arXiv subjects

J. Ph. Guillet

Publications and source records attributed to J. Ph. Guillet.

At least 19 recordsLinked to original sources

arXiv2025

Isolated photon cross section and resummation of the logarithms of the cone radius

The cone isolation criterion used in large-pt photon experiments generates logarithms of the cone radius in the theoretical cross sections. When a small radius is used, unitarity is violated as the inclusive cross section is smaller than the isolated one. We show that unitarity is restored when these logarithms are resummed. We also study a criterion which offers a more precise description of the experimental procedure : no isolation is imposed in a very small cone inside the standard one. In this case as well unitarity is violated and the resummation of logarithms is required.

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arXiv2024

A subtraction scheme for processes involving fragmentation functions at NLO

We present a novel subtraction method to remove the soft and collinear divergences at next-to-leading order for processes involving an arbitrary number of fragmentation functions, where this method acts directly in the hadronic centre-of-mass frame. We provide the analytical formulae of the subtraction terms in the general case where all the final state partons fragment to hadrons and for the two special cases when one of the partons of the final state does not fragment, i.e. it is a photon or involved in a jet.

hep-ph↗

arXiv2021

Two-loop scalar functions with $I$ internal lines, $I \le 5$

This article displays a proof of concept of the mixed analytical/numerical method, presented in previous publications, to compute two-loop functions with up to five massive propagators in a scalar theory having three- and four-leg vertices as the Higgs sector of the Standard Model. Several amplitudes are considered with two, three and four external legs. Some of them diverge in the UV region and we demonstrate that the method works in that case. It is shown that all these classes of amplitudes can be generated by four master topologies. Results of a numerical evaluation for some kinematics are presented, they are compared to a public code and agree well within the error bars quoted by the different programs.

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arXiv2020

A novel approach to the computation of one-loop three- and four-point functions. I -- The real mass case

This article is the first of a series of three presenting an alternative method to compute the one-loop scalar integrals. This novel method enjoys a couple of interesting features as compared with the method closely following 't Hooft and Veltman adopted previously. It directly proceeds in terms of the quantities driving algebraic reduction methods. It applies to the three-point functions and, in a similar way, to the four-point functions. It also extends to complex masses without much complication. Lastly, it extends to kinematics more general than the one of physical e.g.\ collider processes relevant at one loop. This last feature may be useful when considering the application of this method beyond one loop using generalised one-loop integrals as building blocks.

hep-ph↗

arXiv2020

Framework for a novel mixed analytical/numerical approach for the computation of two-loop $N$-point Feynman diagrams

A framework to represent and compute two-loop $N$-point Feynman diagrams as double-integrals is discussed. The integrands are 'generalised one-loop type" multi-point functions multiplied by simple weighting factors. The final integrations over these two variables are to be performed numerically, whereas the ingredients involved in the integrands, in particular the "generalised one-loop type" functions, are computed analytically. The idea is illustrated on a few examples of scalar three- and four-point functions.

hep-ph↗

arXiv2019

A novel approach to the computation of one-loop three- and four-point functions. II - The complex mass case

This article is the second of a series of three presenting an alternative method to compute the one-loop scalar integrals. It extends the results of the first article to general complex masses. Let us remind the main features enjoyed by this method. It directly proceeds in terms of the quantities driving algebraic reduction methods. It applies to the four-point functions in the same way as to the three-point functions. Lastly, it extends to kinematics more general than the one of physical e.g. collider processes relevant at one loop.

hep-th↗

arXiv2019

A novel approach to the computation of one-loop three- and four-point functions. III - The infrared divergent case

This article is the third and last of a series presenting an alternative method to compute the one-loop scalar integrals. It extends the results of first two articles to the infrared divergent case. This novel method enjoys a couple of interesting features as compared with the methods found in the literature. It directly proceeds in terms of the quantities driving algebraic reduction methods. It yields a simple decision tree based on the vanishing of internal masses and one-pinched kinematic matrices which avoids a profusion of cases. Lastly, it extends to kinematics more general than the one of physical e.g. collider processes relevant at one loop. This last feature may be useful when considering the application of this method beyond one loop using generalised one-loop integrals as building blocks.

hep-ph↗

arXiv2013

Tools for NLO automation: extension of the golem95C integral library

We present an extension of the program golem95C for the numerical evaluation of scalar integrals and tensor form factors entering the calculation of one-loop amplitudes, which supports tensor ranks exceeding the number of propagators. This extension allows various applications in Beyond the Standard Model physics and effective theories, for example higher ranks due to propagators of spin two particles, or due to effective vertices. Complex masses are also supported. The program is not restricted to the Feynman diagrammatic approach, as it also contains routines to interface to unitarity-inspired numerical reconstruction of the integrand at the tensorial level. Therefore it can serve as a general integral library in automated programs to calculate one-loop amplitudes.

