arXiv subjects

Costas G. Papadopoulos

Publications and source records attributed to Costas G. Papadopoulos.

34 records · Page 2Linked to original sources

arXiv2005

Diagrammatic proof of the BCFW recursion relation for gluon amplitudes in QCD

We present a proof of the Britto-Cachazo-Feng-Witten tree-level recursion relation for gluon amplitudes in QCD, based on a direct equivalence between BCFW decompositions and Feynman diagrams. We demonstrate that this equivalence can be made explicit when working in a convenient gauge. We exhibit that gauge invariance and the particular structure of Yang-Mills vertices guarantees the validity of the BCFW construction.

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arXiv2005

Multi-Particle Processes in the Standard Model without Feynman Diagrams

A method to efficiently compute, in a automatic way, helicity amplitudes for arbitrary scattering processes at leading order in the Standard Model is presented. The scattering amplitude is evaluated recursively through a set of Dyson-Schwinger equations. The computational cost of this algorithm grows asymptotically as 3^n, where n is the number of particles involved in the process, compared to n! in the traditional Feynman graphs approach. Unitary gauge is used and mass effects are available as well. Additionally, the color and helicity structures are appropriately transformed so the usual summation is replaced by Monte Carlo techniques. Some results related to the production of vector bosons and the Higgs boson in association with jets are also presented.

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arXiv2005

Multi-Particle Processes in QCD without Feynman Diagrams

A way to efficiently compute helicity amplitudes for arbitrary tree-level scattering processes in QCD is presented. The scattering amplitude is evaluated recursively through a set of Dyson-Schwinger equations. The computational cost of this algorithm grows asymptotically as 3^n, where n is the number of particles involved in the process, compared to n! in the traditional Feynman graphs approach. Unitary gauge is used and mass effects are available as well. Additionally,the color and helicity structures are appropriately transformed so the usual summation is replaced by the Monte Carlo techniques.

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arXiv2002

Multi-jet Production in Hadron Collisions

The advent of high-energy hadron colliders necessitates efficient and accurate computation of multi-jet production processes, both as QCD processes in their own right and as backgrounds for other physics. The algorithm that performs these tasks and a brief numerical study of multi-jet processes are presented.

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arXiv2000

HELAC-PHEGAS: automatic computation of helicity amplitudes and cross sections

HELAC-PHEGAS is a FORTRAN based package that is able to compute automatically and efficiently tree-order helicity amplitudes and cross sections for arbitrary scattering processes within the standard electroweak theory and QCD. The algorithm for the amplitude computation, HELAC, exploits the virtues of the Dyson-Schwinger equations. The phase-space generation algorithm, PHEGAS, constructs all possible kinematical mappings dictated by the amplitude under consideration. Combined with mutichannel self-optimized Monte Carlo integration it results to efficient cross section evaluation.

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arXiv2000

PHEGAS: a phase-space generator for automatic cross-section computation

A phase-space generation algorithm, capable to efficiently integrate the squared amplitude of any scattering process, is presented. The algorithm has been implemented in a Monte Carlo program, PHEGAS, which, using HELAC, a helicity amplitude computational package,can be used for automatic cross-section computation and event generation. Results for several scattering processes with four, five and six particles in the final state are briefly presented.

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arXiv2000

HELAC: a package to compute electroweak helicity amplitudes

HELAC is a FORTRAN based package that is able to compute efficiently helicity amplitudes for arbitrary scattering processes within the standard electroweak theory. The algorithm exploits the virtues of the Dyson-Schwinger equations as compared to the traditional Feynman graph approach. All electroweak vertices are included in both the unitary and Feynman gauges, and computations including all mass effects are available. A version performing multi-precision computations with arbitrary - user defined - accuracy is also included, allowing access to any phase space point for arbitrary high energies.

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arXiv1999

Single leptoquark production at high-energy e+ e- colliders

A study of the production and the decay of scalar and vector leptoquarks at high-energy e+ e- colliders is presented. All tree-order contributions to single leptoquark production have been calculated and incorporated in the Monte Carlo event generator ERATO-LQ.

