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Walter T. Giele

Publications and source records attributed to Walter T. Giele.

At least 19 recordsLinked to original sources
arXiv2019

A Projective Phase Space Generator for Hadronic Vector Boson Plus One Jet Production

In this paper we use our previously developed projective phase space generator for the calculation of the hadronic production of a vector boson with one additional jet at Next-to-Leading Order. The projective phase space generator allows us to make physical predictions in novel ways, speeding up both evaluation time and attainable accuracy. For the numerical evaluation we explore a computational model which combines the use of both multi-threading and distributed resources through the use of grid or cloud computing without depending on local institutional computer availability. The projective phase space method is well suited for this approach and gives through the use of cloud computing instant access to a large pool of resources.

hep-ph↗
arXiv2018

A Forward Branching Phase Space Generator for Hadron colliders

In this paper we develop a projective phase space generator appropriate for hadron collider geometry. The generator integrates over bremsstrahlung events which project back to a single, fixed Born event. The projection is dictated by the experimental jet algorithm allowing for the forward branching phase space generator to integrate out the jet masses and initial state radiation. When integrating over the virtual and bremsstrahlung amplitudes this results in a single K-factor, assigning an event probability to each Born event. This K-factor is calculable as a perturbative expansion in the strong coupling constant. One can build observables from the Born kinematics, giving identical results to tradi- tional observables as long as the observable does not depend on the infrared sensitive jet mass or initial state radiation.

hep-ph↗
arXiv2016

Z-boson production in association with a jet at next-to-next-to-leading order in perturbative QCD

We present the first complete calculation of Z-boson production in association with a jet in hadronic collisions through next-to-next-to-leading order in perturbative QCD. Our computation uses the recently-proposed N-jettiness subtraction scheme to regulate the infrared divergences that appear in the real-emission contributions. We present phenomenological results for 13 TeV proton-proton collisions with fully realistic fiducial cuts on the final-state particles. The remaining theoretical uncertainties after the inclusion of our calculations are at the percent-level, making the Z+jet channel ready for precision studies at the LHC Run II.

hep-ph↗
arXiv2015

Improved partonic event generators at lepton colliders

A method is detailed for the phase space integration of multi-jets cross sections at lepton colliders, applicable to parton level Monte Carlo's at any order in perturbation theory. Other non-jet objects, massless or massive, can be included in the phase space generation. We correlate the bremsstrahlung events in a manner that integrates out all partonic configurations leading to a fixed jet configuration, thereby improving convergence. This also allows the method to extend infra-red safety to the fully differential multi-jet cross section.

hep-ph↗
arXiv2015

A Multi-Threaded Version of MCFM

We report on our findings modifying MCFM using OpenMP to implement multi-threading. By using OpenMP, the modified MCFM will execute on any processor, automatically adjusting to the number of available threads. We modified the integration routine VEGAS to distribute the event evaluation over the threads, while combining all events at the end of every iteration to optimize the numerical integration. Special care has been taken that the results of the Monte Carlo integration are independent of the number of threads used, to facilitate the validation of the OpenMP version of MCFM.

physics.comp-ph↗
arXiv2013

Event-by-event weighting at next-to-leading order

We present a general method of associating next-to-leading order weights to leading order phase space configurations at hadron colliders. The method relies on a re-organization of phase space for the real radiation contributions, defining a one-to-many map such that each point in the real phase space is associated with a distinct Born topology. As a result virtual and real singularities cancel at each Born phase space point. The new phase space maps can be used in a traditional approach for NLO calculations. However, interesting applications arise when one instead integrates out the real radiation up to a specified scale. This allows one to define NLO weights for individual phase space points that are present at LO. This method allows for the extension of matrix element methods to next-to-leading order, even in the presence of jets. We discuss some recent applications of the matrix element method to Higgs physics at the LHC.

hep-ph↗
arXiv2013

The Vincia Parton Shower

We summarize recent developments in the VINCIA parton shower. After a brief review of the basics of the formalism, the extension of VINCIA to hadron collisions is sketched. We then turn to improvements of the efficiency of tree-level matching by making the shower history unique and by incorporating identified helicities. We conclude with an overview of matching to one-loop matrix elements.

hep-ph↗
arXiv2013

The Matrix Element Method at Next-to-Leading Order

This paper presents an extension of the matrix element method to next-to-leading order in perturbation theory. To accomplish this we have developed a method to calculate next-to-leading order weights on an event-by-event basis. This allows for the definition of next-to-leading order likelihoods in exactly the same fashion as at leading order, thus extending the matrix element method to next-to-leading order. A welcome by-product of the method is the straightforward and efficient generation of unweighted next-to-leading order events. As examples of the application of our next-to-leading order matrix element method we consider the measurement of the mass of the Z boson and also the search for the Higgs boson in the four lepton channel.