hep-ph↗

arXiv2013

An algebraic/numerical formalism for one-loop multi-leg amplitudes

We present a formalism for the calculation of multi-particle one-loop amplitudes, valid for an arbitrary number N of external legs, and for massive as well as massless particles. A new method for the tensor reduction is suggested which naturally isolates infrared divergences by construction. We prove that for N>4, higher dimensional integrals can be avoided. We derive many useful relations which allow for algebraic simplifications of one-loop amplitudes. We introduce a form factor representation of tensor integrals which contains no inverse Gram determinants by choosing a convenient set of basis integrals. For the evaluation of these basis integrals we propose two methods: An evaluation based on the analytical representation, which is fast and accurate away from exceptional kinematical configurations, and a robust numerical one, based on multi-dimensional contour deformation. The formalism can be implemented straightforwardly into a computer program to calculate next-to-leading order corrections to multi-particle processes in a largely automated way.

hep-ph↗

arXiv2013

Stable One-Dimensional Integral Representations of One-Loop N-Point Functions in the General Massive Case: I - Three Point Functions

In this article we provide representations for the one-loop three point functions in 4 and 6 dimensions in the general case with complex masses. The latter are part of the GOLEM library used for the computation of one-loop multileg amplitudes. These representations are one-dimensional integrals designed to be free of instabilites induced by inverse powers of Gram determinants, therefore suitable for stable numerical implementations.

hep-ph↗

arXiv2007

Algebraic evaluation of rational polynomials in one-loop amplitudes

One-loop amplitudes are to a large extent determined by their unitarity cuts in four dimensions. We show that the remaining rational terms can be obtained from the ultraviolet behaviour of the amplitude, and determine universal form factors for these rational parts by applying reduction techniques to the Feynman diagrammatic representation of the amplitude. The method is valid for massless and massive internal particles. We illustrate this method by evaluating the rational terms of the one-loop amplitudes for gg --> H, the 4-photon and 4-gluon amplitude, the amplitude for 2 photons plus 3 gluons and the 6-photon amplitude.

hep-ph↗

arXiv2005

New NLO Parametrizations of the Parton Distributions in Real Photons

We present new NLO sets of parton distributions in real photons based on a scheme invariant definition of the non-perturbative input. We compare the theoretical predictions with LEP data and a best fit allows us to constrain the parameters of the distributions. The shape of the gluon distribution is poorly constrained and we consider the possibility to measure it in photoproduction experiments. Three parametrizations which aim to take into account the scattering of LEP data are proposed. They are compared to other NLO parametrizations.

hep-ph↗

arXiv2004

A compact representation of the 2 photon 3 gluon amplitude

A compact representation of the loop amplitude gamma gamma ggg -> 0 is presented. The result has been obtained by using helicity methods and sorting with respect to an irreducible function basis. We show how to convert spinor representations into a field strength representation of the amplitude. The amplitude defines a background contribution for Higgs boson searches at the LHC in the channel H -> gamma gamma + jet which was earlier extracted indirectly from the one-loop representation of the 5-gluon amplitude.

hep-ph↗

arXiv2002

A NLO calculation of the hadron-jet cross section in photoproduction reactions

We study the photoproduction of large-p_T charged hadrons in e p collisions, both for the inclusive case and for the case where a jet in the final state is also measured. Our results are obtained by a NLO generator of partonic events. We discuss the sensitivity of the cross section to the renormalisation and factorisation scales, and to various fragmentation function parametrisations. The possibility to constrain the parton densities in the proton and in the photon is assessed. Comparisons are made with H1 data for inclusive charged hadron production.

hep-ph↗

arXiv2002

Cross section of isolated prompt photons in hadron-hadron collisions

We consider the production of isolated prompt photons in hadronic collisions. We present a general discussion in QCD perturbation theory of the isolation criterion used by hadron collider experiments. The isolation criterion is implemented in a computer programme of the Monte Carlo type, which evaluates the production cross section at next-to-leading order accuracy in perturbative QCD. The calculation includes both the direct and the fragmentation components of the cross section, without any approximation of the dependence on the radius R of the isolation cone. We examine the scale dependence of the isolated cross section, the sensitivity of the cross section to the values of the isolation parameters, and we provide a quantitative comparison between the full R dependence and its small-R approximation.

hep-ph↗

arXiv2001

A NLO calculation of the large p_T photon + photon -> photon + jet cross section

We study the production of an isolated large p_T photon as well as the production of an isolated prompt photon plus a jet in e^+e^- collisions. Our results are obtained by a NLO Monte Carlo program of partonic event generator type. We discuss the possibilities to constrain the parton densities in the real photon and compare to preliminary OPAL data.

hep-ph↗

arXiv2001

Is a Large Intrinsic k_T Needed to Describe Photon + Jet Photoproduction at HERA?

We study the photoproduction of an isolated photon and a jet based on a code of partonic event generator type which includes the full set of next-to-leading order corrections. We compare our results to a recent ZEUS analysis in which an effective k_T of the incoming partons has been determined. We find that no additional intrinsic k_T is needed to describe the data.

hep-ph↗

arXiv2001

Isolated prompt photon photoproduction at NLO

We present a full next-to-leading order code to calculate the photoproduction of prompt photons. The code is a general purpose program of partonic event generator type with large flexibility. We study the possibility to constrain the photon structure functions and comment on isolation issues. A comparison to ZEUS data is also shown.

hep-ph↗