arXiv subjects

W. Placzek

Publications and source records attributed to W. Placzek.

At least 37 records · Page 2Linked to original sources

arXiv2016

Parton distribution functions in Monte Carlo factorisation scheme

A next step in development of the KrkNLO method of including complete NLO QCD corrections to hard processes in a LO parton-shower Monte Carlo (PSMC) is presented. It consists of generalisation of the method, previously used for the Drell-Yan process, to Higgs-boson production. This extension is accompanied with the complete description of parton distribution functions (PDFs) in a dedicated, Monte Carlo (MC) factorisation scheme, applicable to any process of production of one or more colour-neutral particles in hadron-hadron collisions.

hep-ph↗

arXiv2016

Precision Studies of Observables in pp->W->l nu and pp->gamma,Z->l+l- processes at the LHC

This report was prepared in the context of the LPCC "Electroweak Precision Measurements at the LHC WG" and summarizes the activity of a subgroup dedicated to the systematic comparison of public Monte Carlo codes, which describe the Drell-Yan processes at hadron colliders, in particular at the CERN Large Hadron Collider (LHC). This work represents an important step towards the definition of an accurate simulation framework necessary for very high-precision measurements of electroweak (EW) observables such as the $W$ boson mass and the weak mixing angle. All the codes considered in this report share at least next-to-leading-order (NLO) accuracy in the prediction of the total cross sections in an expansion either in the strong or in the EW coupling constant. The NLO fixed-order predictions have been scrutinized at the technical level, using exactly the same inputs, setup and perturbative accuracy, in order to quantify the level of agreement of different implementations of the same calculation. A dedicated comparison, again at the technical level, of three codes that reach next-to-next-to-leading-order (NNLO) accuracy in quantum chromodynamics (QCD) for the total cross section has also been performed. These fixed-order results are a well-defined reference that allows a classification of the impact of higher-order sets of radiative corrections. Several examples of higher-order effects due to the strong or the EW interaction are discussed in this common framework. Also the combination of QCD and EW corrections is discussed, together with the ambiguities that affect the final result, due to the choice of a specific combination recipe.

hep-ph↗

arXiv2016

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.

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arXiv2015

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 $Z/γ^*$-boson production at the LHC, are presented.

hep-ph↗

arXiv2013

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.

hep-ph↗

arXiv2013

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.

hep-ph↗

arXiv2013

Observable phi-eta-star at LHC and second-order QED matrix element in Z/gamma to l+ l- decays

In a recent publication by ATLAS collaboration a new observable, the so-called phi-star angle, was used for precise measurement of transverse Z momentum. One of the dominant systematic errors for this measurement originates from the theoretical control of QED final-state bremsstrahlung. At present, it is estimated at the 0.3% level for the shape of the phi-star distribution. In this paper we discuss the possible effects of the second-order QED matrix element for that quantity. For that purpose, results from simulations based on the Yennie--Frautchi--Suura (YFS) exponentiation and featuring the second-order matrix elements are used and compared with the case when the matrix element is restricted to the first order. From this study we conclude that in order to reach the precision below 0.3% for the phi-star distribution at the LHC, inclusion of the second-order QED matrix element in a respective Monte Carlo event generator is necessary.

hep-ph↗

arXiv2012

NLO corrections to hard process in QCD shower -- proof of concept

The concept of new methodology of adding QCD NLO corrections in the initial state Monte Carlo parton shower (hard process part) is tested numerically using, as an example, the process of the heavy boson production at hadron--hadron colliders such as LHC. In spite of the use of a simplified model of the process, all presented numerical results prove convincingly that the basic concept of the new methodology works correctly in practice, that is in the numerical environment of the Monte Carlo parton shower event generator. The differences with the other well established methods, like MC@NLO and POWHEG, are briefly discussed and future refinements of the implementation of the new method are also outlined.

hep-ph↗

arXiv2012

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.

hep-ph↗

arXiv2011

Fully NLO Parton Shower in QCD

The project of constructing a complete NLO-level Parton Shower Monte Carlo for the QCD processes developed in IFJ PAN in Krakow is reviewed. Four issues are discussed: (1) the extension of the standard inclusive collinear factorization into a new, fully exclusive scheme; (2) reconstruction of the LO Parton Shower in the new scheme; (3) inclusion of the exclusive NLO corrections into the hard process and (4) inclusion of the exclusive NLO corrections into the evolution (ladder) part.

hep-ph↗

arXiv2010

ΔM_W < 10 MeV/c^2 at the LHC: a forlorn hope?

