We present the Monte Carlo event generator WINHAC for Drell-Yan processes in proton-proton, proton-antiproton, proton-ion and ion-ion collisions. It features multiphoton radiation within the Yennie-Frautschi-Suura exclusive exponentiation scheme with
O(alpha) electroweak corrections for the charged-current (W+/W-) processes and multiphoton radiation generated by PHOTOS for neutral-current (Z+gamma) ones. For the initial-state QCD/QED parton shower and hadronisation it is interfaced with PYTHIA. It includes several options, e.g. for the polarized W-boson production, generation of weighted/unweighted events, etc. WINHAC was cross-checked numerically at the per-mille level with independent Monte Carlo programs, such as HORACE and SANC. It has been used as a basic tool for developing and testing some new methods of precise measurements of the Standard Model parameters at the LHC, in particular the W-boson mass. Recently, it has been applied to simulations of double Drell-Yan processes resulting from double-parton scattering, in order to assess their influence on the Higgs-boson detection at the LHC in its ZZ and W+W- decay channels.
Challenges for precision measurements at the LHC are discussed and a proposal how to move forward to overcome the LHC-specific precision brick-walls is presented.
Charged lepton transverse momenta in the Drell-Yan processes play an important role at the LHC in precision measurements of the Standard Model parameters, such as the W-boson mass and width, their charge asymmetries and sin^2(theta_W). Therefore, the
ir distributions should be described as accurate as possible by the Monte Carlo event generators. In this paper we discuss the problem of matching the hard-process kinematics of the Monte Carlo generator WINHAC with the parton-shower kinematics of the PYTHIA 6.4 generator while interfacing these two programs. We show that improper assignment of the quark and antiquark effective momenta in the LO matrix element computations may affect considerably the predicted lepton transverse momenta and even completely reverse their charge asymmetries at the LHC. We propose two matching schemes in which the NLO QCD distributions of the leptonic kinematical variables can be well reproduced by the LO WINHAC generator.
