RacoonWW1.3: A Monte Carlo program for four-fermion production at e^+ e^- colliders

TL;DR

RacoonWW is a Monte Carlo generator for $e^+e^- \to 4f$ and $e^+e^- \to 4f+\gamma$ that delivers complete ${O}(\alpha)$ electroweak radiative corrections to $e^+e^- \to W W \to 4f$ in double-pole approximation, along with initial-state radiation and QCD effects. It combines a multi-channel, phase-space–aware MC integration with three evaluation modes (tree-level, IBA, RC) and two IR-splitting branches (slicing and subtraction) to manage soft/collinear singularities consistently. The code also supports anomalous triple and quartic gauge couplings and offers interfaces to Pythia for hadronization, enabling realistic event generation and distributions suitable for LEP2 and future linear-collider studies. The authors report typical cross-section uncertainties at the ${\sim}0.4{-}0.7\%$ level across relevant energies, and emphasize the method’s applicability away from threshold with controlled accuracy, while noting areas for improvement beyond DPA. Overall, RacoonWW provides a comprehensive, high-precision framework for four-fermion processes in $e^+e^-$ annihilation with realistic phenomenological features and extensible options for future explorations.

Abstract

We present the Monte Carlo generator RacoonWW that computes cross sections to all processes e^+ e^- -> 4f and e^+ e^- -> 4f + gamma and calculates the complete O(alpha) electroweak radiative corrections to e^+ e^- -> W W -> 4f in the electroweak Standard Model in double-pole approximation. The calculation of the tree-level processes e^+ e^- -> 4f and e^+ e^- -> 4f + gamma is based on the full matrix elements for massless (polarized) fermions. When calculating radiative corrections to e^+ e^- -> W W -> 4f the complete virtual doubly-resonant electroweak corrections are included, i.e. the factorizable and non-factorizable virtual corrections in double-pole approximation, and the real corrections are based on the full matrix elements for e^+ e^- -> 4f + gamma. The matching of soft and collinear singularities between virtual and real corrections is done alternatively in two different ways, namely by using a subtraction method or by applying phase-space slicing. Higher-order initial-state photon radiation and naive QCD corrections are taken into account. RacoonWW also provides anomalous triple gauge-boson couplings for all processes e^+ e^- -> 4f and anomalous quartic gauge-boson couplings for all processes e^+ e^- -> 4f + gamma.

Figures (5)

• Figure 1: Generic diagram of a specific channel
• Figure 2: The generic phase-space generators symbolized by the topologies of the corresponding diagrams. The arcs indicate the intermediate $2\to2$ scattering processes used in the phase-space parametrizations. Topologies with an $s$-channel $2\to 2$ scattering process are obtained from the ones with only one $t$-channel scattering process by contracting the $t$-channel propagator.
• Figure 3: The basic phase-space decompositions from which all channels for the processes $\mathrm{e^+}$e^+$\mathrm{e^-}$e^-$\to 4f$ and $\mathrm{e^+}$e^+$\mathrm{e^-}$e^-$\to 4f\gamma$ can be constructed, as used in the slicing branch
• Figure 4: The generic diagrams of the $2\to 2$ scattering process and $1\to 2$ decay and the arguments of the corresponding subroutines PROCESS and DECAY in subtraction.f
• Figure 5: Generic diagrams for $\mathrm{e^+}$e^+$\mathrm{e^-}$e^-$\to 4f$