluon densities, higher-twist effects, and non-perturbative QCD mechanisms. Overall, LEPTO 6.5 delivers a versatile, publicly usable framework for simulating complete DIS events with tunable physics models and efficient computation strategies.

Abstract

Physics and programming aspects are discussed for a Fortran 77 Monte Carlo program to simulate complete events in deep inelastic lepton-nucleon scattering. The parton level interaction is based on the standard model electroweak cross sections, which are fully implemented in leading order for any lepton of arbitrary polarization, and different parametrizations of parton density functions can be used. First order QCD matrix elements for gluon radiation and boson-gluon fusion are implemented and higher order QCD radiation is treated using parton showers. Hadronization is performed using the Lund string model, implemented in {\sc Jetset}/{\sc Pythia}. Rapidity gap events are generated through a model based on soft colour interactions.