Searcharxiv⌕ Search

arXiv subjects

R. S. Thorne

Publications and source records attributed to R. S. Thorne.

At least 73 records · Page 4Linked to original sources
arXiv2007

The role of uncertainties in parton distribution functions

I consider the uncertainties in parton distributions and the consequences for hadronic cross-sections. There is ever-increasing sophistication in the relationship between the uncertainties of the distributions and the errors on the experimental data used to extract them. However, I demonstrate that this uncertainty is frequently subsumed by that due to the choice of data used in fits, and more surprisingly by the precise details of the theoretical framework used. Variations in heavy flavour prescriptions provide striking examples.

hep-ph↗
arXiv2007

Update of Parton Distributions at NNLO

We present a new set of parton distributions obtained at NNLO. These differ from the previous sets available at NNLO due to improvements in the theoretical treatment. In particular we include a full treatment of heavy flavours in the region near the quark mass. In this way, an essentially complete set of NNLO partons is presented for the first time. The improved treatment leads to a significant change in the gluon and heavy quark distributions, and a larger value of the QCD coupling at NNLO, alpha_S(M_Z^2) = 0.1191 +/- 0.002(expt.) +/- 0.003(theory). Indirectly this also leads to a change in the light partons at small x and modifications of our predictions for W and Z production at the LHC. As well as the best-fit set of partons, we also provide 30 additional sets representing the uncertainties of the partons obtained using the Hessian approach.

hep-ph↗
arXiv2007

Uncertainties of predictions from parton distributions. I: experimental errors

We determine the uncertainties on observables arising from the errors on the experimental data that are fitted in the global MRST2001 parton analysis. By diagonalizing the error matrix we produce sets of partons suitable for use within the framework of linear propagation of errors, which is the most convenient method for calculating the uncertainties. Despite the potential limitations of this approach we find that it can be made to work well in practice. This is confirmed by our alternative approach of using the more rigorous Lagrange multiplier method to determine the errors on physical quantities directly. As particular examples we determine the uncertainties on the predictions of the charged-current deep-inelastic structure functions, on the cross-sections for W production and for Higgs boson production via gluon--gluon fusion at the Tevatron and the LHC, on the ratio of W-minus to W-plus production at the LHC and on the moments of the non-singlet quark distributions. We discuss the corresponding uncertainties on the parton distributions in the relevant x,Q^2 domains. Finally, we briefly look at uncertainties related to the fit procedure, stressing their importance and using sigma_W, sigma_H and extractions of alpha_S(M_Z^2) as examples. As a by-product of this last point we present a slightly updated set of parton distributions, MRST2002.

hep-ph↗
arXiv2007

A Global Fit to Scattering Data with NLL BFKL Resummations

We perform a global parton fit to DIS and related data, including next-to-leading logarithmic (NLL) BFKL resummations in both the massless and massive sectors. The resummed fit improves over a standard next-to-leading order (NLO) DGLAP fit, with a positive definite gluon at the input scale as opposed to the negative gluon seen at NLO. Furthermore, the predicted longitudinal structure function is free of perturbative instability at small x, and the reduced cross-section shows a turnover at high y (absent in the NLO fit) consistent with the HERA data.

hep-ph↗
arXiv2007

Parton Distributions for LO Calculations

We present a study of the results obtained combining LO partonic matrix elements with different orders of partons distributions. These are compared to the best prediction using NLO for both matrix elements and parton distributions. The aim is to determine which parton distributions are most appropriate to use in those cases where only LO matrix elements are available, e.g. as in many Monte Carlo generators. Both LO and NLO parton distributions have faults so a modified optimal LO set is suggested.

hep-ph↗
arXiv2007

Parton Distributions for the LHC

We present a preliminary set of updated NLO parton distributions. For the first time we have a quantitative extraction of the strange quark and antiquark distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections. Additional jet data from HERA and the Tevatron improve our gluon extraction. Lepton asymmetry data and neutrino structure functions improve the flavour separation, particularly constraining the down quark valence distribution.

hep-ph↗
arXiv2006

A Global Fit to Scattering Data with NLL BFKL Resummations

We calculate DIS-scheme splitting and coefficient functions for electromagnetic deep inelastic scattering with small x resummations, as given by the NLL BFKL equation with running coupling and approximate NLL resummed impact factors. The resummations are combined with a NLO fixed order expansion, and the improved quantities thus obtained are stable at small x and significantly suppressed with respect to LL results. These results are implemented in a global fit to DIS and related data, and the results compared to a NLO fixed order DIS-scheme fit and with a previous LL resummed fit. The NLL resummed fit quality is excellent, and constitutes a marked improvement over the purely fixed order approach. The input gluon, at Q_0^2=1GeV^2, obtained from the resummed fit is positive and slightly increasing as x-->0, in contrast to the result obtained at fixed order. A resummed prediction for the longitudinal structure function F_L is presented. It is positive definite and growing with energy at low x and Q^2, where the fixed order results show a significant perturbative instability.

