B. R. Webber

University of Cambridge, Cambridge, ENG, United Kingdom

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Publications (161)542.67 Total impact

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    ABSTRACT: We compute the resummed hadronic transverse-energy ($E_T$) distribution due to initial-state QCD radiation in the production of a Standard Model Higgs boson of mass 126 GeV by gluon fusion at the Large Hadron Collider, with matching to next-to-leading order calculations at large $E_T$. Effects of hadronization, underlying event and limited detector acceptance are estimated using aMC@NLO with the Herwig++ event generator.
    Journal of High Energy Physics 03/2014; 2014(9). · 5.62 Impact Factor
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    11/2013;
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    ABSTRACT: We point out that QCD coherence effects can help to identify the colour structure of possible new physics contributions to the anomalously large forward-backward asymmetry in top quark pair production. New physics models that yield the same inclusive asymmetry make different predictions for its dependence on the transverse momentum of the pair, if they have different colour structures. From both a fixed-order effective field theory approach and Monte Carlo studies of specific models, we find that an s-channel octet structure is preferred.
    09/2013;
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    Jan Winter, Peter Z. Skands, Bryan R. Webber
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    ABSTRACT: The leading-order accurate description of top quark pair production, as usually employed in standard Monte Carlo event generators, gives no rise to the generation of a forward--backward asymmetry. Yet, non-negligible -- differential as well as inclusive -- asymmetries may be produced if coherent parton showering is used in the hadroproduction of top quark pairs. In this contribution we summarize the outcome of our study of this effect. We present a short comparison of different parton shower implementations and briefly comment on the phenomenology of the colour coherence effect at the Tevatron.
    02/2013;
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    ABSTRACT: We derive generating functions, valid to next-to-double logarithmic accuracy, for QCD jet rates according to the inclusive forms of the kt, Cambridge/Aachen and anti-kt algorithms, which are equivalent at this level of accuracy. We compare the analytical results with jet rates and average jet multiplicities from the SHERPA event generator, and study the transition between Poisson-like and staircase-like behaviour of jet ratios.
    Journal of High Energy Physics 12/2012; 2013(4). · 5.62 Impact Factor
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    ABSTRACT: We propose a method for reconstructing the mass of a particle, such as the Higgs boson, decaying into a pair of tau leptons, of which one subsequently undergoes a 3-prong decay. The kinematics is solved using information from the visible decay products, the missing transverse momentum, and the 3-prong tau decay vertex, with the detector resolution taken into account using a likelihood method. The method is shown to give good discrimination between a 125 GeV Higgs boson signal and the dominant backgrounds, such as Z decays to tau tau and W plus jets production. As a result, we find an improvement, compared to existing methods for this channel, in the discovery potential, as well as in measurements of the Higgs boson mass and production cross section times branching ratio.
    Journal of High Energy Physics 10/2012; 2013(3). · 5.62 Impact Factor
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    Peter Z. Skands, Bryan R. Webber, Jan Winter
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    ABSTRACT: Coherent QCD radiation in the hadroproduction of top quark pairs leads to a forward--backward asymmetry that grows more negative with increasing transverse momentum of the pair. This feature is present in Monte Carlo event generators with coherent parton showering, even though the production process is treated at leading order and has no intrinsic asymmetry before showering. In addition, depending on the treatment of recoils, showering can produce a positive contribution to the inclusive asymmetry. We explain the origin of these features, compare them in fixed-order calculations and the Herwig++, Pythia and Sherpa event generators, and discuss their implications.
    Journal of High Energy Physics 05/2012; · 5.62 Impact Factor
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    Paolo Nason, Bryan Webber
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    ABSTRACT: We review the methods developed for combining the parton shower approximation to QCD with fixed-order perturbation theory, in such a way as to achieve next-to-leading-order (NLO) accuracy for inclusive observables. This has made it possible to generate fully-simulated hadronic final states with the precision and stability of NLO calculations. We explain the underlying theory of the existing methods, MC@NLO and POWHEG, together with their similarities, differences, achievements and limitations. For illustration we mainly compare results on Higgs boson production at the LHC, with particular emphasis on the residual uncertainties arising from the different treatment of effects beyond NLO. We also briefly summarize the difference between these NLO + parton shower methods and matrix-element + parton shower matching, and current efforts to combine the two approaches.