hep-ph↗
arXiv2013

Finding the Higgs boson in decays to Z gamma using the matrix element method at Next-to-Leading Order

We illustrate how the Matrix Element Method at Next-to-Leading Order (MEM@NLO) can be used to discriminate between events arising from the production of a Higgs boson, which subsequently decays to a final state consisting of ell^+ell^-gamma, and the background production of the same final state. We illustrate how the method could be used in an experimental analysis by devising cuts on the signal (P_S) and background (P_B) weights that are computed event-by-event in this approach. We find that we can increase the S/sqrt(B) ratio by around 50\% compared to an invariant mass fit on its own. Considering only statistical uncertainty, this is equivalent to recording a factor of around two times more integrated luminosity.

hep-ph↗
arXiv2011

A Forward Branching Phase-Space Generator

We develop a forward branching phase-space generator for use in next-to-leading order parton level event generators. By performing 2 -> 3 branchings from a fixed jet phase-space point, all bremsstrahlung events contributing to the given jet configuration are generated. The resulting phase-space integration is three-dimensional irrespective of the considered jet multiplicity. In this first study, we use the forward branching phase-space generator to calculate in the leading-color approximation next-to-leading order corrections to fully differential gluonic jet configurations.

hep-ph↗
arXiv2009

Calculating gluon one-loop amplitudes numerically

This note reports on an independent implementation of calculating one-loop amplitudes semi-numerically using generalized unitarity techniques. The algorithm implemented in form of a C++ code closely follows the method by Ellis, Giele, Kunszt and Melnikov. For the case of gluons, the algorithm is briefly reviewed. Double-precision results are presented documenting the accuracy and efficiency of this computation.

hep-ph↗
arXiv2008

A Numerical Unitarity Formalism for One-Loop Amplitudes

The unitarity method for calculating one-loop amplitudes provides algorithms of polynomial complexity. This is primarily beneficial for the computation of multi-leg one loop amplitudes and it is therefore of great interest to develop a numerical implementation of the unitarity method. We describe a recently-developed, efficient, semi-numerical unitarity method for the computation of the cut-constructible part of one-loop amplitudes.

hep-ph↗
arXiv2008

Full one-loop amplitudes from tree amplitudes

We establish an efficient polynomial-complexity algorithm for one-loop calculations, based on generalized $D$-dimensional unitarity. It allows automated computations of both cut-constructible {\it and} rational parts of one-loop scattering amplitudes from on-shell tree amplitudes. We illustrate the method by (re)-computing all four-, five- and six-gluon scattering amplitudes in QCD at one-loop.

hep-ph↗
arXiv2007

A simple shower and matching algorithm

We present a simple formalism for parton-shower Markov chains. As a first step towards more complete uncertainty bands, we incorporate a comprehensive exploration of the ambiguities inherent in such calculations. To reduce this uncertainty, we then introduce a matching formalism which allows a generated event sample to simultaneously reproduce any infrared safe distribution calculated at leading or next-to-leading order in perturbation theory, up to sub-leading corrections. To enable a more universal definition of perturbative calculations, we also propose a more general definition of the hadronization cutoff. Finally, we present an implementation of some of these ideas for final-state gluon showers, in a code dubbed VINCIA.

hep-ph↗
arXiv2001

Parton Distribution Function Uncertainties

We present parton distribution functions which include a quantitative estimate of its uncertainties. The parton distribution functions are optimized with respect to deep inelastic proton data, expressing the uncertainties as a density measure over the functional space of parton distribution functions. This leads to a convenient method of propagating the parton distribution function uncertainties to new observables, now expressing the uncertainty as a density in the prediction of the observable. New measurements can easily be included in the optimized sets as added weight functions to the density measure. Using the optimized method nowhere in the analysis compromises have to be made with regard to the treatment of the uncertainties.

hep-ph↗
arXiv2001

Hard Scattering Based Luminosity Measurement at Hadron Colliders

A strategy to determine the luminosity at Hadron Colliders is discussed using the simultaneous W-boson and Z-boson event counts. The emphasis of the study will be on the uncertainty induced by the parton density functions. Understanding this source of uncertainties is crucial for a reliable luminosity determination using the W-boson and Z-boson events. As an example we will use the D0 run 1 results to extract the luminosity using the vector boson events and compare the result with the traditional method. Subsequently we will look at the implications for the top cross section uncertainties using the extracted luminosity.

hep-ph↗
arXiv2000

A Next-to-Leading Order Calculation of Hadronic Three Jet Production

We present results of a next-to-leading order calculation of three jet production at hadron colliders. This calculation will have many applications. In addition to computing such three-jet observables as spectra, mass distributions, this calculation permits the first next-to-leading order studies at hadron colliders of jet and event shape variables.

hep-ph↗