Extended parameterisations for MSTW PDFs and their effect on lepton charge asymmetry from W decays

  • A. D. Martin
  • A. J. Th. M. Mathijssen
  • W. J. Stirling
  • R. S. Thorne
  • B. J. A. Watt
  • G. Watt
Regular Article - Theoretical Physics

Abstract

We investigate the effect of extending the standard MSTW parameterisation of input parton distribution functions (PDFs) using Chebyshev polynomials, rather than the usual expressions which involve a factor of the form (1+ϵx 0.5+γx). We find evidence that four powers in the polynomial are generally sufficient for high precision. Applying this to valence and sea quarks, the gluon already being sufficiently flexible and needing only two powers, we find an improvement in the global fit, but a significant change only in the small-x valence up-quark PDF, u V . We investigate the effect of also extending, and making more flexible, the ‘nuclear’ correction to deuteron structure functions. We show that the extended ‘Chebyshev’ parameterisation results in an improved stability in the deuteron corrections that are required for the best fit to the ‘global’ data. The resulting PDFs have a significantly, but not dramatically, altered valence down-quark distribution, d V . It is shown that, for the extended set of MSTW PDFs, their uncertainties can be obtained using 23, rather than the usual 20, orthogonal ‘uncertainty’ eigenvectors. This is true both without and with extended deuteron corrections. Since the dominant effect is on the valence quarks, we present a detailed study of the dependence of the valence–sea separation on the predictions for the decay lepton charge asymmetry which results from W ± production at the LHC, illustrating the PDFs and the x range probed for different experimental scenarios. We show that the modified MSTW PDFs make significantly improved predictions for these data at the LHC, particularly for high values of the p T cut of the decay lepton. However, this is a special case, since the asymmetry is extremely sensitive to valence–sea details, and in particular to the combination u V d V of valence PDFs for \(x \sim M_{W}/\sqrt{s}\) at low lepton rapidities. We show that the predictions for a wide variety of total cross sections are very similar to those obtained using the MSTW2008 PDFs, with changes being much smaller than the PDF uncertainties.

Keywords

Chebyshev Polynomial Gluon Distribution Valence Quark Lepton Asymmetry Asymmetry Data 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgements

We would like to thank Stefano Forte and Jon Pumplin for discussion on some of the issues in this article. The work of R.S.T. is supported partly by the London Centre for Terauniverse Studies (LCTS), using funding from the European Research Council via the Advanced Investigator Grant 267352.

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Copyright information

© Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica 2013

Authors and Affiliations

  • A. D. Martin
    • 1
  • A. J. Th. M. Mathijssen
    • 2
  • W. J. Stirling
    • 3
  • R. S. Thorne
    • 4
  • B. J. A. Watt
    • 4
  • G. Watt
    • 5
  1. 1.Institute for Particle Physics PhenomenologyUniversity of DurhamDurhamUK
  2. 2.Rudolf Peierls Centre for Theoretical PhysicsOxfordUK
  3. 3.Cavendish LaboratoryUniversity of CambridgeCambridgeUK
  4. 4.Department of Physics and AstronomyUniversity College LondonLondonUK
  5. 5.Institut für Theoretische PhysikUniversität ZürichZürichSwitzerland

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