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Michael H. Seymour

Publications and source records attributed to Michael H. Seymour.

At least 55 records · Page 3Linked to original sources
arXiv2008

A model of non-perturbative gluon emission in an initial state parton shower

We consider a model of transverse momentum production in which non-perturbative smearing takes place throughout the perturbative evolution, by a simple modification to an initial state parton shower algorithm. Using this as the important non-perturbative ingredient, we get a good fit to data over a wide range of energy. Combining it with the non-perturbative masses and cutoffs that are a feature of conventional parton showers also leads to a reasonable fit. We discuss the extrapolation to the LHC.

hep-ph↗
arXiv2008

TeVJet: A general framework for the calculation of jet observables in NLO QCD

In this paper we present the parton level Monte Carlo program TeVJet, a direct implementation of the dipole subtraction method for calculating jet cross sections in NLO QCD. It has been written so as to allow the inclusion of new processes in as straightforward a way as possible. The user must provide the usual ingredients for an NLO calculation and from these the process-independent parts required to make the phase space integrals finite in 4 dimensions are automatically generated. These integrals are then performed using Monte Carlo techniques. We present the results for a few example processes.

hep-ph↗
arXiv2007

Breakdown of Coherence ?

In a recent paper, Albrecht Kyrieleis, Jeff Forshaw and I discovered a new tower of super-leading logarithms in gaps between jets cross sections. After discussions with the referee of our paper and further investigation, we have come to view this as a breakdown of naive coherence for initial state radiation. In this talk I illustrate this statement in a simple way, and show how it results in the super-leading logarithms.

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arXiv2006

A Comparison of two different jet algorithms for the top mass reconstruction at the LHC

We compare the abilities of the cluster-type jet algorithm, KtJet, and a mid-point iterating cone algorithm to reconstruct the top mass at the LHC. We discuss the information contained in the merging scales of cluster-type algorithms, and how this can be used in experimental analyses, as well as the different sources of systematic errors for the two algorithms. We find that the sources of systematic error are different for the two algorithms, which may help to better constrain the systematic error on the top mass at the LHC.

hep-ph↗
arXiv2005

Symmetry of Anomalous Dimension Matrices for Colour Evolution of Hard Scattering Processes

In a recent paper, Dokshitzer and Marchesini rederived the anomalous dimension matrix for colour evolution of $gg \to gg$ scattering, first derived by Kidonakis, Oderda and Sterman. They noted a weird symmetry that it possesses under interchange of internal (colour group) and external (scattering angle) degrees of freedom and speculated that this may be related to an embedding into a context that correlates internal and external variables such as string theory. In this short note, I point out another symmetry possessed by all the colour evolution anomalous dimension matrices calculated to date. It is more prosaic, but equally unexpected, and may also point to the fact that colour evolution might be understood in some deeper theoretical framework. To my knowledge it has not been pointed out elsewhere, or anticipated by any of the authors calculating these matrices. It is simply that, in a suitably chosen colour basis, they are complex symmetric matrices.

hep-ph↗
arXiv2005

News from HERWIG

I review the current status and future plans of the HERWIG collaboration.

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arXiv2002

The Dipole Formalism for Next-to-Leading Order QCD Calculations with Massive Partons

The dipole subtraction method for calculating next-to-leading order corrections in QCD was originally only formulated for massless partons. In this paper we extend its definition to include massive partons, namely quarks, squarks and gluinos. We pay particular attention to the quasi-collinear region, which gives rise to terms that are enhanced by logarithms of the parton masses, M. By ensuring that our subtraction cross section matches the exact real cross section in all quasi-collinear regions we achieve uniform convergence both for hard scales Q ~= M and Q >> M. Moreover, taking the masses to zero, we exactly reproduce the previously-calculated massless results. We give all the analytical formulae necessary to construct a numerical program to evaluate the next-to-leading order QCD corrections to arbitrary observables in an arbitrary process.

hep-ph↗
arXiv2000

HERWIG for Top Physics at the Linear Collider

We discuss recent improvement in the treatment of gluon radiation in top production and decay in e+e- processes according to the HERWIG event generator and show studies on the top mass reconstruction at the future Linear Collider.

hep-ph↗
arXiv2000

Parton-Shower Simulations of R-parity Violating Supersymmetric Models

We study the colour connection structure of R-parity violating decays and production cross sections, and construct a Monte Carlo simulation of these processes including colour coherence effects. We then present some results from the implementation of these processes in the HERWIG Monte Carlo event generator. We include the matrix elements for the two-body sfermion and three-body gaugino and gluino decays as well as the two-to-two resonant hard production processes in hadron-hadron collisions.

