arXiv subjects

Peter Uwer

Publications and source records attributed to Peter Uwer.

At least 37 records · Page 2Linked to original sources

arXiv2013

Next-to-leading order QCD corrections to five jet production at the LHC

We present theoretical predictions for five jet production in proton-proton collisions at next-to-leading order accuracy in QCD. Inclusive as well as differential observables are studied for collision energies of 7 and 8 TeV. In general the next-to-leading order corrections stabilize the theoretical predictions with respect to scale variations. In case of the inclusive jet cross sections, we compare with experimental data where possible and find reasonable agreement. We observe that the four-to-three and five-to-four jet ratios show better perturbative convergence than the known three-to-two ratio and are promising candidates for future alpha_s measurements. Furthermore, we present a detailed analysis of uncertainties related to parton distribution functions. The full colour virtual matrix elements used in the computation were obtained with the NJet package, a publicly available library for the evaluation of one-loop amplitudes in massless QCD.

hep-ph↗

arXiv2013

A new observable to measure the top-quark mass at hadron colliders

A new method to measure the top-quark mass in high energetic hadron collisions is presented. We use theoretical predictions calculated at next-to-leading order accuracy in quantum chromodynamics to study the (normalized) differential distribution of the \ttbaronejet cross section with respect to its invariant mass $\sqrt{\sttj}$. The sensitivity of the method to the top-quark mass together with the impact of various theoretical and experimental uncertainties has been investigated and quantified. The new method allows for a complementary measurement of the top-quark mass parameter and has a high potential to become competitive in precision with respect to established approaches. Furthermore we emphasize that in the proposed method the mass parameter is uniquely defined through one-loop renormalization.

hep-ph↗

arXiv2013

NGluon: A Package to Calculate One-loop Multi-gluon Amplitudes

We present a computer library for the numerical evaluation of colour-ordered n-gluon amplitudes at one-loop order in pure Yang-Mills theory. The library uses the recently developed technique of generalised unitarity. Running in double precision the library yields reliable results for up to 14 gluons with only a small fraction of events requiring a re-evaluation using extended floating point arithmetic. We believe that the library presented here provides an important contribution to future LHC phenomenology. The program may also prove useful in cross checking results obtained by other methods. In addition, the code provides a sample implementation which may serve as a starting point for further developments.

hep-ph↗

arXiv2013

NLO QCD corrections to multi-jet production at the LHC with a centre-of-mass energy of sqrt(s)=8 TeV

We study three and four jet production in hadronic collisions at next-to-leading order accuracy in massless QCD. We cross check results previously obtained by the BlackHat collaboration for the LHC with a centre-of-mass energy of sqrt(s)=7 TeV and present new results for the LHC operating at 8 TeV. We find large negative NLO corrections reducing the leading-order cross sections by about 40-50%. Furthermore we observe an important reduction of the scale uncertainty. In addition to the cross sections we also present results for differential distributions. The dynamical renormalization/factorization scale used in the calculation leads to a remarkably stable K-factor. The results presented here were obtained with the NJet package, a publicly available library for the evaluation of one-loop amplitudes in massless QCD.

hep-ph↗

arXiv2012

Numerical evaluation of virtual corrections to multi-jet production in massless QCD

We present a C++ library for the numerical evaluation of one-loop virtual corrections to multi-jet production in massless QCD. The pure gluon primitive amplitudes are evaluated using NGluon. A generalized unitarity reduction algorithm is used to construct arbitrary multiplicity fermion-gluon primitive amplitudes. From these basic building blocks the one-loop contribution to the squared matrix element, summed over colour and helicities, is calculated. No approximation in colour is performed. While the primitive amplitudes are given for arbitrary multiplicities we provide the squared matrix elements only for up to 7 external partons allowing the evaluation of the five jet cross section at next-to-leading order accuracy. The library has been recently successfully applied to four jet production at next-to-leading order in QCD.

hep-ph↗

arXiv2012

Comparing efficient computation methods for massless QCD tree amplitudes: Closed Analytic Formulae versus Berends-Giele Recursion

Recent advances in our understanding of tree-level QCD amplitudes in the massless limit exploiting an effective (maximal) supersymmetry have led to the complete analytic construction of tree-amplitudes with up to four external quark-anti-quark pairs. In this work we compare the numerical efficiency of evaluating these closed analytic formulae to a numerically efficient implementation of the Berends-Giele recursion. We compare calculation times for tree-amplitudes with parton numbers ranging from 4 to 25 with no, one, two and three external quark lines. We find that the exact results are generally faster in the case of MHV and NMHV amplitudes. Starting with the NNMHV amplitudes the Berends-Giele recursion becomes more efficient. In addition to the runtime we also compared the numerical accuracy. The analytic formulae are on average more accurate than the off-shell recursion relations though both are well suited for complicated phenomenological applications. In both cases we observe a reduction in the average accuracy when phase space configurations close to singular regions are evaluated. We believe that the above findings provide valuable information to select the right method for phenomenological applications.

hep-ph↗

arXiv2012

Top-quark pair-production with one jet and parton showering at hadron colliders

We present heavy-flavor production in association with one jet in hadronic collisions matched to parton shower Monte Carlo predictions at next-to-leading order QCD with account of top-quark decays and spin correlations. We use the POWHEGBOX for the interface to the parton shower programs PYTHIA or HERWIG. Phenomenological studies for the LHC and the Tevatron are presented with particular emphasis on the inclusion of spin-correlation effects in top decay and the impact of the parton shower on the top-quark charge asymmetries. As a novel application of the present calculation the measurement of the top-quark mass is discussed.

