arXiv subjects

Stefan Kallweit

Publications and source records attributed to Stefan Kallweit.

At least 19 recordsLinked to original sources

arXiv2026

Higgs Boson Pair Production via Gluon Fusion: Higher-Order Corrections and Theoretical Uncertainties

In this contribution, the higher-order QCD and electroweak corrections to Standard Model Higgs boson pair production via the gluon-fusion mechanism, $gg\to hh$, are summarized and the different sources of theoretical uncertainty are assessed. The discussion includes finite top quark mass effects, matching to parton showers, approximate NNLO and N$^3$LO QCD corrections, NLO electroweak effects, and uncertainties associated with the top quark mass scheme and perturbative scale choices. In addition, we provide an updated state-of-the-art recommendation for the inclusive gluon-fusion Higgs boson pair production cross section and the corresponding Higgs boson pair invariant-mass distribution.

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arXiv2026

NNLO QCD corrections to $WWγ$ production at the LHC

Triboson production processes are crucial to study quartic gauge-boson couplings. We present the computation of the radiative corrections to $WWγ$ production at the next-to-next-to-leading order (NNLO) in QCD. The leptonic decays of the $W$ bosons and off-shell effects are fully included. The calculation is exact, apart from the finite part of the two-loop amplitudes, which is evaluated in a soft-photon approximation. We validate our approach by using $Zγ$ production as a reference process, where we take advantage of the availability of the exact two-loop amplitudes to derive a conservative error estimate for our approximation, and then apply it to $WWγ$ production. For typical selection cuts, at the centre-of-mass energy $\sqrt{s}=13$ TeV, the NNLO corrections increase the next-to-leading order (NLO) result by about $16\%$, and the perturbative uncertainties are reduced to the $\pm4\%$ level. The uncertainty from the soft approximation turns out to be at the few per mille level, largely subdominant compared to the residual perturbative uncertainties, both for the fiducial cross section and the most relevant differential distributions.

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arXiv2026

NNLO QCD predictions for $t\bar t W$ production at hadron colliders

The production of a top-antitop quark pair in association with a $W$ boson constitutes one of the heaviest final states currently studied at the Large Hadron Collider (LHC) at CERN. Measurements of its production rate have consistently exceeded Standard Model predictions. Owing to the complexity of the two-loop amplitudes entering the double-virtual correction, next-to-next-to-leading-order (NNLO) QCD calculations for this process have so far employed dynamical approximations for the two-loop contribution. We present NNLO QCD predictions based, for the first time, on a direct computation of the required two-loop amplitudes in the generalised leading-colour limit.

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arXiv2025

Jet Production at NNLO: Exploring a New Scheme

We consider dijet production in $e^+e^-$ collisions and in $H\to b{\bar b}$ decays at next-to-next-to-leading order (NNLO) in perturbative QCD. A new non-local subtraction scheme is applied, for the first time, to obtain the fully differential cross section for these benchmark processes. We discuss and explicitly evaluate the perturbative ingredients needed in the computation, and we compare the performance of different slicing variables to obtain the NNLO corrections.

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arXiv2025

NNLO QCD predictions for $Wγγ$ production at the LHC

Triboson production processes play a crucial role in probing the electroweak sector of the Standard Model, as they involve quartic gauge-boson couplings already at the tree level. With these measurements entering the precision era at the Large Hadron Collider (LHC), accurate theoretical predictions become indispensable. We present the computation of the next-to-next-to-leading-order (NNLO) QCD radiative corrections to the production of a $W$ boson in association with two photons ($Wγγ$) at the LHC. The calculation is exact, except for the finite part of the two-loop contribution, which is included in the leading-colour approximation. Predictions for the fiducial cross section and selected kinematic distributions are provided at a centre-of-mass energy of $\sqrt{s}=13$ TeV, under standard experimental selection cuts. In line with observations for other multiboson processes involving direct photons, we find sizable NNLO corrections that enhance the next-to-leading-order predictions by about $23\%$, with residual perturbative uncertainties that can be roughly estimated to be at the $5\%$ level.

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arXiv2025

State-of-the-art cross sections for ttH: NNLO predictions matched with NNLL resummation and EW corrections

We present new, state-of-the-art predictions for the associated production of the SM Higgs boson with top quarks, computed in accordance with the recommendations of the LHC Higgs Working Group. The NNLO QCD predictions, derived through suitable approximations of the two-loop virtual contribution, are supplemented with soft-gluon resummation up to NNLL accuracy. Two distinct resummation frameworks are employed - one based on direct QCD and the other on soft-collinear effective theory - and their features are compared in detail. These results are further combined with the complete-NLO corrections, yielding the most precise SM predictions for this process to date. The relevant sources of theoretical uncertainties are thoroughly estimated and discussed.

