arXiv subjects

Bryan R. Webber

Publications and source records attributed to Bryan R. Webber.

33 records · Page 2Linked to original sources

arXiv2009

Phenomenology of Production and Decay of Spinning Extra-Dimensional Black Holes at Hadron Colliders

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.

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arXiv2009

Perturbative QCD effects and the search for a H->WW->l nu l nu signal at the Tevatron

The Tevatron experiments have recently excluded a Standard Model Higgs boson in the mass range 160 - 170 GeV at the 95% confidence level. This result is based on sophisticated analyses designed to maximize the ratio of signal and background cross-sections. In this paper we study the production of a Higgs boson of mass 160 GeV in the gg -> H -> WW -> l nu l nu channel. We choose a set of cuts like those adopted in the experimental analysis and compare kinematical distributions of the final state leptons computed in NNLO QCD to lower-order calculations and to those obtained with the event generators PYTHIA, HERWIG and MC@NLO. We also show that the distribution of the output from an Artificial Neural Network obtained with the different tools does not show significant differences. However, the final acceptance computed with PYTHIA is smaller than those obtained at NNLO and with HERWIG and MC@NLO. We also investigate the impact of the underlying event and hadronization on our results.

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arXiv2008

The MC@NLO 3.4 Event Generator

This is the user's manual of MC@NLO 3.4. 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, single top in association with a W, 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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arXiv2008

Distinguishing Spins in Decay Chains at the Large Hadron Collider

If new particles are discovered at the LHC, it will be important to determine their spins in as model-independent a way as possible. We consider the case, commonly encountered in models of physics beyond the Standard Model, of a new scalar or fermion $D$ decaying sequentially into other new particles $C,B,A$ via the decay chain $D\to C q$, $C\to B l^{near}$, $B\to A l^{far}$, $l^{near}$ and $l^{far}$ being opposite-sign same-flavour charged leptons and $A$ being invisible. We compute the observable 2- and 3-particle invariant mass distributions for all possible spin assignments of the new particles, and discuss their distinguishability using a quantitative measure known as the Kullback-Leibler distance.

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arXiv2008

QCD radiation effects on the H->WW->l nu l nu signal at the LHC

The discovery of a Standard Model Higgs boson is possible when experimental cuts are applied which increase the ratio of signal and background cross-sections. In this paper we study the pp->H->WW signal cross-section at the LHC which requires a selection of Higgs bosons with small transverse momentum. We compare predictions for the efficiency of the experimental cuts from a NNLO QCD calculation, a calculation of the resummation of logarithms in the transverse momentum of the Higgs boson at NNLL, and the event generator MC@NLO. We also investigate the impact of various jet-algorithms, the underlying event and hadronization on the signal cross-section.

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arXiv2007

Distinguishing Spins in Supersymmetric and Universal Extra Dimension Models at the Large Hadron Collider

An interesting alternative to supersymmetry (SUSY) for extending physics beyond the Standard Model is a model with universal extra dimensions (UED), in which the SUSY superpartners are replaced by Kaluza-Klein excitations of the Standard Model particles. If new particles are discovered at the LHC, even if their mass spectrum favours SUSY or UED, it will be vital to distinguish between their spin assignments in the two models as far as possible. We extend the method proposed by Barr [hep-ph/0405052] to the UED case and investigate the angular and charge asymmetries of decay distributions for sample mass spectra of both SUSY and UED types. For hierarchical (`SUSY-type') mass spectra there is a good chance of distinguishing the spin structures of the two models. However, a mass spectrum of the quasi-degenerate type expected in UED would make it difficult to observe spin correlations.

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arXiv2006

Addendum to "Distinguishing Spins in Decay Chains at the Large Hadron Collider"

We extend our earlier study of spin correlations in the decay chain $D\to C q$, $C\to B l^{near}$, $B\to A l^{far}$, where $A,B,C,D$ are new particles with known masses but undetermined spins, $l^{near}$ and $l^{far}$ are opposite-sign same-flavour charged leptons and $A$ is invisible. Instead of looking at the observable 2- and 3-particle invariant mass distributions separately, we compare the full three-dimensional phase space distributions for all possible spin assignments of the new particles, and show that this enhances their distinguishability using a quantitative measure known as the Kullback-Leibler distance.

