Searcharxiv⌕ Search

arXiv subjects

A. Gehrmann-De Ridder

Publications and source records attributed to A. Gehrmann-De Ridder.

68 records · Page 4Linked to original sources
arXiv2004

Infrared Structure of $e^+e^- \to 2$ jets at NNLO

The production of two jets is the simplest exclusive quantum chromodynamics process in electron-positron annihilation. Using this process, we examine the structure of next-to-next-to-leading order (NNLO) corrections to jet production observables. We derive a subtraction formalism including double real radiation at tree level and single real radiation at one loop. For two-jet production, these subtraction terms coincide with the full matrix elements, thus highlighting the phase space structure of the subtraction procedure. We then analytically compute the infrared singularities arising from each partonic channel. For the purely virtual (two-parton) NNLO corrections, these take the well known form predicted by Catani's infrared factorization formula. We demonstrate that individual terms in the infrared factorization formula can be identified with infrared singular terms from three- and four-parton final states, leaving only single poles and a contribution from the one-loop soft gluon current, which subsequently cancels between the three- and four-parton final states. Summing over all different final states, we observe an explicit cancellation of all infrared poles and recover the known two-loop correction to the hadronic $R$-ratio.

hep-ph↗
arXiv2003

Four-Particle Phase Space Integrals in Massless QCD

The inclusive four-particle phase space integral of any $1\to 4$ matrix element in massless QCD contains divergences due to the soft and collinear emission of up to two particles in the final state. We show that any term appearing in this phase space integral can be expressed as linear combination of only four master integrals. These four master integrals are all computed in dimensional regularisation up to their fourth order terms, relevant to next-to-next-to-leading order jet calculations, both in an analytic form and purely numerically. New analytical and numerical techniques are developed in this context. We introduce the tripole parametrisation of the four-parton phase space. Furthermore, we exploit unitarity relations between multi-parton phase space integrals and multi-loop integrals. For the numerical calculation, the iterated sector decomposition of loop integrals is extended to phase space integrals. The results in this paper lead to infrared subtraction terms needed for the double real radiation contributions to jet physics in $e^+e^-$ annihilation at the next-to-next-to-leading order.

hep-ph↗
arXiv2002

The QCD/SM Working Group: Summary Report

This Report documents the results obtained by the Working Group on Quantum ChromoDynamics and the Standard Model for the Workshop ``Physics at TeV Colliders'', Les Houches, France, 21 May - 1 June 2001. The account of uncertainties in Parton Distribution Functions is reviewed. Progresses in the description of multiparton final states at Next-to-Leading Order and the extension of calculations for precision QCD observables beyond this order are summarized. Various issues concerning the relevance of resummation for observables at TeV colliders is examined. Improvements to algorithms of jet reconstruction are discussed and predictions for diphoton and photon pi-zero production at the LHC are made for kinematic variables of interest regarding searches for a Higgs boson decaying into two photons. Finally, several improvements implemented in Monte-Carlo event generators are documented.

hep-ph↗
arXiv2000

Photon Plus Jet Cross Sections in Deep Inelastic ep Collisions at Order O(alpha^2 alpha_s)

The production of a hard and isolated photon accompanied by one or two jets in large-Q2 deep inelastic ep scattering is calculated at next-to-leading order. We include consistently contributions from quark-to-photon fragmentation and study various differential cross sections and their dependence on isolation cut parameters. Numerical results relevant for HERA experiments are presented.

hep-ph↗
arXiv1999

Deep Inelastic e-gamma Scattering at High Q2

The electromagnetic and weak structure functions of the photon can be studied in the deep inelastic electron-photon processes e+gamma -> e+X and e+gamma -> nu+X. While at low energies only virtual photon exchange is operative in the neutral-current process, additional Z-exchange becomes relevant for high Q2 of order MZ^2 at e+e- linear colliders. Likewise the charged-current process can be observed at these high energy colliders. By measuring the electroweak neutral- and charged-current structure functions, the up- and down-type quark content of the photon can be determined separately.

hep-ph↗
arXiv1999

Photon Fragmentation in Large-Q2 ep Collisions at Next-to-Leading Order QCD

We study the production of photons accompanied by jets in large-Q2 deep inelastic scattering. Numerical results for the cross section differential with respect to the fraction of momentum z_gamma carried by a photon inside a jet at large z_gamma, up to O(alpha alpha_s) in perturbative QCD, are presented. The sensitivity to the fragmentation contribution allows one to study the quark-to-photon fragmentation function. Our results can be confronted with future experimental data from HERA.

hep-ph↗
arXiv1999

Deep-inelastic Electron-Photon Scattering at High Q^2 : Neutral and Charged Current Reactions

We present the results of a calculation of deep inelastic electron-photon scattering at a linear collider for very high virtuality of the intermediate gauge boson up to NLO in perturbative QCD. The real photon is produced unpolarized via the Compton back scattering of laser light of the incoming beam. For $Q^2$ values close to the masses squared of the Z and W gauge bosons, the deep inelastic electron-photon scattering process receives important contributions not only from virtual photon exchange but also from the exchange of a Z-boson and a W-boson. We find that the total cross section for center of mass energies above $500 \rm{GeV}$ is at least of ${\cal O}(pb)$ and has an important charged current contribution.

