arXiv subjects

Jean-Nicolas Lang

Publications and source records attributed to Jean-Nicolas Lang.

At least 19 recordsLinked to original sources

arXiv2022

Precise predictions for double-Higgs production via vector-boson fusion

Theoretical predictions with next-to-next-to-leading order (NNLO) QCD accuracy combined with the next-to-leading order (NLO) electroweak (EW) corrections are presented for differential observables of the double-Higgs production process via vector-boson fusion. While the QCD corrections were previously known, the EW ones are computed here for the first time. The numerical results are obtained for a realistic experimental set-up at the LHC and are presented in the form of fiducial cross sections and differential distributions. Within this setup we find that the VBF approximation employed in the NNLO QCD correction is accurate at the sub-percent level. We find that the NLO EW corrections within the fiducial volume are $-6.1\%$, making them of almost the same order as the NLO QCD corrections. In some kinematic regions they can grow as large as $-30\%$ making them the dominant radiative corrections. When the EW corrections are combined with the NNLO QCD corrections we find a total correction of $-14.8\%$. The results presented here thus comprise the state-of-the-art theoretical predicition for the double-Higgs production via vector-boson fusion, which will be of value to the high-luminosity programme at the LHC.

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arXiv2022

Two-Loop Rational Terms for Spontaneously Broken Theories

Rational counterterms are a key ingredient for the automation of loop calculations through numerical methods. Building on the recently established properties of rational terms of UV origin at two loops, in this paper we present a systematic method for the determination of rational counterterms within spontaneously broken theories. In particular we introduce a generalised vev-expansion approach that makes it possible to obtain the rational counterterms of UV origin for a spontaneously broken theory by means of calculations in the unbroken phase. The drastic simplifications that result from the underlying symmetry open the door to the efficient determination of rational counterterms for the full Standard Model at two loops. The renormalisation-scheme dependence is analysed in detail, and we show that rational counterterms need to be determined only once and for all in a generic renormalisation scheme for the symmetric phase and, a posteriori, they can be easily adapted to a wide range of physical renormalisation schemes for the spontaneously broken phase. As a first application we determine the full set of $\mathcal{O}(α_{\mathrm s}^2)$ rational counterterms of UV origin for the full Standard Model, i.e. for all superficially UV-divergent two-loop vertex functions involving combinations of gluons, quarks, electroweak vector bosons and scalar bosons.

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arXiv2021

Full NLO QCD corrections to off-shell ttbb production

In this article, we report on the computation of the NLO QCD corrections to ${\rm p}{\rm p} \to μ^-\barν_μ{\rm e}^+ν_{\rm e}\bar{\rm b}{\rm b}\bar{\rm b}{\rm b}$ at the LHC, which is an irreducible background to ${\rm p}{\rm p}\to{\rm t}\bar{\rm t}{\rm H}\bigl(\to{\rm b}\bar{\rm b}\bigr)$. This is the first time that a full NLO computation for a $2\to8$ process with 6 external strongly-interacting partons is made public. No approximations are used, and all off-shell and interference effects are taken into account. Cross sections and differential distributions from the full computation are compared to results obtained by using a double-pole approximation for the top quarks. The difference between the full calculation and the one using the double-pole approximation is in general below 5\% but can reach 10\% in some regions of phase space.

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arXiv2020

Electroweak $t \bar t$ hadroproduction in the presence of heavy $Z'$ and $W'$ bosons at NLO QCD in POWHEG

We extend and improve upon our previous calculation of electroweak top-quark pair hadroproduction in extensions of the Standard Model with extra heavy neutral and charged spin-1 resonances. In particular, we allow for flavour-non-diagonal $Z'$ couplings and take into account non-resonant production in the SM and beyond including the contributions with $t$-channel $W$ and $W'$ bosons. All amplitudes are generated using the Recola2 package. As in our previous work, we include NLO QCD corrections and consistently match to parton showers with the POWHEG method fully taking into account the interference effects between SM and new physics amplitudes. We consider the Sequential Standard Model, the Topcolour model, as well as the Third Family Hypercharge Model featuring non-flavour-diagonal $Z'$ couplings which has been proposed recently to explain the anomalies in $B$ decays. We present numerical results for $t \bar t$ cross sections at hadron colliders with a centre-of-mass energy up to 100 TeV.

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arXiv2020

Two-Loop Rational Terms in Yang-Mills Theories

Scattering amplitudes in $D$ dimensions involve particular terms that originate from the interplay of UV poles with the $D-4$ dimensional parts of loop numerators. Such contributions can be controlled through a finite set of process-independent rational counterterms, which make it possible to compute loop amplitudes with numerical tools that construct the loop numerators in four dimensions. Building on a recent study [1] of the general properties of two-loop rational counterterms, in this paper we investigate their dependence on the choice of renormalisation scheme. We identify a nontrivial form of scheme dependence, which originates from the interplay of mass and field renormalisation with the $D-4$ dimensional parts of loop numerators, and we show that it can be controlled through a new kind of one-loop counterterms. This guarantees that the two-loop rational counterterms for a given renormalisable theory can be derived once and for all in terms of generic renormalisation constants, which can be adapted a posteriori to any scheme. Using this approach, we present the first calculation of the full set of two-loop rational counterterms in Yang-Mills theories. The results are applicable to SU(N) and U(1) gauge theories coupled to $n_{f}$ fermions with arbitrary masses.

