arXiv subjects

S. Alekhin

Publications and source records attributed to S. Alekhin.

At least 73 records · Page 4Linked to original sources

arXiv2010

Update of the NNLO PDFs in the 3-, 4-, and 5-flavour scheme

We report on an update of the next-to-next-to-leading order (NNLO) ABKM09 parton distributions functions. They are obtained with the use of the combined HERA collider Run I inclusive deep-inelastic scattering (DIS) data and the partial NNLO corrections to the heavy quark electro-production taken into account. The value of the strong couplig constant $α^{\rm NNLO}_{\rm s}(M_{\rm Z})=0.1147(12)$ is obtained. The standard candle cross sections for the Tevatron collider and the LHC estimated with the updated PDFs are provided.

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arXiv2010

The 3-, 4-, and 5-flavor NNLO Parton from Deep-Inelastic-Scattering Data and at Hadron Colliders

We determine the parton distribution functions (PDFs) in a next-to-next-to-leading order (NNLO) QCD-analysis of the inclusive neutral-current deep-inelastic-scattering (DIS) world data combined with the neutri{-}no-nucleon DIS di-muon data and the fixed-target Drell-Yan data. The PDF-evolution is performed in the $N_f = 3$ fixed-flavor scheme and supplementary sets of PDFs in the 4- and 5-flavor schemes are derived from the results in the 3-flavor scheme using matching conditions. The charm-quark DIS contribution is calculated in a general-mass variable-flavor-number (GMVFN) scheme interpolating between the zero-mass 4-flavor scheme at asymptotically large values of momentum transfer $Q^2$ and the 3-flavor scheme prescription of Buza-Matiounine-Smith-van Neerven (BMSN) at the value of $Q^2 = m_c^2$. The results in the GMVFN scheme are compared with those of the fixed-flavor scheme and other prescriptions used in global fits of PDFs. The strong coupling constant is measured at an accuracy of $\approx 1.5%$. We obtain at NNLO $α_s(M_Z^2) = 0.1135 \pm 0.0014$ in the fixed-flavor scheme and $α_s(M_Z^2) = 0.1129 \pm 0.0applying the BMSN-prescription. The implications for important standard candle and hard scattering processes at hadron colliders are illustrated. Predictions for cross sections of $W^{\pm}$- and $Z$-boson, the top-quark pair- and Higgs-boson production at the Tevatron and the LHC based on the 5-flavor PDFs of the present analysis are provided.014$

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arXiv2009

Strange and Anti-strange Sea Distributions from Neutrino-Nucleon Deep Inelastic Scattering

We perform QCD fit of the nucleon strange and anti-strange sea distributions to the neutrino and anti-neutrino dimuon data by the CCFR and NuTeV collaborations, supplemented by the inclusive charged lepton-nucleon Deep Inelastic Scattering (DIS) and Drell-Yan data. The effective semi-leptonic charmed-hadron branching ratio is constrained from the inclusive charmed hadron measurements performed by the FNAL-E531 and CHORUS neutrino emulsion experiments as $B_μ=(8.8\pm0.5)%$. We obtain a strange sea suppression factor {$κ(20 {\rm GeV}^2)=0.62\pm0.04({\rm exp.})\pm0.03({\rm QCD})$}. An $x$-distribution of total strange sea obtained in the fit is slightly softer than the non-strange sea, and an asymmetry between strange and anti-strange quark distributions is consistent with zero (integrated over $x$ it is equal to {$0.0013\pm 0.0009({\rm exp.}) \pm 0.0002({\rm QCD})$ at the scale of $20 {\rm GeV^2}$}).

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arXiv2009

Variable-Flavor-Number Scheme in Analysis of Heavy-Quark Electro-Production Data

We check the impact of the factorization scheme employed in the calculation of the heavy-quark deep-inelastic scattering (DIS) electro-production on the PDFs determined in the NNLO QCD analysis of the world inclusive neutral-current DIS data combined with the ones on the neutrino-nucleon DIS di-muon production and the fixed-target Drell-Yan process. The charm-quark DIS contribution is calculated in the general-mass variable-flavor-number (GMVFN) scheme: At asymptotically large values of the momentum transfer $Q$ it is given by the zero-mass 4-flavor scheme and at the value of $Q$ equal to the charm-quark mass it is smoothly matched with the 3-flavor scheme using the Buza-Matiounine-Smith-van Neerven prescription. The PDFs obtained in this variant of the fit are very similar to the ones obtained in the fit with a 3-flavor scheme employed. Our 5-flavor PDFs derived from the 3-flavor ones using the NNLO matching conditions are used to calculate the rates of $W^{\pm}/Z$ and $t\bar{t}$ production at the Tevatron collider and the LHC at NNLO.