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arXiv1997

Studying Trilinear Gauge Couplings at Linear Collider Energies

We study the sensitivity of the processes `e+ e- -> lepton (l) neutrino (v) quark (u) antiquark (d)', where the lepton is an electron or a muon, on the non-standard trilinear gauge couplings (TGC), using the optimal observables method at Linear Collider energies. Our study is based on the four-fermion generator ERATO. Taking into account all possible correlations between the different trilinear gauge coupling parameters, we show that they can be measured with an accuracy of $10^{-3}$ to $10^{-4}$ for typical Linear Collider energies and luminosities.

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arXiv1996

ERATO: event generator for four-fermion production at LEP2 energies and beyond

This paper is a long write-up of ERATO. ERATO is a Monte Carlo event generator describing four fermion production at LEP2 and beyond. All tree-order processes leading to four fermions are included, taking into account all relevant QCD contributions. QED higher order corrections are introduced via a structure function approach, whereas weak corrections related to fermion loops are also included. Special attention has been paid in studying the trilinear gauge couplings, and all possible contributions, including the CP-violating ones, have been implemented.

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arXiv1996

Studying trilinear gauge couplings at LEP2 using optimal observables

We study the sensitivity of the processes `e+ e- -> lepton (l) neutrino (v) quark (u) antiquark (d)' at LEP2 energies on the non-standard trilinear gauge couplings (TGC), using the optimal observables method. All relevant leading logarithmic corrections to the tree-order cross section, as well as experimental resolution effects have been studied. Taking into account correlations among the different TGC parameters we show that the limits on the TGC can reach the level of 0.15~(1sd) at 161 GeV with 100 pb$^{-1}$, a challenge for the first LEP2 phase. At higher energies this can be improved drastically, reaching the level of 0.02~(1sd).

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arXiv1995

Stable Calculations for Unstable Particles: Restoring Gauge Invariance

We discuss theoretical and phenomenological aspects of the use of boson propagators with energy-dependent widths in predictions for high-energy scattering processes. In general, gauge invariance is violated in such calculations. We discuss several approaches to restore gauge invariance, necessary for a reliable result. The most promising method is the addition of the relevant parts of the fermionic corrections, which fulfills all Ward identities. The numerical difference between this and other approaches is studied. A number of recommendations are given for LEP2 computations.

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arXiv1995

On the determination of the trilinear boson couplings in e^-e^+ --> lepton anti-neutrino quark anti-quark at LEPII

We present the full tree-order calculation of the process $e^+e^-\to\ell\barν_\ell\; q \bqp$ including the width effects in a way which is consistent with gauge invariance and high energy unitarity and which has a natural interpretation within standard perturbation theory. We also include C- and P-preserving deviations from Standard Model trilinear vector boson couplings. An analysis of the sensitivity of these channels on the trilinear couplings, using well measured angular variables of the four fermion final state, is also presented.

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arXiv1994

Single-$W$ versus $W$-pair production at LEP~II

This paper is a study of the single-$W$ and $W$-pair production at LEP~II. I show that the sensitivity of the single-$W$ production channel on the anomalous trilinear couplings of $W$ is essentially the same as the sensitivity of the channel $e^+e^-\to W^+W^-$. Therefore, even without the reconstruction of the $W$ boson, deviations form the SM predictions on trilinear couplings can be measured at the level of $0.2 - 1$, using single-$W$ production at LEP~II.

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arXiv1993

Partial wave amplitudes and multiparticle production

Recursion relations for integrals of amplitudes over the phase space, i.e. for partial wave amplitudes, are introduced. In their simplest form these integrals are proportional to the s-wave amplitudes and represent rigorous lower bounds on the total cross sections. The connection with classical field equations in $D$ dimensions is established. Previous results on multiparticle amplitudes are easily reproduced.

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arXiv1993

Multiscalar amplitudes to all orders in perturbation theory

A method for calculating loop amplitudes at the multiboson threshold is presented, based on Feynman-diagram techniques. We explicitly calculate the one-loop amplitudes in both $ϕ^4$-symmetric and broken symmetry cases, using dimensional regularization. We argue that, to all orders in the perturbation expansion, the unitarity-violating behaviour of the tree-order amplitudes persists.

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