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arXiv2020

An event generator for same-sign W-boson scattering at the LHC including electroweak corrections

In this article we present an event generator based on the Monte Carlo program Powheg in combination with the matrix-element generator Recola. We apply it to compute NLO electroweak corrections to same-sign W-boson scattering, which have been shown to be large at the LHC. The event generator allows for the generation of unweighted events including the effect of the NLO electroweak corrections matched to a QED parton shower and interfaced to a QCD parton shower. In view of the expected experimental precision of future measurements, the use of such a tool will be indispensable.

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arXiv2020

NLO QCD + electroweak predictions for off-shell ttH production at the LHC

In these proceedings, next-to-leading-order (NLO) QCD and electroweak (EW) corrections to ${\rm p}{\rm p} \to {\rm e}^+ ν_{\rm e} μ^- \bar ν_μ{\rm b} \bar{\rm b} {\rm H}$ at the LHC are presented. In these computations the top quarks are considered off their mass shell and all non-resonant contributions are included. The results are presented in the form of fiducial cross sections and differential distributions. Moreover, two prescriptions to combine QCD and EW corrections are examined.

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arXiv2019

Renormalization schemes for mixing angles in extended Higgs sectors

The proper renormalization of mixing angles in quantum field theories is a long-standing problem. It is relevant for the renormalization of the quark mixing matrix in the Standard Model and for various mixing scenarios in theories beyond. In this contribution we specifically consider theories with extended scalar sectors. We describe renormalization schemes for mixing angles based on combinations of observables or symmetry requirements such as rigid or background-field gauge invariance and compare their properties to previous approaches such as $\bar{MS}$ schemes. We formulate specific renormalization conditions for the mixing angles in the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model and calculate electroweak corrections to Higgs-boson decays via W- or Z-boson pairs within these models for a selection of (new and old) renormalization schemes.

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arXiv2019

OpenLoops 2

We present the new version of OpenLoops, an automated generator of tree and one-loop scattering amplitudes based on the open-loop recursion. One main novelty of OpenLoops 2 is the extension of the original algorithm from NLO QCD to the full Standard Model, including electroweak (EW) corrections from gauge, Higgs and Yukawa interactions. In this context, among several new features, we discuss the systematic bookkeeping of QCD-EW interferences, a flexible implementation of the complex-mass scheme for processes with on-shell and off-shell unstable particles, a special treatment of on-shell and off-shell external photons, and efficient scale variations. The other main novelty is the implementation of the recently proposed on-the-fly reduction algorithm, which supersedes the usage of external reduction libraries for the calculation of tree-loop interferences. This new algorithm is equipped with an automated system that avoids Gram-determinant instabilities through analytic methods in combination with a new hybrid-precision approach based on a highly targeted usage of quadruple precision with minimal CPU overhead. The resulting significant speed and stability improvements are especially relevant for challenging NLO multi-leg calculations and for NNLO applications.

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arXiv2019

Renormalization of mixing angles

We discuss the renormalization of mixing angles for theories with extended scalar sectors. Motivated by shortcomings of existing schemes for mixing angles, we review existing renormalization schemes and introduce new ones based on on-shell conditions or symmetry requirements such as rigid or background-field gauge invariance. Considering in particular the renormalization of the mixing angles in the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model, we compare electroweak corrections within these models for a selection of renormalization schemes. As specific examples, we present next-to-leading-order results on the four-fermion decays of heavy and light CP-even Higgs bosons, $H_1/H_2\to WW/Z Z\to4f$, and on electroweak Higgs-boson production processes, i.e. Higgs-strahlung and vector-boson fusion. We find that our new proposals for on-shell and symmetry-based renormalization conditions are well-behaved for the considered benchmark scenarios in both models.

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arXiv2019

Proposal for the validation of Monte Carlo implementations of the standard model effective field theory

We propose a procedure to cross-validate Monte Carlo implementations of the standard model effective field theory. It is based on the numerical comparison of squared amplitudes computed at specific phase-space and parameter points in pairs of implementations. Interactions are fully linearised in the effective field theory expansion. The squares of linear effective field theory amplitudes and their interference with standard-model contributions are compared separately. Such pairwise comparisons are primarily performed at tree level and a possible extension to the one-loop level is also briefly considered. We list the current standard model effective field theory implementations and the comparisons performed to date.