hep-ph↗

arXiv2012

Hadronic top-quark pair-production with one jet and parton showering

We present a calculation of heavy-flavor production in hadronic collisions in association with one jet matched to parton shower Monte Carlo programs at next-to-leading order in perturbative QCD. Top-quark decays are included and spin correlations in the decay products are taken into account. The calculation builds on existing results for the radiative corrections to heavy-quark plus one jet production and uses the POWHEG BOX for the interface to the parton shower programs PYTHIA or HERWIG. A broad phenomenological study for the Large Hadron Collider and the Tevatron is presented. In particular we study---as one important sample application---the impact of the parton shower on the top-quark charge asymmetry.

hep-ph↗

arXiv2012

One-Loop Amplitudes for Multi-Jet Production at Hadron Colliders

We present a numerical implementation for virtual corrections to multi-jet production at Next-to-Leading order. Using the algorithm of generalised unitarity we compute primitive amplitudes from tree-level input. These basic ingredients are then used to compute full colour and helicity summed corrections.

hep-ph↗

arXiv2011

Numerical evaluation of one-loop QCD amplitudes

We present the publicly available program NGluon allowing the numerical evaluation of primitive amplitudes at one-loop order in massless QCD. The program allows the computation of one-loop amplitudes for an arbitrary number of gluons. The focus of the present article is the extension to one-loop amplitudes including an arbitrary number of massless quark pairs. We discuss in detail the algorithmic differences to the pure gluonic case and present cross checks to validate our implementation. The numerical accuracy is investigated in detail.

hep-ph↗

arXiv2010

Measuring the running Top quark mass

In this contribution we discuss conceptual issues of current mass measurements performed at the Tevatron. In addition we propose an alternative method which is theoretically much cleaner and to a large extend free from the problems encountered in current measurements. In detail we discuss the direct determination of the top-quark's running mass from the cross section measurements performed at the Tevatron.

hep-ph↗

arXiv2010

NLO QCD corrections to WW+jet production including leptonic W decays at hadron colliders

We report on the calculation of the next-to-leading order QCD corrections to the production of W-boson pairs in association with a hard jet at the Tevatron and the LHC, which is an important source of background for Higgs and new-physics searches. Leptonic decays of the W bosons are included by applying an improved version of the narrow-width approximation that treats the W bosons as on-shell particles, but keeps the information on the W spin. A selection of differential NLO QCD cross sections is provided both for the LHC and the Tevatron. The QCD corrections stabilize the LO prediction for the cross section with respect to scale variations. The differential LO cross sections are generally not simply rescaled by the corrections. Their shapes are particularly distorted if an additional energy scale is involved.

hep-ph↗

arXiv2008

Theoretical status and prospects for top-quark pair production at hadron colliders

We present an update of the theoretical predictions for the cross section of top-quark pair production at Tevatron and LHC. In particular we employ improvements due to soft gluon resummation at next-to-next-to-leading logarithmic accuracy. We expand the resummed results and derive analytical finite-order cross sections through next-to-next-to-leading order which are exact in all logarithmically enhanced terms near threshold. These results are the best present estimates for the top-quark pair production cross section. We investigate the scale dependence as well as the sensitivity on the parton luminosities.

hep-ph↗

arXiv2008

NLO QCD corrections to pp->WW+jet+X

We report on a calculation of the next-to-leading order QCD corrections to the production of W-boson pairs in association with a hard jet at hadron colliders, which is an important source of background for Higgs and new-physics searches at the LHC. If a veto against the emission of a second hard jet is applied, the corrections stabilize the leading-order prediction for the cross section considerably.

hep-ph↗

arXiv2008

NLO QCD corrections to pp -> top anti-top + jet + X

We discuss the production of top--anti-top quark pairs in association with a hard jet at the Tevatron and at the LHC and we report on the calculation of the next-to-leading order QCD corrections to this process. Numerical results for the top--anti-top+jet cross section and the forward--backward charge asymmetry are presented. The corrections stabilize the leading-order prediction for the cross section. In contrast, the charge asymmetry receives large corrections. The dependence of the cross section as well as the asymmetry on the minimum transverse momenta used to define the additional jet is studied in detail for the Tevatron.

hep-ph↗

arXiv2007

NLO QCD corrections to pp->WW+jet+X

We report on the calculation of the next-to-leading order QCD corrections to the production of W-boson pairs in association with a hard jet at hadron colliders, which is an important source of background for Higgs and new-physics searches. If a veto against the emission of a second hard jet is applied, the corrections stabilize the leading-order prediction for the cross section considerably.

hep-ph↗

arXiv2007

EasyNData: A simple tool to extract numerical values from published plots

The comparison of numerical data with published plots is a frequently occurring task. In this article I present a short computer program written in Java(TM) helping in those cases where someone wants to get the numbers out of a plot but is not able to read the plot with a decent accuracy and cannot contact the author of the plot directly for whatever reason. The accuracy reached by this method depends on many factors. For the examples illustrated in this paper a precision at the level of a few per mille could be reached. The tool might help in improving the quality of future publications.

physics.comp-ph↗

arXiv2004

Maximizing the spin correlation of top quark pairs produced at the LHC

The measurement of top quark spin correlation is an important tool for precise studies of top quark interactions. In this letter I construct a quantization axis maximizing the spin correlation at the LHC within the Standard Model. Using this axis a spin correlation of 48% or even more, on applying additional cuts, can be reached. This represents a significant improvement compared to the helicity bases studied thus far.

hep-ph↗