The precision Monte Carlo event generator for two-fermion final states in collisions

Abstract

We present the Monte Carlo event generator KK version 4.13 for precision predictions of the Electroweak Standard Model for the process e + e − → ff + nγ, f = µ, τ, d, u, s, c, b, at centre-of-mass energies from τ lepton threshold to 1 TeV, that is for LEP, SLC, future Linear Colliders, b, c, τ -factories, etc. Effects due to photon emission from initial beams and outgoing fermions are calculated in QED up to second order, including all interference effects, within Coherent Exclusive Exponentiation (CEEX), which is based on Yennie-Frautschi-Suura exponentiation. Electroweak corrections are included in first order, with higher-order extensions, using the DIZET 6.21 library. Final-state quarks hadronize according to the parton shower model using JETSET. Beams can be polarized longitudinally and transversely. Decay of the τ leptons is simulated using the TAUOLA library, taking into account spin polarization effects as well. In particular the complete spin correlations density matrix of the initial-state beams and final state τ 's is incorporated in an exact manner. Effects due to beamstrahlung are simulated in a realistic way. The main improvements with respect to KORALZ are: (a) inclusion of the initial-final state QED interference, (b) inclusion of the exact matrix element for two photons, and (c) inclusion of the transverse spin correlations in τ decays (as in KORALB). To appear in Computer Physics Nature of the physical problem:The fermion pair production is and will be used as an important data point for precise tests of the standard electroweak theory at LEP and future linear colliders at higher energies. QED corrections to fermion pair production (especially τ leptons) at c-quark and b-quark factories has to be known to second order, including spin polarization effects. The Standard Model predictions at the per mille precision level, taking into account multiple emission of photons for realistic experimental acceptance, can only be obtained using a Monte Carlo event generator. Method of solution:The Monte Carlo methods are used to simulate most of the twofermion final-state processes in e + e − collisions in the presence of multiphoton initial-state radiation. The latter is described in the framework of exclusive coherent exponentiation (CEEX) based on Yennie-Frautschi-Suura exclusive exponentiation (YFS/EEX). CEEX treats correctly to infinite order not only infrared cancellations but also QED interferences and narrow resonances. The matrix element according to standard YFS exponentiation is also provided for tests. For quarks and τ leptons, the appropriate simulation of hadronization or decay is included. Beam polarization and spin effects, both longitudinal and transverse, in tau decays are properly taken into account. Restrictions on the complexity of the problem: In the present version, electron (Bhabha), neutrino and top quark final states are not included (they will be in a future version). Additional fermion pair production is not included. Third-order QED corrections in leading-logarithmic ap...

Search forZc(3900)±→ωπ±Ablikim1, Ачасов2, Ai3 et al. 2015Phys. Rev. D181110View full textAdd to dashboardCite

“…The decays of ω → π + π − π 0 are generated with the OMEGA DALITZ model in evtgen [27,28]. Initial state radiation (ISR) is simulated with kkmc [29,30], where the Born cross section of e + e − → Z c (3900) ± π ∓ is assumed to follow a Y (4260) Breit-Wigner (BW) line shape with resonance parameters taken from the Particle Data Group (PDG) [31], in which listed as X(4260). Final state radiation (FSR) effects associated with charged particles are handled with PHOTOS [29].…”

“…Initial state radiation (ISR) is simulated with kkmc [29,30], where the Born cross section of e + e − → Z c (3900) ± π ∓ is assumed to follow a Y (4260) Breit-Wigner (BW) line shape with resonance parameters taken from the Particle Data Group (PDG) [31], in which listed as X(4260). Final state radiation (FSR) effects associated with charged particles are handled with PHOTOS [29]. For studies of possible backgrounds, inclusive Y (4260) MC samples with luminosity equivalent to the experimental data at √ s = 4.23 and √ s = 4.26 GeV are generated, where the main known decay channels are generated using evtgen [27,28] with branching fractions taken from the PDG [31].…”

New hadronic currents in TAUOLA: for confrontation with the experimental data

“…In those tests, results from the second-order matrix element calculations embedded in KKMC [32] Monte Carlo are used in case of Z decay. For W decays com-parisons with electroweak calculations of Refs.…”

A High Statistics Study of the Decay τ−→π−π0ντ

“…The KORALB/TAUOLA program [20,21] is used for τ + τ − -pair generation, the QQ generator [22] forBB andqq continuum processes, the BHLUMI [23] program for radiative Bhabha events, the KKMC [24] program for radiative µ + µ − -pair events, and the AAFHB [25] program for two-photon processes. The BHLUMI and KKMC programs include higher-order radiative corrections and are among the most accurate programs available.…”