Publications (173)622.61 Total impact
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ABSTRACT: We show that in studies of light quark and gluoninitiated jet discrimination, it is important to include the information on softer reconstructed jets (associated jets) around a primary hard jet. This is particularly relevant while adopting a small radius parameter for reconstructing hadronic jets. The probability of having an associated jet as a function of the primary jet transverse momentum ($p_T$) and radius, the minimum associated jet $p_T$ and the association radius is computed upto nexttodouble logarithmic accuracy (NDLA), and the predictions are compared with results from Herwig++, Pythia6 and Pythia8 Monte Carlos (MC). We demonstrate the improvement in quarkgluon discrimination on using the associated jet rate variable with the help of a multivariate analysis. The associated jet rates are found to be only mildly sensitive to the choice of parton shower and hadronization algorithms, as well as to the effects of initial state radiation and underlying event. In addition, the number of $k_T$ subjets of an anti$k_T$ jet is found to be an observable that leads to a rather uniform prediction across different MC's, broadly being in agreement with predictions in NDLA, as compared to the often used number of charged tracks observable.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the resummed hadronic transverseenergy ($E_T$) distribution due to initialstate 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 nexttoleading 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.  [Show abstract] [Hide abstract]
ABSTRACT: We point out that QCD coherence effects can help to identify the colour structure of possible new physics contributions to the anomalously large forwardbackward 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 fixedorder effective field theory approach and Monte Carlo studies of specific models, we find that an schannel octet structure is preferred.  [Show abstract] [Hide abstract]
ABSTRACT: The leadingorder accurate description of top quark pair production, as usually employed in standard Monte Carlo event generators, gives no rise to the generation of a forwardbackward asymmetry. Yet, nonnegligible  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.  [Show abstract] [Hide abstract]
ABSTRACT: We derive generating functions, valid to nexttodouble logarithmic accuracy, for QCD jet rates according to the inclusive forms of the kt, Cambridge/Aachen and antikt 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 Poissonlike and staircaselike behaviour of jet ratios.  [Show abstract] [Hide abstract]
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 3prong decay. The kinematics is solved using information from the visible decay products, the missing transverse momentum, and the 3prong 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.  [Show abstract] [Hide abstract]
ABSTRACT: Coherent QCD radiation in the hadroproduction of top quark pairs leads to a forwardbackward 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 fixedorder calculations and the Herwig++, Pythia and Sherpa event generators, and discuss their implications.  [Show abstract] [Hide abstract]
ABSTRACT: We review the methods developed for combining the parton shower approximation to QCD with fixedorder perturbation theory, in such a way as to achieve nexttoleadingorder (NLO) accuracy for inclusive observables. This has made it possible to generate fullysimulated 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 matrixelement + parton shower matching, and current efforts to combine the two approaches.  [Show abstract] [Hide abstract]
ABSTRACT: These are the proceedings of the "Workshop on Precision Measurements of alphas" held at the MaxPlanckInstitute for Physics, Munich, February 911, 2011. The workshop explored in depth the determination of alphas(mZ) in the MSbar scheme from the key categories where high precision measurements are currently being made, including DIS and global PDF fits, taudecays, electroweak precision observables and Zdecays, eventshapes, and lattice QCD. These proceedings contain a short summary contribution from the speakers, as well as the lists of authors, conveners, participants, and talks.  [Show abstract] [Hide abstract]
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 thirdgeneration leptoquarks in composite models of electroweak symmetry breaking, and Higgs boson decay into two tau leptons.  [Show abstract] [Hide abstract]
ABSTRACT: We review the physics basis, main features and use of generalpurpose Monte Carlo event generators for the simulation of protonproton collisions at the Large Hadron Collider. Topics included are: the generation of hardscattering matrix elements for processes of interest, at both leading and nexttoleading 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; nonperturbative 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 beyondStandardModel 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 partonlevel predictions are translated into hadronlevel events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.  [Show abstract] [Hide abstract]
ABSTRACT: Fermion masses may arise via mixing of elementary fermions with composite fermions of a strong sector in scenarios of stronglycoupled 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 gaugeinvariant nonderivative and singlederivative 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. 
Article: The MC@NLO 4.0 Event Generator
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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 selfcontained, but we emphasise the main differences with respect to previous versions. Comment: 36 pages, no figures  [Show abstract] [Hide abstract]
ABSTRACT: We present the calculations necessary to obtain nexttoleading 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 
Article: Erratum to: Nonperturbative contribution to the thrust distribution in e+eā annihilation
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ABSTRACT: We reevaluate the nonperturbative 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 nonperturbative physics on this observable. In particular, we investigate how well a model based on a lowscale 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 nonperturbative 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 nontrivial because a part of the 1/Qdependent contribution (the infrared renormalon) is included in the NNLO perturbative correction. 
Article: Review of Particle Physics
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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 quarkmixing matrix, Vud & Vus, Vcb & Vub, fragmentation functions, particle detectors for accelerator and nonaccelerator 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: http://pdg.1b1.gov.  [Show abstract] [Hide abstract]
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 ā Y + jet, Y ā X + l 1, X ā N + l 2, where l 1, l 2 are oppositesign sameflavour 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 3dimensional space of masssquared 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.  [Show abstract] [Hide abstract]
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 finalstate 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 partonlevel Monte Carlo results for top quark and gluino pairproduction 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 Wbosons). 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 
Article: Resummation of transverse energy in vector boson and Higgs boson production at hadron colliders
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ABSTRACT: We compute the resummed hadronic transverse energy (E T ) distribution due to initialstate QCD radiation in vector boson and Higgs boson production at hadron colliders. The resummed exponent, parton distributions and coefficient functions are treated consistently to nexttoleading order. The results are matched to fixedorder calculations at large E T and compared with partonshower Monte Carlo predictions at Tevatron and LHC energies. 
Article: HERWIG 6.5 release note.
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ABSTRACT: A new release of the Monte Carlo program HERWIG (version 6.5) is now available. The main new features are: support for the Les Houches interface to matrix element generators; additional SM and MSSM Higgs processes in lepton collisions; additional matrix elements for the spin correlation algorithm; a new version of the ISAWIG interface; interface to the MC@NLO program for heavy quark, Higgs and vector boson production in hadron collisions. This is planned to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.
Publication Stats
22k  Citations  
622.61  Total Impact Points  
Top Journals
Institutions

19812015

University of Cambridge
 Department of Physics: Cavendish Laboratory
Cambridge, England, United Kingdom


2010

UniversitĆ¤t Siegen
Siegen, North RhineWestphalia, Germany


2008

Utrecht University
 Institute for Theoretical Physics
Utrecht, Utrecht, Netherlands


19842003

CERN
 Theoretical Physics Unit (TH)
GenĆØve, Geneva, Switzerland


2000

Durham University
 Department of Physics
Durham, England, United Kingdom
