arXiv subjects

B. R. Webber

Publications and source records attributed to B. R. Webber.

At least 19 recordsLinked to original sources

arXiv2025

QCD-inspired description of multiplicity distributions in jets

We suggest a universal QCD-motivated expression for the Polyakov-KNO multiplicity distributions of hadrons in jets and compare it with data from $e^+e^-$ annihilation experiments. The moments and overall shape of the distributions in full events and quark and gluon jets, over a range of energies, are described with reasonable quantitative precision. In particular, the scaling violation predicted by QCD is seen clearly in the moments and high-multiplicity fluctuations.

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arXiv2025

Hadron multiplicity fluctuations in perturbative QCD

We examine hadron multiplicity fluctuations in hard processes and confront analytic QCD predictions with the pattern of multiplicity fluctuations observed in $e^+e^-$ annihilation and high-$p_t$ jets produced in $pp$ collisions at the LHC. Special emphasis is placed on high-multiplicity fluctuations in jets. Selecting events with hadronic multiplicity exceeding the average value by a factor of 3 or more in various processes has been a source of conundrums for many years. We discuss two recent high-multiplicity puzzles and attempt to reveal their common origin.

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arXiv2008

Herwig++ Physics and Manual

In this paper we describe Herwig++ version 2.3, a general-purpose Monte Carlo event generator for the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron collisions. A number of important hard scattering processes are available, together with an interface via the Les Houches Accord to specialized matrix element generators for additional processes. The simulation of Beyond the Standard Model (BSM) physics includes a range of models and allows new models to be added by encoding the Feynman rules of the model. The parton-shower approach is used to simulate initial- and final-state QCD radiation, including colour coherence effects, with special emphasis on the correct description of radiation from heavy particles. The underlying event is simulated using an eikonal multiple parton-parton scattering model. The formation of hadrons from the quarks and gluons produced in the parton shower is described using the cluster hadronization model. Hadron decays are simulated using matrix elements, where possible including spin correlations and off-shell effects.

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arXiv2008

Non-Perturbative Contribution to the Thrust Distribution in $e^+ e^-$ Annihilation

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 $α_s(M_Z) = 0.1164^{+0.0028}_{-0.0026}$ and $α_0(2 GeV)=0.59+/-0.03$ are obtained with $χ^2/d.o.f.=1.09$. This is consistent with NLO+NLL results but the quality of fit is improved. The agreement in $α_0$ is non-trivial because a part of the 1/Q-dependent contribution (the infrared renormalon) is included in the NNLO perturbative correction.

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arXiv2008

Herwig++ Status Report

Herwig++ is the successor of the event generator HERWIG. In its present version 2.2.1 it provides a program for full LHC event generation which is superior to the previous program in many respects. We briefly summarize its features and describe present work and some future plans.

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arXiv2008

Herwig++ 2.2 Release Note

A new release of the Monte Carlo program Herwig++ (version 2.2) is now available. This version includes a number of improvements including: matrix elements for the production of an electroweak gauge boson, W and Z, in association with a jet; several new processes for Higgs production in association with an electroweak gauge boson; and the matrix element correction for QCD radiation in Higgs production via gluon fusion.

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arXiv2007

Herwig++ 2.1 Release Note

A new release of the Monte Carlo program Herwig++ (version 2.1) is now available. This version includes a number of significant improvements including: an eikonal multiple parton-parton scattering model of the underlying event; the inclusion of Beyond the Standard Model physics; and a new hadronic decay model tuned to LEP data. This version of the program is now fully ready for the simulation of events in hadron-hadron collisions.

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arXiv2007

Angular correlations of lepton pairs from vector boson and top quark decays in Monte Carlo simulations

We explain how angular correlations in leptonic decays of vector bosons and top quarks can be included in Monte Carlo parton showers, in particular those matched to NLO QCD computations. We consider the production of $n$ pairs of leptons, originating from the decays of $n$ electroweak vector bosons or of $n$ top quarks, in the narrow-width approximation. In the latter case, the information on the $n$ $b$ quarks emerging from the decays is also retained. We give results of implementing this procedure in MC@NLO

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arXiv2006

The MC@NLO 3.3 Event Generator

This is the user's manual of MC@NLO 3.3. This package is a practical implementation, based upon the HERWIG event generator, 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, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations are included for all processes except ZZ and WZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions.