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arXiv2018

Effects of anomalous triple-gauge-boson interactions in diboson production with RECOLA2

Diboson production processes are of great importance in high-energy physics and a precise theoretical knowledge of VV production is mandatory not only in view of precision tests of the Standard Model but also in the one of new physics searches. We present results of arXiv:1804.01477, where we computed the NLO QCD and NLO electroweak corrections to diboson production processes at the LHC including the effect of the anomalous triple-gauge-boson interactions at NLO QCD accuracy. The anomalous triple-gauge-boson interactions are parametrized in terms of higher-dimensional operators in the effective field theory framework. Our calculation is the first application of Recola2 to effective field theory models.

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arXiv2018

On-the-fly reduction of open loops

We describe new developments in the OpenLoops framework based on the recently introduced on-the-fly method. The on-the-fly approach exploits the factorisation of one-loop diagrams into segments in order to perform various operations, such as helicity summation, diagram merging and the reduction of Feynman integrands in between the recursion steps for the amplitude construction. This method significantly reduces the complexity of scattering amplitude calculations for multi-particle processes, leading to a major increase in CPU efficiency and numerical stability. The unification of the reduction to scalar integrals with the amplitude construction in a single algorithm, allows to identify problematic kinematical configurations and cure numerical instabilities in single recursion steps. A simple permutation trick in combination with a one-parameter expansion for a single topology, which is now implemented to any order, eliminate rank-two Gram determinant instabilities altogether. Due to this any-order expansion, the numerical accuracy of the algorithm can be determined with a rescaling test. The on-the-fly algorithm is fully implemented for double and quadruple precision, which allows for true quadruple precision benchmarks with up to 32 correct digits as well as a powerful rescue system for unstable points. We present first speed and stability results for these new features. The on-the-fly algorithm is part of the forthcoming release of OpenLoops 2.

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arXiv2018

Anomalous triple-gauge-boson interactions in diboson production

Diboson production is of prime interest at the LHC particularly due to its sensitivity to the gauge-boson self interaction, allowing to test its Standard Model prediction with high precision and to search for possible deviations with respect to the Standard Model. We report on the results of arXiv:1804.01477, where we computed WW, WZ and ZZ production (including the leptonic decays of the vector bosons) in the effective field theory framework at NLO QCD accuracy. The impact of the higher-dimensional operators is compared to the NLO QCD and NLO EW corrections in the Standard Model. Our calculation is the first application of RECOLA2 in the effective field theory framework.

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arXiv2018

Anomalous triple-gauge-boson interactions in vector-boson pair production with RECOLA2

Diboson production at the LHC is a process of great importance both in the context of tests of the SM and for direct searches for new physics. In this paper we present a phenomenological study of WW, WZ, and ZZ production considering event selections of interest for the anomalous triple-gauge-boson-coupling searches at the LHC: we provide theoretical predictions within the Standard Model at NLO QCD and NLO EW accuracy and study the effect of the anomalous triple-gauge-boson interactions at NLO QCD. For WW and ZZ production, the contribution of the gluon-initiated loop-induced processes is included. Anomalous triple-gauge-boson interactions are parametrized in the EFT framework. This paper is the first application of RECOLA2 in the EFT context.

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arXiv2018

Recola2: REcursive Computation of One-Loop Amplitudes 2

We present the Fortran95 program Recola2 for the perturbative computation of next-to-leading-order transition amplitudes in the Standard Model of particle physics and extended Higgs sectors. New theories are implemented via model files in the 't Hooft-Feynman gauge in the conventional formulation of quantum field theory and in the Background-Field method. The present version includes model files for the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model. We support standard renormalization schemes for the Standard Model as well as many commonly used renormalization schemes in extended Higgs sectors. Within these models the computation of next-to-leading-order polarized amplitudes and squared amplitudes, optionally summed over spin and colour, is fully automated for any process. Recola2 allows the computation of colour- and spin-correlated leading-order squared amplitudes that are needed in the dipole subtraction formalism. Recola2 is publicly available for download at http://recola.hepforge.org.

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arXiv2017

Recola2: a one-loop matrix-element generator for BSM theories and SM effective field theory

We present the Recola2 library for the efficient generation and computation of one-loop amplitudes in Beyond-Standard-Model theories. Recola2 is based on Recola, an efficient one-loop amplitude generator for the Standard Model, and Rept1l, a newly developed tool to generate one-loop model files for Recola2 in a fully automated way. Recola2 is able to operate with non-trivial extensions of the SM, e.g. extended Higgs sectors and effective field theories. We discuss first applications to extended Higgs sectors and their renormalization.

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arXiv2017

NLO QCD and EW corrections to processes involving off-shell top quarks

We review recent results on next-to-leading order (NLO) QCD and electroweak (EW) corrections for processes involving off-shell top quarks. For the off-shell production of two top quarks that decay leptonically, the full NLO EW corrections have been computed. For the very same process in association with a Higgs boson, EW corrections have been calculated and combined with existing NLO QCD corrections. In both cases, selected differential distributions are shown. In these proceedings, particular emphasis is put on the effect of the EW corrections as well as the off-shell contributions.

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