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arXiv2005

The MC@NLO 3.1 Event Generator

This is the user's manual of MC@NLO 3.1. 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, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations in decays are included for all processes except ttbar, ZZ, and WZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions.

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arXiv2005

Coalescence model for Theta_c pentaquark formation

We present a model for the formation of the charmed pentaquark Theta_c in hard scattering processes such as deep inelastic scattering, e^+e^- annihilation, and high-energy p pbar collisions. The model assumes that the cross section for Theta_c formation is proportional to the rate of production of p D^{*-} (or pbar D^{*+}) pairs in close proximity both in momentum space and in coordinate space. The constant of proportionality is determined from the Theta_c cross section in deep inelastic scattering as reported by the H1 experiment. The HERWIG Monte Carlo is used to generate simulated DIS events and also to model the space-time structure of the final state. Requiring the proton and the D^* be within a 100 MeV mass window and separated by a spacelike distance of no more than 2 fm, we find that a large "coalescence enhancement factor" F_co \sim 10 is required to account for the H1 signal. The same approach is then applied in order to estimate the number and characteristics of Theta_c events produced at LEP and the Tevatron.

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arXiv2003

The MC@NLO 2.2 Event Generator

This is the user's manual of MC@NLO 2.2. 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 Higgs bosons, single vector bosons, vector boson pairs, and heavy quark-antiquark pairs. This document is self-contained, but we emphasise the main differences with respect to previous versions.

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arXiv2001

Tau Neutrinos from Astrophysical and Cosmological Sources

Previous work on the neutrino spectra from high energy sources has not included the tau neutrinos directly produced by the decays in the source. Here we consider the tau neutrino component and discuss how its inclusion modifies the expected neutrino spectra. We discuss implications for interpreting any observed tau neutrino component in TeV - UHE events as evidence of nu_mu --> nu_tau oscillations.

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arXiv2000

Two-loop enhancement factor for 1/Q corrections to event shapes in deep inelastic scattering

We compute the two-loop enhancement factors for our earlier one-loop calculations of leading (1/Q) power corrections to the mean values of some event shape variables in deep inelastic lepton scattering. The enhancement is found to be equal to the universal ``Milan factor'' for those shape variables considered, provided the one-loop calculation is performed in a particular way. As a result, the phenomenology of power corrections to DIS event shapes remains largely unaffected.

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arXiv1999

BFKL dynamics at hadron colliders

Hadron colliders can provide important tests of BFKL `small-x' dynamics. We discuss two examples of such tests, the inclusive dijet jet cross section at large rapidity separation and the number of associated `mini-jets' in Higgs boson production.

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arXiv1999

Multiplicity of (Mini-)Jets at Small x

We derive closed expressions for the mean and variance of the (mini-)jet multiplicity distribution in hard scattering processes at low x. Here (mini-)jets are defined as those due to initial-state radiation of gluons with transverse momenta greater than some resolution scale m_R, where Lambda^2 << m_R^2 << Q^2, Lambda being the intrinsic QCD scale and Q the momentum transfer scale of the hard scattering. Our results are valid to leading order in log(1/x) but include all sub-leading logarithms of Q^2/m_R^2. As an illustration, we predict the mini-jet multiplicity in Higgs boson production at the Large Hadron Collider.

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arXiv1999

Jet Rates at Small x to Single-Logarithmic Accuracy

We present predictions of jet rates in deep inelastic scattering at small x to leading-logarithmic order in x, including all sub-leading logarithms of Q^2/m_R^2 where m_R is the transverse momentum scale at which jets are resolved. We give explicit results for up to three jets, and a perturbative expansion for multi-jet rates and jet multiplicities.

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