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R. G. Roberts

Publications and source records attributed to R. G. Roberts.

At least 19 recordsLinked to original sources
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↗
arXiv2004

Parton distributions incorporating QED contributions

We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${\cal O}(α_{\rm QED})$ corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p \neq d^n$, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2θ_W$ anomaly. A second consequence is the appearance of photon parton distributions $γ(x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to eγX$; our predictions are in agreement with the present data.

hep-ph↗
arXiv2004

Physical Gluons and High $E_T$ Jets

We propose a more physical parameterization of the gluon distribution for global parton analyses of deep inelastic and related hard scattering data. In the new parameterization the gluon distribution at large x in the MSbar-scheme is driven by the valence quarks, which naturally produces a shoulder-like form at high x, and hence produces a better description of the Tevatron inclusive jet data. We perform the new analysis at both NLO and NNLO. The improvement is found to be even better at NNLO than at NLO. We make available the new sets of NLO and NNLO partons, which we denote by MRST2004.

hep-ph↗
arXiv2004

Update of MRST Parton Distributions

I present a brief summary of the most important updates in the MRST determinations of the NLO and NNLO parton distributions. I concentrate on uncertainties due to input assumptions and possible theoretical corrections, particularly isospin violation, QED corrections, NNLO and corrections at low Q^2 and small x.

hep-ph↗
arXiv2003

Uncertainties of predictions from parton distributions. I: Theoretical errors

We study the uncertainties in parton distributions, determined in global fits to deep inelastic and related hard scattering data, due to so-called theoretical errors. Amongst these, we include potential errors due to the change of perturbative order (NLO to NNLO), ln(1/x) and ln(1-x) effects, absorptive corrections and higher-twist contributions. We investigate these uncertainties both by including explicit corrections to our standard global analysis and by examining the sensitivity to changes of the x,Q^2,W^2 cuts on the data that are fitted. In this way we expose those kinematic regions where the conventional DGLAP description is inadequate. As a consequence we obtain a set of NLO, and of NNLO, conservative partons where the data are fully consistent with DGLAP evolution, but over a restricted kinematic domain. We also examine the potential effects of such issues as the choice of input parameterization, heavy target corrections, assumptions about the strange quark sea and isospin violation. Hence we are able to compare the theoretical errors with those uncertainties due to errors on the experimental measurements, which we studied previously. We use W and Higgs boson production at the Tevatron and the LHC as explicit examples of the uncertainties arising from parton distributions. For many observables the theoretical error is dominant, but for the cross section for W production at the Tevatron both the theoretical and experimental uncertainties are small, and hence the NNLO prediction may serve as a valuable luminosity monitor.

hep-ph↗
arXiv2003

MRST partons and uncertainties

We discuss uncertainties in the extraction of parton distributions from global analyses of DIS and related data. We present conservative sets of partons, at both NLO and NNLO, which are stable to x,Q^2,W^2 cuts on the data. We give the corresponding values of alpha(M_Z^2) and the cross sections for W production at the Tevatron.

hep-ph↗
arXiv2002

The interplay between Sudakov resummation, renormalons and higher twist in deep inelastic scattering

We claim that factorization implies that the evolution kernel, defined by the logarithmic derivative of the N-th moment of the structure function d ln F_2^N / d ln Q^2, receives logarithmically enhanced contributions (Sudakov logs) from a single source, namely the constrained invariant mass of the jet. Available results from fixed-order calculations facilitate Sudakov resummation up to the next-to-next-to-leading logarithmic accuracy. We use additional all-order information on the physical kernel from the large-beta_0 limit to model the behaviour of further subleading logs and explore the uncertainty in extracting alpha_s and in determining the magnitude of higher-twist contributions from a comparison with data on high moments.

hep-ph↗
arXiv2002

Update of MRST Parton Distributions

We discuss the latest update of the MRST parton distributions in response to the most recent data. We discuss the areas where there are hints of difficulties in the global fit, and compare to some other updated sets of parton distributions, particularly CTEQ6. We briefly discuss the issue of uncertainties associated with partons.

