LUSIFER: a LUcid approach to SIx-FERmion production

TL;DR

Lusifer introduces a fast, flexible Monte Carlo event generator for all tree-level $e^+e^-\to 6f$ processes, incorporating the full set of diagrams, massless external fermions with polarization, Weyl--van der Waerden helicity amplitudes, and leading-log initial-state radiation via structure functions. The paper details the matrix-element calculation, including eight-fermion amplitudes, fermion-permutation bookkeeping, and color treatment, along with multiple finite-width schemes and cross-checks against MadGraph. A robust multi-channel phase-space integration framework is developed to efficiently sample the high-dimensional, resonance-rich phase space, with propagator mappings and generic phase-space generators tailored to diagram topology. Numerical results at $\sqrt{s}=500$ GeV showcase comprehensive cross sections for all $6f$ final states, comparison with Whizard and Madgraph, and phenomenological distributions for top-quark and Higgs-boson production, plus gauge-invariance considerations under different width schemes. The study demonstrates good agreement with existing tools and highlights gauge-invariance issues, recommending the complex-mass scheme for gauge-consistent predictions and outlining a path toward incorporating non-universal radiative corrections in future work.

Abstract

LUSIFER is a Monte Carlo event generator for all processes e+e-->6fermions, which is based on the multi-channel Monte Carlo integration technique and employs the full set of tree-level diagrams. External fermions are taken to be massless, but can be arbitrarily polarized. The calculation of the helicity amplitudes and of the squared matrix elements is presented in a compact way. Initial-state radiation is included at the leading logarithmic level using the structure-function approach. The discussion of numerical results contains a comprehensive list of cross sections relevant for a 500GeV collider, including a tuned comparison to results obtained with the combination of the WHIZARD and MADGRAPH packages as far as possible. Moreover, for off-shell top-quark pair production and the production of a Higgs boson in the intermediate mass range we additionally discuss some phenomenologically interesting distributions. Finally, we numerically analyze the effects of gauge-invariance violation by comparing various ways of introducing decay widths of intermediate top quarks, gauge and Higgs bosons.

Figures (11)

• Figure 1: Generic diagrams with 4 fermion currents
• Figure 3: Generic diagrams with 2 fermion currents
• Figure 4: Topological diagram for the generic phase-space generator
• Figure 5: Topological diagram for a specific phase-space generator
• Figure 6: Diagram for ${\rm t}$ t$\bar{{\rm t}$ t$}$ production: ${\rm e^+}$ e^+${\rm e^-}$ e^-$\to{\rm t}$ t$\bar{{\rm t}$ t$}\to{\rm b}$ b${\rm W}$ W$^+\bar{{\rm b}$ b$}{\rm W}$ W$^-\to6f$