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arXiv2006

Herwig++ 2.0 Release Note

A new release of the Monte Carlo program Herwig++ (version 2.0) is now available. This is the first version of the program which can be used for hadron-hadron physics and includes the full simulation of both initial- and final-state QCD radiation.

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arXiv2006

A standard format for Les Houches Event Files

A standard file format is proposed to store process and event information, primarily output from parton-level event generators for further use by general-purpose ones. The information content is identical with what was already defined by the Les Houches Accord five years ago, but then in terms of Fortran commonblocks. This information is embedded in a minimal XML-style structure, for clarity and to simplify parsing.

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arXiv2006

Single-top production in MC@NLO

We match next-to-leading order QCD results for single-top hadroproduction with parton shower Monte Carlo simulations, according to the prescription of the MC@NLO formalism. In this way, we achieve the first practical implementation in MC@NLO of a process that has both initial- and final-state collinear singularities. We show that no difficulties of principle arise from this complication, and present selected results relevant to the Tevatron.

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arXiv2006

Herwig++ 2.0beta Release Note

A new release of the Monte Carlo program Herwig++ (version 2.0beta) is now available. The main new feature is the extension of the program to include simple hadron-hadron processes including the initial-state parton shower.

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arXiv2006

The MC@NLO 3.2 Event Generator

This is the user's manual of MC@NLO 3.2. This package is a practical implementation, based upon the HERWIG event generator, 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, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations in decays are included for all processes except ttbar, single-t, ZZ, and WZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions.

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arXiv2004

Exploring Higher Dimensional Black Holes at the Large Hadron Collider

In some extra dimension theories with a TeV fundamental Planck scale, black holes could be produced in future collider experiments. Although cross sections can be large, measuring the model parameters is difficult due to the many theoretical uncertainties. Here we discuss those uncertainties and then we study the experimental characteristics of black hole production and decay at a typical detector using the ATLAS detector as a guide. We present a new technique for measuring the temperature of black holes that applies to many models. We apply this technique to a test case with four extra dimensions and, using an estimate of the parton-level production cross section error of 20%, determine the Planck mass to 15% and the number of extra dimensions to +-0.75.

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arXiv2004

Les Houches Guidebook to Monte Carlo Generators for Hadron Collider Physics

Recently the collider physics community has seen significant advances in the formalisms and implementations of event generators. This review is a primer of the methods commonly used for the simulation of high energy physics events at particle colliders. We provide brief descriptions, references, and links to the specific computer codes which implement the methods. The aim is to provide an overview of the available tools, allowing the reader to ascertain which tool is best for a particular application, but also making clear the limitations of each tool.

hep-ph↗

arXiv2004

The MC@NLO 2.3 Event Generator

This is the user's manual of MC@NLO 2.3. This package is a practical implementation, based upon the HERWIG event generator, 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, and lepton pairs. This document is self-contained, but we emphasise the main differences with respect to previous versions.

hep-ph↗

arXiv2003

Matching NLO QCD and parton showers in heavy flavour production

We apply the MC@NLO approach to the process of heavy flavour hadroproduction. MC@NLO is a method for matching next-to-leading order (NLO) QCD calculations and parton shower Monte Carlo (MC) simulations, with the following features: fully exclusive events are generated, with hadronisation according to the MC model; total rates are accurate to NLO; NLO results for distributions are recovered upon expansion in $\as$; hard emissions are treated as in NLO computations while soft/collinear emissions are handled by the MC simulation, with the same logarithmic accuracy as the MC; matching between the hard and soft regions is smooth, and no intermediate integration steps are necessary. The method was applied previously to the hadroproduction of gauge boson pairs, which at NLO involves only initial-state QCD radiation and a unique colour structure. In heavy flavour production, it is necessary to include contributions from final-state QCD radiation and different colour flows. We present illustrative results on top and bottom production at the Tevatron and LHC.

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