45 research outputs found
Numerical Loop-Tree Duality: contour deformation and subtraction
We introduce a novel construction of a contour deformation within the
framework of Loop-Tree Duality for the numerical computation of loop integrals
featuring threshold singularities in momentum space. The functional form of our
contour deformation automatically satisfies all constraints without the need
for fine-tuning. We demonstrate that our construction is systematic and
efficient by applying it to more than 100 examples of finite scalar integrals
featuring up to six loops. We also showcase a first step towards handling
non-integrable singularities by applying our work to one-loop infrared
divergent scalar integrals and to the one-loop amplitude for the ordered
production of two and three photons. This requires the combination of our
contour deformation with local counterterms that regulate soft, collinear and
ultraviolet divergences. This work is an important step towards computing
higher-order corrections to relevant scattering cross-sections in a fully
numerical fashion.Comment: 87 page
Automation of One-Loop Computations for Scattering Amplitudes and Applications to Collider Phenomenology
This thesis presents the automation of loop computations for the calculation of Next-to-Leading Order contributions to theoretical predictions for particle colliders. We start with the general techniques for performing such predictions and how they can be expressed as perturbative expansions in the coupling constants controlling the strength of particle interactions. This leads to the discussion of the subtleties arising when considering higher order corrections, in particular the methods employed for isolating and canceling the infrared divergences occurring at intermediate steps of the computation. We then introduce the Passarino-Veltman and Ossola-Papadopolous-Pittau loop reduction algorithms. The latter is used by the computer code MadLoop that we wrote specifically for the automation of loop computations. The main part of the thesis focuses on the description of this program, starting with its original loop diagram generation algorithm and how it is embedded within the MadGraph5 environment. An initiation to the usage of the code is given, followed by a discussion of its optimizations where particular attention is paid to the implementation of the open-loop technique. Great details about the validation of MadLoop results against those of other codes are given in appendix B for specific kinematic configurations. We also list in the main text the total rates obtained for various processes. Quantitative information on the runtime speed and numerical stability performances are presented for many processes, each representative of a certain class of complexity. This serves as a comparison benchmark, and shows that realistic studies can be performed for any 2 → 3 and most 2 → 4 processes in the Standard Model. We finish by providing two examples of phenomenology study at the Large Hadron Collider using the tools we developed. The first treats the production of a scalar or pseudo-scalar in association with a top quark pair while the second addresses the tri-boson production channel Z W+ W− with MadSpin simulating the subsequent decay to leptons. We conclude with some insights on MadLoop prospects
The complete NLO corrections to dijet hadroproduction
We study the production of jets in hadronic collisions, by computing all
contributions proportional to , with and .
These correspond to leading and next-to-leading order results, respectively,
for single-inclusive and dijet observables in a perturbative expansion that
includes both QCD and electroweak effects. We discuss issues relevant to the
definition of hadronic jets in the context of electroweak corrections, and
present sample phenomenological predictions for the 13-TeV LHC. We find that
both the leading and next-to-leading order contributions largely respect the
relative hierarchy established by the respective coupling-constant
combinations.Comment: 30 pages, 14 figures; v2 contains minor changes to the text and to
one label in fig.1, plus additional material in the form of an ancillary file
that reports cross sections computed in various HT range
Local Unitarity: a representation of differential cross-sections that is locally free of infrared singularities at any order
We propose a novel representation of differential scattering cross-sections
that locally realises the direct cancellation of infrared singularities
exhibited by its so-called real-emission and virtual degrees of freedom. We
take advantage of the Loop-Tree Duality representation of each individual
forward-scattering diagram and we prove that the ensuing expression is locally
free of infrared divergences, applies at any perturbative order and for any
process without initial-state collinear singularities. Divergences for loop
momenta with large magnitudes are regulated using local ultraviolet
counterterms that reproduce the usual Lagrangian renormalisation procedure of
quantum field theories. Our representation is especially suited for a numerical
implementation and we demonstrate its practical potential by computing fully
numerically and without any IR counterterm the next-to-leading order accurate
differential cross-section for the process . We
also show first results beyond next-to-leading order by computing interference
terms part of the N4LO-accurate inclusive cross-section of a
scalar scattering process.Comment: 88 page
Loop Tree Duality for multi-loop numerical integration
Loop Tree Duality (LTD) offers a promising avenue to numerically integrate
multi-loop integrals directly in momentum space. It is well-established at one
loop, but there have been only sparse numerical results at two loops. We
provide a formal derivation for a novel multi-loop LTD expression and study its
threshold singularity structure. We apply our findings numerically to a diverse
set of up to four-loop finite topologies with kinematics for which no contour
deformation is needed. We also lay down the ground work for constructing such a
deformation. Our results serve as an important stepping stone towards a
generalised and efficient numerical implementation of LTD, applicable to the
computation of virtual corrections.Comment: 13 page
Risk Factors for Mortality after COVID-19 in Patients with Preexisting Interstitial Lung Disease.
International audienc
Four-lepton production at hadron colliders: aMC@NLO predictions with theoretical uncertainties
We use aMC@NLO to study the production of four charged leptons at the LHC,
performing parton showers with both HERWIG and Pythia6. Our underlying matrix
element calculation features the full next-to-leading order
result and the contribution of the channel, and it
includes all off-shell, spin-correlation, virtual-photon-exchange, and
interference effects. We present several key distributions together with the
corresponding theoretical uncertainties. These are obtained through a
process-independent technique that allows aMC@NLO to compute scale and PDF
uncertainties in a fully automated way and at no extra CPU-time costComment: 24 pages, 6 figure
Spin polarisation oft ̄tγγproduction at NLO+PS with GOSAM interfaced to MADGRAPH5_AMC@NLO
We present an interface between the multipurpose Monte Carlo toolMadGraph5_aMC@NLOand the automated amplitude generator GoSam. As a first application of this novel framework, we compute the NLO corrections topp→t ̄tHandpp→t ̄tγγmatched to a parton shower. In the phenomenological analyses of these processes, we focus our attention on observables which are sensitive to the polarisation of the top quarks