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arXiv2009

Higher order QCD corrections to charged-lepton deep-inelastic scattering and global fits of parton distributions

We study the perturbative QCD corrections to heavy-quark structure functions of charged-lepton deep-inelastic scattering and their impact on global fits of parton distributions. We include the logarithmically enhanced terms near threshold due to soft gluon resummation in the QCD corrections at next-to-next-to-leading order. We demonstrate that this approximation is sufficient to describe the available HERA data in most parts of the kinematic region. The threshold-enhanced next-to-next-to-leading order corrections improve the agreement between predictions based on global fits of the parton distribution functions and the HERA collider data even in the small-x region.

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arXiv2008

Determination of Strange Sea Distributions from Neutrino-Nucleon Deep Inelastic Scattering

We present an analysis of the nucleon strange sea extracted from a global Parton Distribution Function fit including the neutrino and anti-neutrino dimuon data by the CCFR and NuTeV collaborations, the inclusive charged lepton-nucleon Deep Inelastic Scattering and Drell-Yan data. The (anti-)neutrino induced dimuon analysis is constrained by the semi-leptonic charmed-hadron branching ratio $B_μ=(8.8\pm0.5)%$, determined from the inclusive charmed hadron measurements performed by the FNAL-E531 and CHORUS neutrino emulsion experiments. Our analysis yields a strange sea suppression factor $κ(Q^2=20{\rm GeV}^2)=0.62\pm 0.04$, the most precise value available, an $x$-distribution of total strange sea that is slightly softer than the non-strange sea, and an asymmetry between strange and anti-strange quark distributions consistent with zero (integrated over $x$ it is equal to $0.0013 \pm 0.0009$ at $Q^2=20{\rm GeV}^2$).

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arXiv2008

Update of the global fit of PDFs including the low-Q DIS data

We perform the next-to-next-leading-order (NNLO) QCD global fit of PDFs using inclusive charged-lepton and neutrino DIS data down to Q = 1 GeV. We also consider the data on neutrino-nucleon dimuon production, that allows us to disentangle the strange sea distribution. The fit results in χ^2/NDP = 5150/4338 = 1.2 that demonstrates a good consistency of the data sets used in analysis. The resulting value of α_s(M_Z) = 0.1136+- 0.0007(exp.) is in a good agreement with the previous version of the fit with a more stringent cut on Q. This analysis allows us to improve the accuracy of PDFs. The HT terms of the neutrino-nucleon structure functions F_2 and xF_3 are determined, the former is found to be consistent with one for the charged-leptons if the charge factor is taken into account.

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arXiv2007

Modeling Lepton-Nucleon Inelastic Scattering from High to Low Momentum Transfer

We present a model for inclusive charged lepton-nucleon and (anti)neutrino-nucleon cross sections at momentum transfer squared, $Q^2$, $\sim1 {\rm GeV}^2$. We quantify the impact of existing low-Q charged-lepton deep-inelastic scattering (DIS) data on effects due to high-twist operators and on the extraction of parton distribution functions (PDFs). No evidence is found for twist-6 contributions to structure functions (SF), and for a twist-4 term in the logitudinal SF at $x\gtrsim0.1$. We find that DIS data are consistent with the NNLO QCD approximation with the target mass and phenomenological high twist corrections. For $Q^2<1 {\rm GeV}^2$, we extend extrapolation of the operator product expansion, preserving the low-$Q$ current-conservation theorems. The procedure yields a good description of data down to $Q^2\sim 0.5 {\rm GeV}^2$. An updated set of PDFs with reduced uncertainty and applicable down to small momentum transfers in the lepton-nucleon scattering is obtained.