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arXiv2025

Mixed QCD-EW corrections to the neutral-current Drell-Yan process

We report on the complete computation of the mixed QCD-electroweak corrections to the neutral-current Drell-Yan process. Our calculation holds in the entire range of dilepton invariant masses. We present phenomenological results for several kinematical distributions in the case of bare muons both in the resonant region and for high invariant masses. We also consider the forward-backward asymmetry, which is a key observable to measure the weak mixing angle. We finally extend our calculation to dressed leptons and compare our results in the massless limit to those available in the literature.

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arXiv2025

Towards NNLO QCD predictions for off-shell top-quark pair production and decays

We consider QCD radiative corrections to $W^+W^-b {\bar b}$ production with leptonic decays and massive bottom quarks at the LHC. We perform an exact next-to-leading order (NLO) calculation within the $q_T$-subtraction formalism and validate it against an independent computation in the dipole subtraction scheme. Non-resonant and off-shell effects related to the top quarks and the leptonic decays of the $W^\pm$ bosons are consistently included. We also consider the approximation in which the real-emission contribution is computed exactly while the virtual is evaluated in the double-pole approximation (DPA), which formally requires the inclusion of both factorisable and non-factorisable corrections. We evaluate such contributions and show that the DPA performs remarkably well at both the inclusive and differential levels. We then extend our calculation to the next-to-next-to-leading order (NNLO). All tree-level and one-loop amplitudes are evaluated exactly, while the missing two-loop virtual contribution is estimated using the DPA. The factorisable two-loop corrections are explicitly computed by relying on available results for the polarised two-loop on-shell top-quark pair production amplitudes and the corresponding top-quark decays. The non-factorisable contributions are inferred by exploiting the cancellation of logarithmic singularities in the $Γ_t\to 0$ limit through an on-shell matching procedure. The NNLO corrections for the inclusive cross section are found to increase the NLO prediction by approximately $11\%$, with a numerical uncertainty that is conservatively estimated to be below the $2\%$ level $\unicode{x2013}$ significantly smaller than the $5\%$ residual perturbative uncertainties.

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arXiv2025

Precise predictions for $t \bar t H$ production at the LHC: inclusive cross section and differential distributions

We present the first fully differential next-to-next-to-leading order (NNLO) QCD calculation for the production of a top-antitop quark pair in association with a Higgs boson ($t \bar t H$) at hadron colliders. The computation is exact, except for the finite part of the two-loop virtual contribution, which we estimate using two different methods that yield consistent results within their respective uncertainties. The first method relies on a soft-Higgs factorisation formula that we develop up to the three-loop order. The second is based on a high-energy expansion in the small top-mass limit. Combining the newly computed corrections with the complete set of next-to-leading order (NLO) QCD+EW results provides the most advanced perturbative prediction currently available at the LHC for both inclusive and differential $t \bar t H$ cross sections. The uncertainties due to the missing exact two-loop contribution are conservatively estimated to be at the percent level, both for the total cross section and for most of the differential distributions, and therefore subleading compared to the residual perturbative uncertainties.

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arXiv2024

Matching NNLO to parton shower using N$^3$LL colour-singlet transverse momentum resummation in GENEVA

We extend the GENEVA Monte Carlo framework using the transverse momentum of a colour-singlet system as the resolution variable. This allows us to use next-to-next-to-next-to leading logarithm (N$^3$LL) resummation via the \radish formalism to obtain precise predictions for any colour-singlet production process at the fully exclusive level. Thanks to the implementation of two different resolution variables within the GENEVA framework, we are able to assess the impact of such a choice on differential observables for the first time. As a first application we present predictions for Drell-Yan lepton pair production at next-to-next-to-leading order (NNLO) in QCD interfaced to a parton shower simulation that includes additional all-order radiative corrections. We provide fully showered and hadronised events using PYTHIA8, while retaining the NNLO QCD accuracy for observables which are inclusive over the additional radiation. We also show that it is possible to obtain a numerically good agreement between showered GENEVA predictions and the N$^3$LL resummation for the transverse momentum spectrum by choosing a more local recoil scheme. We compare our final predictions to LHC data at 13 TeV, finding good agreement across several distributions.

