Charge asymmetries of lepton transverse momenta in Drell-Yan processes at the LHC

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, their 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.Comment: 18 pages, 8 figure

Multiphoton Radiation in Leptonic W-Boson Decays

We present the calculation of multiphoton radiation effects in leptonic W-boson decays in the framework of the Yennie-Frautschi-Suura exclusive exponentiation. This calculation is implemented in the Monte Carlo event generator WINHAC for single W-boson production in hadronic collisions at the parton level. Some numerical results obtained with the help of this program are also presented.Comment: 26 pages, 16 Postscript figure

Drell-Yan processes with WINHAC

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.Comment: 8 pages; presented at the XXXVII International Conference of Theoretical Physics "Matter To The Deepest", Ustron, Poland, 1-6 September 2013; to appear in Acta Physica Polonica

Markovian Monte Carlo program EvolFMC v.2 for solving QCD evolution equations

We present the program EvolFMC v.2 that solves the evolution equations in QCD for the parton momentum distributions by means of the Monte Carlo technique based on the Markovian process. The program solves the DGLAP-type evolution as well as modified-DGLAP ones. In both cases the evolution can be performed in the LO or NLO approximation. The quarks are treated as massless. The overall technical precision of the code has been established at 0.05% precision level. This way, for the first time ever, we demonstrate that with the Monte Carlo method one can solve the evolution equations with precision comparable to the other numerical methods.Comment: 38 pages, 9 Postscript figure

e^+ (Anti)Quark Scattering in the Presence of the Anomalous HERA Positron-Jet Event Phenomenon

We discuss the leptoquark interpretation of the anomalous HERA positron-jet events in the context of the YFS exponentiated Monte Carlo event generator treatment of the attendant multiple photon radiative effects for both the would-be signal and the SM background, wherein finite-$p_T$ photon effects are properly taken into account and wherein infrared singularities are cancelled to all orders in $\alpha$. We show that H1 and ZEUS data are consistent with such an interpretation for a leptoquark coupling $\lesssim 0.3g_W$, mass $\sim 200$ GeV, and width $\lesssim 2$GeV. Possible future tests are proposed.Comment: 9 pages, 3 figures, Latex and bbl file

NLO corrections in the initial-state parton shower Monte Carlo

The decade-old technique of combining NLO-corrected hard process with LO-level parton shower Monte Carlo is now mature and used in practice of the QCD calculations in the LHC data analysis. The next step, its extension to an NNLO-corrected hard process combined with the NLO-level parton shower Monte Carlo, will require development of the latter component. It does not exist yet in a complete form. In this note we describe recent progress in developing the NLO parton shower for the initial-state hadron beams. The technique of adding NLO corrections in the fully exclusive form (defined in recent years) is now simplified and tested numerically, albeit for a limited set of NLO diagrams in the evolution kernels.Comment: 8 pages, 5 figure

On the dependence of QCD splitting functions on the choice of the evolution variable

We show that already at the NLO level the DGLAP evolution kernel Pqq starts to depend on the choice of the evolution variable. We give an explicit example of such a variable, namely the maximum of transverse momenta of emitted partons and we identify a class of evolution variables that leave the NLO Pqq kernel unchanged with respect to the known standard MS-bar results. The kernels are calculated using a modified Curci-Furmanski-Petronzio method which is based on a direct Feynman-graphs calculation.Comment: 16 pages, 4 figure

Matching NLO QCD with parton shower in Monte Carlo scheme - the KrkNLO method

A new method of including the complete NLO QCD corrections to hard processes in the LO parton-shower Monte Carlo (PSMC) is presented. This method, called KrkNLO, requires the use of parton distribution functions in a dedicated Monte Carlo factorization scheme, which is also discussed in this paper. In the future, it may simplify introduction of the NNLO corrections to hard processes and the NLO corrections to PSMC. Details of the method and numerical examples of its practical implementation, as well as comparisons with other calculations, such as MCFM, MC@NLO, POWHEG, for single $Z/\gamma^*$-boson production at the LHC, are presented.Comment: 50 pages, 14 figures. Version published in JHEP: added a comparison of krkNLO results and the fixed-order NNLO result from the DYNNLO program and addressed several points raised by the refere