Multi-photon Monte Carlo event generator KKMCee for lepton and quark pair production in lepton colliders

• Published in: Computer physics communications 2023-02, Vol.283, p.108556, Article 108556
• Main Authors: Jadach, S., Ward, B.F.L., Wa̧s, Z., Yost, S.A., Siodmok, A.
• Format: Article
• Language: English
• Subjects: Electroweak radiative corrections; Event record interface; Lepton tau decays; Monte Carlo event generation; Multiphoton emission; Spin polarization
• ISSN: 0010-4655; 1879-2944
• DOI: 10.1016/j.cpc.2022.108556

We present the KKMCee 5.00.2 Monte Carlo event generator for lepton and quark pair production for the high energy electron-positron annihilation process. It is still the most sophisticated event generator for such processes. Its entire source code is re-written in the modern C++ language. It reproduces all features of the older KKMC code in Fortran 77. However, a number of improvements in the Monte Carlo algorithm are also implemented. Most importantly, it is intended to be a starting point for the future improvements, which will be mandatory for the future high precision lepton collider projects. As in the older version, in addition to higher order QED corrections, it includes so-called O(α1.5) genuine weak corrections using a version of the classic DIZET library and polarized τ decays using TAUOLA program. Both DIZET and TAUOLA external libraries are still in Fortran 77. In addition, a HEPMC3 interface to other MC programs, like parton showers and detector simulation, replaces the older HepEvt interface. The HEPMC3 interface is also exploited in the implementation of the additional photon final state emissions in τ decays using an external PHOTOS library rewritten in C. Program title:KKMCee 5.00.2 CPC Library link to program files:https://doi.org/10.17632/7drvvhbw92.1 Licensing provisions: GPL-3.0 Programming languages:C++, FORTRAN77 External routines:CERN ROOT library, PHOTOS, HepMC v.3.0, TAUOLA, PHOTOS, FOAM, HEPMC3 Nature of the problem: Fermion pair production is and will be used as an important data source for precise tests of the standard electroweak theory at a high luminosity future circular collider near the Z resonance and above and/or at the future linear lepton colliders of higher energies than those at LEP. The QED corrections to fermion pair production (especially τ leptons) have to be known to at least second order, including spin polarization effects, with 4-5 digit precision. The Standard Model predictions at the sub-permille precision level, taking into account multiple emission of photons for realistic experimental acceptances, can only be obtained using a Monte Carlo event generator. The realistic and precise simulation of τ lepton decays taking into account spin effects is an indispensable ingredient in the Monte Carlo event generator for the fermion pair production process. Solution method: Monte Carlo methods are used to simulate most of the two-fermion final-state processes in e+e− collisions in the presence of multiphoton initia