hep-ph↗
arXiv1998

Photon Fragmentation at LEP

The production of final state photons in hadronic $Z$-boson decays can be used to study the quark-to-photon fragmentation function $D_{q\to γ}(z,μ_{F})$. Currently, two different observables are used at LEP to probe this function: the `photon' +~1 jet rate and the inclusive photon energy distribution. We outline the results of a calculation of the `photon' +~1 jet rate at fixed ${\cal O}(αα_{s})$, which yield a next-to-leading order determination of the quark-to-photon fragmentation function $D_{q\to γ}(z,μ_{F})$. The resulting predictions for the isolated photon rate and the inclusive photon spectrum at the same, fixed order, are found to be in good agreement with experimental data. Furthermore, we outline the main features of conventional approaches using parameterizations of the resummed solutions of the evolution equation and point out deficiencies of these currently available parameterizations in the large $z$-region. We finally demonstrate that the ALEPH data on the `photon' +~1 jet rate are able to discriminate between different parameterizations of the quark-to-photon fragmentation function, which are equally allowed by the OPAL photon energy distribution data.

hep-ph↗
arXiv1998

Isolated and non-isolated photon rates at LEP

We present the results of the calculation of the `photon' +~1 jet rate at ${\cal O}(αα_{s})$ for LEP energies. By comparing these results with the data from the ALEPH Collaboration we make a next-to-leading order determination of the quark-to-photon fragmentation function $D_{q\to γ}(z,μ_{F})$ at ${\cal O}(αα_{s})$. The predictions obtained using this fragmentation function for the isolated rate, defined as the `photon' +~1 jet rate for $z>0.95$, are found in good agreement with the ALEPH data. The next-to-leading order corrections are moderate demonstrating the perturbative stability of this particular isolated photon definition. We have also computed the inclusive photon energy distribution and found good agreement with the OPAL data.

hep-ph↗
arXiv1998

Final state photon production at LEP

We present a detailed study of photon production in hadronic events in electron-positron annihilation at LEP energies. We show that estimates of the inclusive photon spectrum using the quark-to-photon fragmentation function determined using the ALEPH `photon' +~1 jet data agree well with the observations of the OPAL collaboration. This agreement shows that the photon fragmentation function determined in this way can be used for inclusive observables. We also compare next-to-leading order and beyond leading logarithm predictions obtained using the numerically resummed solutions of the fragmentation function evolution equation of Bourhis, Fontannaz and Guillet and Glück, Reya and Vogt with the data. Moreover, in order to check the general behaviour of the fragmentation function, we consider an analytic series expansion in the strong coupling. We find that the parameterizations are inaccurate at large $x$ values. While the OPAL data is in broad agreement with estimates based on any of these approaches, the ALEPH data prefers the resummed BFG parameterization. Finally, there is some ambiguity as to whether the fragmentation function is treated as ${\cal O}(α)$ or ${\cal O}(α/α_s)$. We show that at present this ambiguity affects mainly the prediction for the `photon' +~1 jet rate at large $z$.

hep-ph↗
arXiv1998

A complete ${\cal O}(αα_s)$ calculation of the Photon + 1 Jet rate in $e^+e^-$ annihilation

We present a complete calculation of the photon +~1 jet rate in $e^+e^-$ annihilation up to ${\cal O}(αα_{s})$. Although formally of next-to-leading order in perturbation theory, this calculation contains several ingredients appropriate to a next-to-next-to-leading order calculation of jet observables. In particular, we describe a generalization of the commonly used phase space slicing method to isolate the singularities present when more than one particle is unresolved. Within this approach, we analytically evaluate the singularities associated with the following double unresolved regions; triple collinear, soft/collinear and double single collinear configurations as well as those from the collinear limit of virtual graphs. By comparing the results of our calculation with the existing data on the photon +~1 jet rate from the ALEPH Collaboration at CERN, we make a next-to-leading order determination of the process-independent non-perturbative quark-to-photon fragmentation function $D_{q \to γ}(z,μ_{F})$ at ${\cal O}(αα_{s})$. As a first application of this measurement allied with our improved perturbative calculation, we determine the dependence of the isolated photon +~1 jet cross section in a democratic clustering approach on the jet resolution parameter $\ycut$ at next-to-leading order. The next-to-leading order corrections to this observable are moderate but improve the agreement between theoretical prediction and experimental data.

hep-ph↗
arXiv1997

Radiative Corrections to the Photon + 1 Jet Rate at LEP

We present the results of a calculation of the rate for the annihilation of an $e^+e^-$ pair into a jet containing an energetic photon and a single further jet at order $αα_{s}$. By comparing these results with the existing data on the photon + 1 jet rate from the ALEPH Collaboration, we make a first determination of the process independent quark-to-photon fragmentation function $D_{q \to γ}(z,μ_{F})$ at order $αα_{s}$. We also compare our prediction for the inclusive photon energy spectrum with the recent OPAL measurement.

hep-ph↗