EXCALIBUR -- a Monte Carlo program to evaluate all four fermion processes at LEP 200 and beyond

TL;DR

EXCALIBUR computes all four-fermion final states in $e^+ e^-$ annihilation, including QED initial-state radiation and QCD backgrounds. Fermions are treated as massless to speed up matrix-element evaluation, and a modular, self-optimizing, multichannel Monte Carlo framework samples the highly peaking integrand across all contributing diagrams. Amplitudes are generated with spinorial techniques, and crossing assembles the full diagram set (up to 144 Abelian and up to 8 non-Abelian) with corresponding phase-space channels. Typical running time is about $100$ events per second on HP hardware, and the approach provides a blueprint for future event generators spanning energies from around $100$ GeV to $1$ TeV, with LL $O(\,\alpha\,)$ and $O(\,\alpha^2\,)$ ISR with exponentiation. The work demonstrates integrated treatment of signal and background four-fermion final states for LEP II and beyond in a single flexible Monte Carlo framework.

Abstract

A Monte Carlo program is presented that computes all four fermion processes in $e^+ e^-$ annihilation. QED initial state corrections and QCD contributions are included. Fermions are taken to be massless, allowing a very fast evaluation of the matrix element. A systematic, modular and self-optimizing strategy has been adopted for the Monte Carlo integration, that serves also as an example for further event generators in high energy particle physics.