    Annual Review of Nuclear and Particle Science 02/2012; · 7.40 Impact Factor
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    ABSTRACT: These are the proceedings of the "Workshop on Precision Measurements of alphas" held at the Max-Planck-Institute for Physics, Munich, February 9-11, 2011. The workshop explored in depth the determination of alphas(mZ) in the MS-bar scheme from the key categories where high precision measurements are currently being made, including DIS and global PDF fits, tau-decays, electroweak precision observables and Z-decays, event-shapes, and lattice QCD. These proceedings contain a short summary contribution from the speakers, as well as the lists of authors, conveners, participants, and talks.
    09/2011;
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    Ben Gripaios, Kazuki Sakurai, Bryan Webber
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    ABSTRACT: We consider the problem of reconstructing energies, momenta, and masses in collider events with missing energy, along with the complications introduced by combinatorial ambiguities and measurement errors. Typically, one reconstructs more than one value and we show how the wrong values may be correlated with the right ones. The problem has a natural formulation in terms of the theory of Riemann surfaces. We discuss examples including top quark decays in the Standard Model (relevant for top quark mass measurements and tests of spin correlation), cascade decays in models of new physics containing dark matter candidates, decays of third-generation leptoquarks in composite models of electroweak symmetry breaking, and Higgs boson decay into two tau leptons.
    Journal of High Energy Physics 03/2011; 9. · 5.62 Impact Factor
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    ABSTRACT: We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hard-scattering matrix elements for processes of interest, at both leading and next-to-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diffractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the ARIADNE, Herwig++, PYTHIA 8 and SHERPA generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.
    Physics Reports 01/2011; · 22.93 Impact Factor
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    ABSTRACT: Fermion masses may arise via mixing of elementary fermions with composite fermions of a strong sector in scenarios of strongly-coupled electroweak symmetry breaking. The strong sector may contain leptoquark states with masses as light as several hundred GeV. In the present study we focus on the scalar modes of such leptoquarks since their bosonic couplings are determined completely and hence their production cross sections only depend on their masses. We study all the possible gauge-invariant non-derivative and single-derivative couplings of the scalar leptoquarks to the quarks and leptons, which turn out to be, predominantly, of the third generation. We examine their phenomenology and outline search strategies for their dominant decay modes at the LHC.
    Journal of High Energy Physics 10/2010; · 5.62 Impact Factor
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    ABSTRACT: This is the user's manual of MC@NLO 4.0. This package is a practical implementation, based upon the Fortran HERWIG and Herwig++ event generators, of the MC@NLO formalism, which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework. Processes available in this version include the hadroproduction of single vector and Higgs bosons, vector boson pairs, heavy quark pairs, single top, single top in association with a W, single top in association with a charged Higgs in type I or II 2HDM models, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations are included for all processes except ZZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions. Comment: 36 pages, no figures
    10/2010;
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    ABSTRACT: We present the calculations necessary to obtain next-to-leading order QCD precision with the Herwig++ event generator using the MC@NLO approach, and implement them for all the processes that were previously available from Fortran HERWIG with MC@NLO. We show a range of results comparing the two implementations. With these calculations and recent developments in the automatic generation of NLO matrix elements, it will be possible to obtain NLO precision with Herwig++ for a much wider range of processes
    Journal of High Energy Physics 10/2010; 2011(1). · 5.62 Impact Factor
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    ABSTRACT: This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2158 new measurements from 551 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on neutrino mass, mixing, and oscillations, QCD, top quark, CKM quark-mixing matrix, V_(ud) & V_(us), V_(cb) & V_(ub), fragmentation functions, particle detectors for accelerator and non-accelerator physics, magnetic monopoles, cosmological parameters, and big bang cosmology. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: pdg.lbl.gov.
    Journal of Physics G Nuclear and Particle Physics 07/2010; · 5.33 Impact Factor
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    Mihoko M. Nojiri, Kazuki Sakurai, Bryan R. Webber
