431 research outputs found
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
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
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- photon effects are
properly taken into account and wherein infrared singularities are cancelled to
all orders in . We show that H1 and ZEUS data are consistent with such
an interpretation for a leptoquark coupling , mass
GeV, and width GeV. Possible future tests are proposed.Comment: 9 pages, 3 figures, Latex and bbl file
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 -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
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