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arXiv2000

Resonant Slepton Production at the LHC

We consider the resonant production of charged sleptons at the LHC via R-parity violation followed by gauge decays to a charged lepton and a neutralino which then decays via R-parity violation. This gives a signature of two like-sign charged leptons. In the simulation we include the full hadronisation via Monte Carlo programs. We find a background, after cuts, of 5.1+/-2.5 events for an integrated luminosity of 10 fb^-1. A preliminary study of the signal suggests that couplings of 2.10^-3 for a smuon mass of 223 GeV and smuon masses of up to 540 GeV for couplings of 10^-2 can be probed.

hep-ph↗
arXiv1999

Simulations of Top Production and Decay at the Linear Collider

We review the present status of simulations of top production and decay in the HERWIG event generator. We show the phenomenological impact of the recently-implemented matrix-element corrections to top decays for e+e- collisions at 360 GeV and discuss possible further improvements for studies of future experiments at the Linear Collider.

hep-ph↗
arXiv1999

Initial State Radiation in Simulations of Vector Boson Production at Hadron Colliders

The production of vector bosons at present and future hadron colliders will provide a crucial test for QCD and Standard Model physics. In this paper we improve parton shower simulations of Drell-Yan processes by implementing matrix-element corrections to the initial-state radiation. We apply our work to the HERWIG Monte Carlo event generator and compare our phenomenological results with the ones obtained using the previous version of HERWIG, with resummed calculations which we match to the exact first-order perturbative result, and with recent Tevatron data. We also make some predictions for jet events at the LHC.

hep-ph↗
arXiv1999

The Forward-Backward Asymmetry in NNLO QCD

We have recently calculated the second-order QCD corrections to the forward-backward asymmetry in e+e- annihilation (hep-ph/9905424). Here we recall the results and compare them to others in the literature.

hep-ph↗
arXiv1999

Corrections of Order α_s^2 to the Forward-Backward Asymmetry

We calculate the second-order QCD corrections to the forward-backward asymmetry in $e^+e^-$ annihilation. Using the quark axis definition, we do not agree with either existing calculation, but the difference relative to one of them is small and understood. In particular, we point out that the forward-backward asymmetry of massive quarks is enhanced by logarithms of the quark mass. This implies that the forward-backward asymmetry of massless quarks is not computable in QCD perturbation theory and affected by non-power-suppressed corrections coming from the non-perturbative fragmentation functions. We also calculate the second-order corrections using the experimentally-preferred thrust axis definition for the first time.

hep-ph↗
arXiv1999

Resonant Slepton Production

We consider the production of resonant sleptons via R-parity Violation followed by gauge decays to a charged lepton and a neutralino which then decays via R-parity Violation. This gives a signature of two like-sign charged leptons. We find a background at run II of 0.14 +/- 0.13 events with an integrated luminosity of 2 fb^-1. This enables us to probe R-parity Violating couplings of 2.10^-3 for slepton mass of 100 GeV and up to slepton masses of 300 GeV for R-parity Violating couplings of 10^-2.

hep-ph↗
arXiv1998

QED Structure Functions of the Photon

In deep inelastic electron-photon scattering in leading order QED, egamma->egamma*gamma->effbar, there are four non-zero structure functions. We calculate them for real photons retaining the full dependence on the fermion mass, and show numerical results of its effect.

hep-ph↗
arXiv1998

Matrix Element Corrections to Parton Shower Simulations of Heavy Quark Decay

Parton showers are accurate for soft and/or collinear emission, but for a good description of the whole of phase space they need to be supplemented by matrix element corrections. In this paper, we discuss matrix element corrections to the decay t->Wb and apply our results to the HERWIG Monte Carlo event generator. The phenomenological results show marked improvement relative to previous versions and agree well with the exact first-order matrix-element calculation.

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