hep-ph↗
arXiv2006

Parton Distributions - DIS06

I discuss the current status of parton distributions. I outline the wide variety of different parton distributions available, and highlight which are either necessary or suitable for use at present.

hep-ph↗
arXiv2006

MRST Parton Distributions -- status 2006

We present the new preliminary MRST parton distributions at NLO and NNLO. The analysis includes some new data and there is an improvement in the theoretical treatment at NNLO. Essentially complete NNLO partons are presented for the first time, together with uncertainties.

hep-ph↗
arXiv2006

The Virtual Photon-Gluon Impact Factor with Massive Quarks and Exact Gluon Kinematics

We calculate the impact factor coupling a virtual photon to a gluon via a massive quark-antiquark pair at LL order, but with the imposition of the correct gluon kinematics. Exact analytical results are presented in triple Mellin space with respect to scaled Bjorken x, gluon transverse momentum and heavy quark mass. The application of these results to the calculation of approximate NLL coefficient functions needed to relate structure functions to the BFKL gluon is presented. The NLL effects with running coupling are seen to lead to a suppression of the small x divergence when compared with the fixed and running coupling LL results, but less than in the massless case.

hep-ph↗
arXiv2006

MRST partons generated in a fixed-flavour scheme

We generate fixed three- and four-light-flavour sets of partons using MRST2004 partons as input. We show that it is important to set n_f=3 in the strong coupling, as well as in the splitting and coefficient functions, in order to obtain a consistent set of fixed-flavour partons. We compare the description of data using partons in both variable- and fixed-flavour-number-schemes.

hep-ph↗
arXiv2006

A Variable-Flavour Number Scheme for NNLO

At NNLO it is particularly important to have a Variable-Flavour Number Scheme (VFNS) to deal with heavy quarks because there are major problems with both the zero mass variable-flavour number scheme and the fixed-flavour number scheme. I illustrate these problems and present a general formulation of a Variable-Flavour Number Scheme (VFNS)for heavy quarks that is explicitly implemented up to NNLO in the strong coupling constant alpha_S, and may be used in NNLO global fits for parton distributions. The procedure combines elements of the ACOT(chi) scheme and the Thorne-Roberts scheme. Despite the fact that at NNLO the parton distributions are discontinuous as one changes the number of active quark flavours, all physical quantities are continuous at flavour transitions and the comparison with data is successful.

hep-ph↗
arXiv2006

The role of F_L(x,Q^2) in parton analyses

We investigate the effect of the structure function F_L in global parton analyses of deep inelastic and related hard scattering data. We perform NLO and NNLO analyses which include the reduced cross section HERA data at high y, as well as earlier direct measurements of F_L. We find that the NNLO analysis gives a better description of F_L at low x than that performed at NLO. Nevertheless the data show evidence of the need of further contributions to F_L, which may be of higher-twist origin. We study such corrections both phenomenologically and theoretically via a renormalon approach. The higher-twist corrections extracted from a successful fit to the data are in general agreement with the theoretical expectations, but there is still room for alternative theoretical contributions, particularly at low x and Q^2. The importance of future measurements of F_L is emphasized.

hep-ph↗
arXiv2006

A Variable Flavour Number Scheme for Heavy Quark Production at Small x

We define a new variable flavour number scheme for use in deep inelastic scattering, motivated by the need to consistently implement high energy resummations alongside a fixed order QCD expansion. We define the DIS(chi) scheme at fixed order, and show how to obtain the small x coefficient functions and heavy flavour matrix elements to leading order in the high energy resummation. We then implement these results in a global fit at LO which includes leading resummations with running coupling corrections. Finally, we address the impact of the resummed results on predictions for the longitudinal structure function. We find that they stabilise the behaviour of F_L at small x. Overall, we find that resummations significantly improve the fit to scattering data in the low x regime, although higher orders in the fixed order expansion are needed to describe current structure function and related data over the complete x range.

hep-ph↗
arXiv2005

Importance of a Measurement of F_L(x,Q^2) at HERA

I investigate what a direct measurement of the longitudinal structure function F_L(x,Q^2) could teach us about the structure of the proton and the best way in which to use perturbative QCD for structure functions. I assume HERA running at a lowered beam energy for approximately 4-5 months and examine how well the measurement could distinguish between different theoretical approaches. I conclude that such a measurement would provide useful information on how to calculate structure functions and parton distributions at small x.

hep-ph↗
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.

hep-ph↗
arXiv2005

Comparison of NNLO DIS scheme splitting functions with results from exact gluon kinematics at small x

We consider the effect of exact gluon kinematics in the virtual photon-gluon impact factor at small x. By comparing with fixed order DIS scheme splitting and coefficient functions, we show that the exact kinematics results match the fixed order results well at each order, which suggests that they allow for an accurate NLL analysis of proton structure functions. We also present, available for the first time, x-space parameterisations of the NNLO DGLAP splitting functions in the DIS scheme, and also the longitudinal coefficients for neutral current scattering.

hep-ph↗