At the LHC, the measurement of the W mass with a precision of \cal{O}(10) MeV/c^2 is both mandatory and difficult. In the analysis strategies proposed so far, shortcuts have been made that are justified for proton-antiproton collisions at the Tevatron, but not for proton-proton collisions at the LHC. The root of the problem lies in the inadequate knowledge of parton density functions of the proton. It is argued that in order to reach a 10 MeV/c^2 precision for the W mass, more precise parton density functions of the proton are needed, and an LHC-specific analysis strategy ought to be pursued. Proposals are made on both issues.

hep-ex↗

arXiv2009

Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low energy e+e- colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on tau decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and tau decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

hep-ph↗

arXiv2009

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.

hep-ph↗

arXiv2009

Measurement of MW+ - MW- at LHC

This paper is the second of the series of papers proposing dedicated strategies for precision measurements of the Standard Model parameters at the LHC. The common feature of these strategies is their robustness with respect to the systematic measurement and modeling error sources. Their impact on the precision of the measured parameters is reduced using dedicated observables and dedicated measurement procedures which exploit flexibilities of the collider and detector running modes. In the present paper we focus our attention on the measurement of the charge asymmetry of the W-boson mass. This measurement is of primordial importance for the LHC experimental program, both as a direct test of the charge-sign-independent coupling of the W-bosons to the matter particles and as a necessary first step towards the precision measurement of the charge-averaged W-boson mass. We propose and evaluate the LHC-specific strategy to measure the mass difference between the positively and negatively charged W-bosons, MW+ - MW-. We show that its present precision can be improved at the LHC by a factor of 20. We argue that such a precision is beyond the reach of the standard measurement and calibration methods imported to the LHC from the Tevatron program.

hep-ph↗

arXiv2008

Solving QCD evolution equations in rapidity space with Markovian Monte Carlo

This work covers methodology of solving QCD evolution equation of the parton distribution using Markovian Monte Carlo (MMC) algorithms in a class of models ranging from DGLAP to CCFM. One of the purposes of the above MMCs is to test the other more sophisticated Monte Carlo programs, the so-called Constrained Monte Carlo (CMC) programs, which will be used as a building block in the parton shower MC. This is why the mapping of the evolution variables (eikonal variable and evolution time) into four-momenta is also defined and tested. The evolution time is identified with the rapidity variable of the emitted parton. The presented MMCs are tested independently, with ~0.1% precision, against the non-MC program APCheb especially devised for this purpose.

hep-ph↗

arXiv2008

Markovian MC simulation of QCD evolution at NLO level with minimum k_T

We present two Monte Carlo algorithms of the Markovian type which solve the modified QCD evolution equations at the NLO level. The modifications with respect to the standard DGLAP evolution concern the argument of the strong coupling constant alpha_S. We analyze the z - dependent argument and then the k_T - dependent one. The evolution time variable is identified with the rapidity. The two algorithms are tested to the 0.05% precision level. We find that the NLO corrections in the evolution of parton momentum distributions with k_T - dependent coupling constant are of the order of 10 to 20%, and in a small x region even up to 30%, with respect to the LO contributions.

hep-ph↗

arXiv2008

Implementation of SANC EW corrections in WINHAC Monte Carlo generator

In this paper we describe a check of the implementation of SANC system generated modules into the framework of the WINHAC Monte Carlo event generator. At this stage of work we limit ourselves to inclusion of complete one-loop electroweak corrections to the charged-current Drell--Yan process. We perform tuned comparisons of the results derived with the aid of two codes: 1) the standard SANC integrator with YFS-inspired treatment of the ISR QED corrections and 2) the WINHAC generator, upgraded with the SANC electroweak modules and downgraded to the O(α) QED corrections. The aim of these comparisons is to prove the correctness of implementation of the SANC electroweak modules into the WINHAC generator. This is achieved through the presented tuned comparisons.

hep-ph↗

arXiv2007

Constrained MC for QCD evolution with rapidity ordering and minimum kT

With the imminent start of LHC experiments, development of phenomenological tools, and in particular the Monte Carlo programs and algorithms, becomes urgent. A new algorithm for the generation of a parton shower initiated by the single initial hadron beam is presented. The new algorithm is of the class of the so called ``constrained MC'' type algorithm (an alternative to the backward evolution MC algorithm), in which the energy and the type of the parton at the end of the parton shower are constrained (predefined). The complete kinematics configurations with explicitly constructed four momenta are generated and tested. Evolution time is identical with rapidity and minimum transverse momentum is used as an infrared cut-off. All terms of the leading-logarithmic approximation in the DGLAP evolution are properly accounted for. In addition, the essential improvements towards the so-called CCFM/BFKL models are also properly implemented. The resulting parton distributions are cross-checked up to the 0.1% precision level with the help of a multitude of comparisons with other MC and non-MC programs. We regard these tests as an important asset to be exploited at the time when the presented MC will enter as a building block in a larger MC program for W/Z production process at LHC.

hep-ph↗