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arXiv2007

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part B

The HERA electron--proton collider has collected 100 pb$^{-1}$ of data since its start-up in 1992, and recently moved into a high-luminosity operation mode, with upgraded detectors, aiming to increase the total integrated luminosity per experiment to more than 500 pb$^{-1}$. HERA has been a machine of excellence for the study of QCD and the structure of the proton. The Large Hadron Collider (LHC), which will collide protons with a centre-of-mass energy of 14 TeV, will be completed at CERN in 2007. The main mission of the LHC is to discover and study the mechanisms of electroweak symmetry breaking, possibly via the discovery of the Higgs particle, and search for new physics in the TeV energy scale, such as supersymmetry or extra dimensions. Besides these goals, the LHC will also make a substantial number of precision measurements and will offer a new regime to study the strong force via perturbative QCD processes and diffraction. For the full LHC physics programme a good understanding of QCD phenomena and the structure function of the proton is essential. Therefore, in March 2004, a one-year-long workshop started to study the implications of HERA on LHC physics. This included proposing new measurements to be made at HERA, extracting the maximum information from the available data, and developing/improving the theoretical and experimental tools. This report summarizes the results achieved during this workshop.

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arXiv2006

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part A

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arXiv2005

Parton Distributions: Summary Report

We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of $F_L$), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques.

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arXiv2005

Parton distribution functions from the precise NNLO QCD fit

We report the parton distribution functions (PDFs) determined from the NNLO QCD analysis of the world inclusive DIS data with account of the precise NNLO QCD corrections to the evolution equations kernel. The value of strong coupling constant α_s^{NNLO}(M_Z)=0.1141(14), in fair agreement with one obtained using the earlier approximate NNLO kernel by van Neerven-Vogt. The intermediate bosons rates calculated in the NNLO using obtained PDFs are in agreement to the latest Run II results.

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arXiv2003

NNLO parton distributions from deep-inelastic-scattering data

The parton distributions functions (PDFs) derived from the NNLO QCD analysis of existing light-targets deep-inelastic-scattering data are presented. The NLO and NNLO PDFs are compared in order to analyze perturbative stability of the analysis and estimate impact of the higher-order QCD corrections. The main theoretical uncertainties and experimental uncertainties in PDFs due to all sources of experimental errors in data are estimated and used to assess corresponding uncertainties in the cross sections of other hadronic processes.

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arXiv2003

The NNLO predictions for the rates of the W/Z production in pp and pbar p collisions

The NNLO rates of the intermediate vector boson production (IVB) are calculated and found to agree with the preliminary results of Run II at the Fermilab pbar p collider. The estimated uncertainties in NNLO predictions for the IVB rates including the errors in PDFs, $α_s$, and the factorization/renormalization scales are about 2% for the Fermilab collider and 3% for the LHC that allows to use these predictions as a competitive benchmark for calibration of the collision luminosity.

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arXiv2003

Value of alpha_s from deep-inelastic-scattering data

We report the value of $α_{\rm s}$ obtained from QCD analysis of existing data on deep-inelastic scattering of charged leptons off proton and deuterium and estimate its theoretical uncertainties with particular attention paid to impact of the high-twist contribution to the deep-inelastic-scattering structure functions. Taking into account the major uncertainties the value $α^{\rm NNLO}_{\rm s}(M_{\rm Z})=0.1143 \pm 0.0014({\rm exp.})\pm 0.0013({\rm theor.})$ is obtained. An extrapolation of the LO--NLO--NNLO results to the higher orders makes it possible to estimate $α^{\rm N^3LO}_{\rm s}(M_{\rm Z})\sim 0.113$.

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arXiv2002

High twists and the NNLO QCD corrections in DIS

We discuss interplay between the high-twist (HT) terms in the operator-product expansion and the next-to-next-to-leading-order (NNLO) QCD corrections to the deep-inelastic-scattering structure functions in analysis of the high-precision data for charged leptons. Under account of the NNLO corrections the observed HT terms change within their experimental errors only and do not vanish in the NNLO.

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arXiv2002

The QCD/SM Working Group: Summary Report

This Report documents the results obtained by the Working Group on Quantum ChromoDynamics and the Standard Model for the Workshop ``Physics at TeV Colliders'', Les Houches, France, 21 May - 1 June 2001. The account of uncertainties in Parton Distribution Functions is reviewed. Progresses in the description of multiparton final states at Next-to-Leading Order and the extension of calculations for precision QCD observables beyond this order are summarized. Various issues concerning the relevance of resummation for observables at TeV colliders is examined. Improvements to algorithms of jet reconstruction are discussed and predictions for diphoton and photon pi-zero production at the LHC are made for kinematic variables of interest regarding searches for a Higgs boson decaying into two photons. Finally, several improvements implemented in Monte-Carlo event generators are documented.

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arXiv2000

QCD

We discuss issues of QCD at the LHC including parton distributions, Monte Carlo event generators, the available next-to-leading order calculations, resummation, photon production, small x physics, double parton scattering, and backgrounds to Higgs production.

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