arXiv subjects

W. J. Stirling

Publications and source records attributed to W. J. Stirling.

At least 91 records · Page 5Linked to original sources

arXiv2003

Non-perturbative effects and the resummed Higgs transverse momentum distribution at the LHC

We investigate the form of the non-perturbative parameterization in both the impact parameter (b) space and transverse momentum (p_T) space resummation formalisms for the transverse momentum distribution of single massive bosons produced at hadron colliders. We propose to analyse data on Upsilon hadroproduction as a means of studying the non-perturbative contribution in processes with two gluons in the initial state. We also discuss the theoretical errors on the resummed Higgs transverse momentum distribution at the LHC arising from the non-perturbative contribution.

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arXiv2003

Energy Consumption and Jet Multiplicity from the Leading Log BFKL Evolution

We study the associated jet multiplicity arising from t-channel BFKL gluon evolution in forward dijet production at hadron colliders. Previous results have shown that the effect of conserving overall energy and momentum is to introduce a pdf suppression that completely compensates the predicted exponential BFKL rise with rapidity difference between the leading dijets. However, we show that there is still expected to be a significant amount of BFKL radiation, especially in the central region, and we give predictions for the multiplicity of the resulting mini-jets at the LHC.

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arXiv2002

A Z-Monitor to Calibrate Higgs Production via Vector Boson Fusion with Rapidity Gaps at the LHC

We study central Z-boson production accompanied by rapidity gaps on either side as a way to gauge Higgs weak boson fusion production at the LHC. We analyse the possible backgrounds for the bb-decay mode and show that these can be substantially reduced. Special attention is paid to the evaluation of the gap survival factor, which is the major source of theoretical uncertainty in the rate of H, Z and W central production events with rapidity gaps.

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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

Questions on Uncertainties in Parton Distributions

A discussion is presented of the manner in which uncertainties in parton distributions and related quantities are determined. One of the central problems is the criteria used to judge what variation of the parameters describing a set of partons is acceptable within the context of a global fit. Various ways of addressing this question are outlined.

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arXiv2002

Multiparticle Production in QCD Jets

We briefly summarise the main results presented at the IPPP Workshop on Multiparticle Production in QCD Jets, held in Durham in December 2001.

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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).

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arXiv2001

Forward jets and forward $W$-boson production at hadron colliders

In this talk we give a short review of forward jets and forward $W$-boson production at hadron colliders, in view of the extraction of footprints of BFKL physics. We argue that at Tevatron energies, dijet production at large rapidity intervals is still subasymptotic with respect to the BFKL regime, thus the cross section is strongly dependent on the various cuts applied in the experimental setup. In addition, the choice of equal transverse momentum cuts on the tagging jets makes the cross section dependent on large logarithms of non-BFKL origin, and thus may spoil the BFKL analysis. For vector boson production in association with two jets, we argue that the configurations that are kinematically favoured tend to have the vector boson forward in rapidity. Thus $W + 2$ jet production lends itself naturally to extensions to the high-energy limit.

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arXiv2001

Non-factorizable corrections and effective field theories

We analyze the structure of higher-order radiative corrections for processes with unstable particles. By subsequently integrating out the various scales that are induced by the presence of unstable particles we obtain a hierarchy of effective field theories. In the effective field theory framework the separation of physically different effects is achieved naturally. In particular, we automatically obtain a separation of factorizable and non-factorizable corrections to all orders in perturbation theory. At one loop this treatment is equivalent to the double-pole approximation (DPA) but generalizes to higher orders and, at least in principle, to beyond the DPA. It is known that one-loop non-factorizable corrections to invariant mass distributions are suppressed at high energy. We study the mechanism of this suppression and obtain estimates of higher-order non-factorizable corrections at high energy.

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

W Boson Production with Associated Jets at Large Rapidities

We analyse W boson production at hadron colliders in association with one or two jets, both with the exact kinematics and in the high-energy limit. We argue that the configurations that are kinematically favoured tend to have the W boson forward in rapidity. Thus W boson production in association with jets lends itself naturally to extensions to the high-energy limit, which we examine both at leading order and by resumming higher-order corrections through the BFKL theory.

hep-ph↗

arXiv2001

Progress Towards a Generator for BFKL Physics

In certain regions of phase space in jet production, large logarithms can arise which are resummed by the BFKL equation. Linear colliders can potentially be excellent places to study BFKL effects in jet production. We discuss an approach to BFKL calculations which incorporates kinematic effects explicitly and can be implemented in an event generator.

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arXiv2001

Mueller-Navelet Jets at Hadron Colliders

We critically examine the definition of dijet cross sections at large rapidity intervals in hadron-hadron collisions, taking proper account of the various cuts applied in a realistic experimental setup. We argue that the dependence of the cross section on the precise definition of the parton momentum fractions and the presence of an upper bound on the momentum transfer cannot be neglected, and we provide the relevant modifications to the analytical formulae by Mueller and Navelet. We also point out that the choice of equal transverse momentum cuts on the tagging jets can spoil the possibility of a clean extraction of signals of BFKL physics.

hep-ph↗

arXiv2000

Recent Results on BFKL Physics

Virtual photon scattering in $e^+e^-$ collisions can result in events with the electron-positron pair produced at large rapidity separation in association with hadrons. The BFKL equation resums large logarithms that dominate the cross section for this process. After a brief overview of analytic BFKL resummation and its experimental status, we report on a Monte Carlo method for solving the BFKL equation that allows kinematic constraints to be taken into account. We discuss results for $e^+e^-$ collisions using both fixed-order QCD and the BFKL approach. We conclude with some brief comments on the status of NLL calculations.

hep-ph↗

arXiv2000

BFKL Physics in Jet Production at e+e- colliders

Virtual photon scattering in $e^+e^-$ collisions can result in events with the electron-positron pair at large rapidity separation with hadronic activity in between. The BFKL equation resums large logarithms that dominate the cross section for this process. We report here on a Monte Carlo method for solving the BFKL equation that allows kinematic constraints to be taken into account and show results for $e^+e^-$ collisions.

hep-ph↗

arXiv2000

Impact of the W boson decay width on photon bremsstrahlung accompanying W pair production

The bremsstrahlung spectrum of photons accompanying $W$ pair production depends on the decay width of the unstable $W$ bosons. The dependence arises from the interference between emission at different stages of the production and decay process. We present a quantitative discussion of this effect, and consider the implications for measurements at LEP2 and LC energies.

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.

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