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    ABSTRACT: A method is proposed for determining the masses of the new particles N,X,Y,Z in collider events containing a pair of effectively identical decay chains Z to Y+jet, Y to X+l_1, X to N+l_2, where l_1, l_2 are opposite-sign same-flavour charged leptons and N is invisible. By first determining the upper edge of the dilepton invariant mass spectrum, we reduce the problem to a curve for each event in the 3-dimensional space of mass-squared differences. The region through which most curves pass then determines the unknown masses. A statistical approach is applied to take account of mismeasurement of jet and missing momenta. The method is easily visualized and rather robust against combinatorial ambiguities and finite detector resolution. It can be successful even for small event samples, since it makes full use of the kinematical information from every event. Comment: 12 pages, 5 figures
    Journal of High Energy Physics 05/2010; · 5.62 Impact Factor
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    Andreas Papaefstathiou, Bryan Webber
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    ABSTRACT: We examine the effects of invisible particle emission in conjunction with QCD initial state radiation (ISR) on quantities designed to probe the mass scale of new physics at hadron colliders, which involve longitudinal as well as transverse final-state momenta. This is an extension of our previous treatment, arXiv:0903.2013, of the effects of ISR on global inclusive variables. We present resummed results on the visible invariant mass distribution and compare them to parton-level Monte Carlo results for top quark and gluino pair-production at the LHC. There is good agreement as long as the visible pseudorapidity interval is large enough (eta ~ 3). The effect of invisible particle emission is small in the case of top pair production but substantial for gluino pair production. This is due mainly to the larger mass of the intermediate particles in gluino decay (squarks rather than W-bosons). We also show Monte Carlo modelling of the effects of hadronization and the underlying event. The effect of the underlying event is large but may be approximately universal. Comment: 22 pages, expanded sections and other minor modifications. Version published in JHEP
    Journal of High Energy Physics 04/2010; · 5.62 Impact Factor
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    ABSTRACT: We compute the resummed hadronic transverse energy (E_T) distribution due to initial-state QCD radiation in vector boson and Higgs boson production at hadron colliders. The resummed exponent, parton distributions and coefficient functions are treated consistently to next-to-leading order. The results are matched to fixed-order calculations at large E_T and compared with parton-shower Monte Carlo predictions at Tevatron and LHC energies. Comment: 24 pages, 15 figures
    Journal of High Energy Physics 02/2010; · 5.62 Impact Factor
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    R. A. Davison, B. R. Webber
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    ABSTRACT: We re-evaluate the non-perturbative contribution to the thrust distribution in $e^+e^-\to$ hadrons, in the light of the latest experimental data and the recent NNLO perturbative calculation of this quantity. By extending the calculation to NNLO+NLL accuracy, we perform the most detailed study to date of the effects of non-perturbative physics on this observable. In particular, we investigate how well a model based on a low-scale QCD effective coupling can account for such effects. We find that the difference between the improved perturbative distribution and the experimental data is consistent with a $1/Q$-dependent non-perturbative shift in the distribution, as predicted by the effective coupling model. Best fit values of $\alpha_s(M_Z) = 0.1164^{+0.0028}_{-0.0026}$ and $\alpha_0(2 GeV)=0.59+/-0.03$ are obtained with $\chi^2/d.o.f.=1.09$. This is consistent with NLO+NLL results but the quality of fit is improved. The agreement in $\alpha_0$ is non-trivial because a part of the 1/Q-dependent contribution (the infrared renormalon) is included in the NNLO perturbative correction.
    European Physical Journal C 01/2010; 69(1):341-341. · 5.25 Impact Factor
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    ABSTRACT: We present results of CHARYBDIS2, a new Monte Carlo simulation of black hole production and decay at hadron colliders in theories with large extra dimensions and TeV-scale gravity. The main new feature of CHARYBDIS2 is a full treatment of the spin-down phase of the decay process using the angular and energy distributions of the associated Hawking radiation. Also included are improved modelling of the loss of angular momentum and energy in the production process as well as a wider range of options for the Planck-scale termination of the decay. The new features allow us to study the effects of black hole spin and the feasibility of its observation in such theories. The code and documentation can be found at http://projects.hepforge.org/charybdis2/.
    Journal of High Energy Physics 10/2009; 2009(10):014. · 5.62 Impact Factor

Publication Stats

9k Citations
542.67 Total Impact Points

Institutions

  • 1984–2012
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 1984–2003
    • CERN
      Genève, Geneva, Switzerland
  • 2000
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 1991–1995
    • Lund University
      • Department of Theoretical Physics
      Lund, Skane, Sweden