hep-ph↗
arXiv2002

NNLO global parton analysis

We perform a NNLO (and a LO) global parton analysis in which we include the new precise data for deep inelastic scattering from HERA and for inclusive jet production at the Tevatron, together with the improved knowledge of the three-loop splitting functions. The results are compared with our recent NLO analyses. The LO fit produces significantly worse results in general, but gives a surprisingly good fit to the Tevatron high-E_T jet data. For the approximate NNLO analysis we notice a slight improvement in the quality of the global fit, and find that the partons are changed by up to 10% at Q^2=10 GeV^2$, in particular in the x<0.01 regime.

hep-ph↗
arXiv2001

MRST2001: partons and alpha_S from precise deep inelastic scattering and Tevatron jet data

We use all the available new precise data for deep inelastic and related hard scattering processes to perform NLO global parton analyses. These new data allow an improved determination of partons and, in particular, the inclusion of the recent measurements of the structure functions at HERA and of the inclusive jets at the Tevatron help to determine the gluon distribution and alpha_S better than ever before. We find a somewhat smaller gluon at low x than previous determinations and that alpha_S(M_Z^2) = 0.119 +/- 0.002 (expt.) +/- 0.003 (theory).

hep-ph↗
arXiv2001

MRST Global Fit Update

We discuss the impact of the most recent data on the MRST global analysis - in particular the new high-E_T jet data and their implications for the gluon and the new small x structure function data. In the light of these new data we also consider the uncertainty in predictions for physical quantities depending on parton distributions, concentrating on the W cross-section at hadron colliders.

hep-ph↗
arXiv2001

Precision test of a Fermion mass texture

Texture zeros in the quark Yukawa matrices generally lead to precise and simple expressions for CKM matrix elements in terms of ratios of quark masses. Using the new data on $b-$decays we test a particularly promising texture zero solution and show that it is at best approximate. We analyse the approximate texture zero structure and show it is consistent with experiment. We investigate the implications for the CKM unitarity triangle, measurements at $BaBar$ and $BELLE$ as well as for the theories which invoke family symmetries.

hep-ph↗
arXiv2001

A Variable Flavour Number Scheme for Charged Current Heavy Flavour Structure Functions

The Thorne-Roberts variable flavour number scheme (VFNS) for heavy quarks is presented in detail for the specific case of charged current DIS. As in neutral current DIS this provides a smooth extrapolation from the fixed flavour number scheme (FFNS) appropriate at low $Q^2$ to the zero-mass variable flavour number scheme (ZM-VFNS) appropriate as $Q^2 \to \infty$, and differs from alternative versions of a VFNS by the definition of the coefficient functions at each order, and the strict ordering of the expansion in $α_S$. However, there are subtle differences from the neutral current case which are addressed here. We discuss both the LO and NLO expressions, the latter unfortunately requiring some (minimal) modelling due to the current lack of some necessary ${\cal O}(α^2_S)$ FFNS coefficient functions.

hep-ph↗
arXiv2000

Estimating the effect of NNLO contributions on global parton analyses

We use the recent estimates of NNLO splitting functions, made by van Neerven and Vogt, to perform exploratory fits to deep inelastic and related hard scattering data. We investigate the hierarchy of parton distributions obtained at LO, NLO and NNLO, and, more important, the stability of the resulting predictions for physical observables. We use the longitudinal structure function $F_L$ and the cross sections $σ_W, σ_Z$ for $W$ and $Z$ hadroproduction as examples. For $F_L$ we find relatively poor convergence, with increasing order, at small $x$; whereas $σ_{W,Z}$ are much more reliably predicted.

hep-ph↗
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.

hep-ph↗
arXiv1999

Parton Distributions and the LHC: W and Z Production

W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledge of parton distributions and their evolution.

hep-ph↗
arXiv1999

Update of MRST partons

We discuss topical issues concerning the MRST parton analysis. We introduce some new parton sets in order to estimate the uncertainty in the cross sections predicted for W and Z production at the Tevatron and the LHC. We compare with CTEQ5 partons.

hep-ph↗