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arXiv2024

Associated production of a $W$ boson and massive bottom quarks at next-to-next-to-leading order in QCD

We present the first calculation for the hadroproduction of a $W$ boson in association with a massive bottom ($b$) quark-antiquark pair at next-to-next-to-leading order (NNLO) in QCD perturbation theory. We exploit the hierarchy between the $b$ quark mass and the characteristic energy scale of the process to obtain a reliable analytic expression for the two-loop virtual amplitude with three massive legs, starting from the corresponding result available for massless bottom quarks. The use of massive $b$ quarks avoids the ambiguities associated with the correct flavour assignment in massless calculations, paving the way to a more realistic comparison with experimental data. We present phenomenological results considering proton-proton collisions at centre-of-mass energy $\sqrt{s}=13.6$ TeV for inclusive $Wb \bar b$ production and within a fiducial region relevant for the associated production of a $W$ boson and a Higgs boson decaying into a $b \bar b$ pair, for which $Wb \bar b$ production represents one of the most relevant backgrounds. We find that the NNLO corrections are substantial and that their inclusion is mandatory to obtain reliable predictions.

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arXiv2023

Precise predictions for the associated production of a $W$ boson with a top-antitop quark pair at the LHC

The production of a top-antitop quark pair in association with a $W$ boson ($t\bar tW$) is one of the heaviest signatures currently probed at the Large Hadron Collider (LHC). Since the first observation reported in 2015 the corresponding rates have been found to be consistently higher than the Standard Model predictions, which are based on next-to-leading order~(NLO) calculations in the QCD and electroweak (EW) interactions. We present the first next-to-next-to-leading order (NNLO) QCD computation of $t\bar tW$ production at hadron colliders. The calculation is exact, except for the finite part of the two-loop virtual corrections, which is estimated using two different approaches that lead to consistent results within their uncertainties. We combine the newly computed NNLO QCD corrections with the complete NLO QCD+EW results, thus obtaining the most advanced perturbative prediction available to date for the \ttW inclusive cross section. The tension with the latest ATLAS and CMS results remains at the $1σ-2σ$ level.

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arXiv2023

Double Higgs production at NNLO interfaced to parton showers in GENEVA

In this work, we study the production of Higgs boson pairs at next-to-next-to-leading order in QCD matched to parton showers, using the \textsc{Geneva} framework and working in the heavy-top-limit approximation. This includes the resummation of large logarithms of the zero-jettiness $\mathcal{T}_0$ up to the next-to-next-to-next-to-leading-log accuracy. This process features an extremely large momentum transfer, which makes its study particularly relevant for matching schemes such as that employed in \textsc{Geneva}, where the resummation of a variable different from that used in the ordering of the parton shower is used. To further study this effect, we extend the original shower interface designed for \textsc{Pythia8} to include other parton showers, such as \textsc{Dire} and \textsc{Sherpa}.

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arXiv2023

Refining the GENEVA method for Higgs boson production via gluon fusion

We describe a number of improvements to the GENEVA method for matching NNLO calculations to parton shower programs. In particular, we detail changes to the resummed calculation used in the matching procedure, including disentangling the cross section dependence on factorisation and beam scales, and an improved treatment of timelike logarithms. We also discuss modifications in the implementation of the splitting functions which serve to make the resummed calculation differential in the higher multiplicity phase space. These changes improve the stability of the numerical cancellation of the nonsingular term at small values of the resolution parameter. As a case study, we consider the gluon-initiated Higgs boson production process $gg\to H$. We validate the NNLO accuracy of our predictions against independent calculations, and compare our showered and hadronised results with recent data taken at the ATLAS and CMS experiments in the diphoton decay channel, finding good agreement.

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arXiv2023

Higgs boson production in association with a top-antitop quark pair in next-to-next-to leading order QCD

The associated production of a Higgs boson with a top-antitop quark pair is a crucial process at the LHC since it allows for a direct measurement of the top-quark Yukawa coupling. We present the computation of the radiative corrections to this process at the next-to-next-to-leading order (NNLO) in QCD perturbation theory. This is the very first computation for a $2 \to 3$ process with massive coloured particles at this perturbative order. We develop a soft Higgs boson approximation for loop amplitudes, which enables us to reliably quantify the impact of the yet unknown two-loop contribution. At the centre-of-mass energy $\sqrt{s}=13$ TeV the NNLO corrections increase the next-to-leading order result for the total cross section by about 4% and lead to a significant reduction of perturbative uncertainties.

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arXiv2022

Linear power corrections for two-body kinematics in the $q_T$ subtraction formalism

Transverse-momentum cuts on undistinguished particles in two-body final states induce an enhanced sensitivity to low momentum scales. This undesirable feature, which ultimately leads to an instability of the fixed-order series, poses additional challenges to non-local subtraction schemes. In this letter, we address this issue for general colour-singlet processes within the $q_T$-subtraction formalism, focussing on neutral-current Drell-Yan production. We present a simple procedure to reduce the dependence on the slicing parameter from linear to quadratic, by accounting for the linear power corrections through an appropriate recoil prescription. We observe a dramatical improvement of the numerical convergence and a reduction of the systematic uncertainties. We also discuss how a linear dependence in $q_T$ can be avoided for Drell-Yan production by using staggered cuts, which, to the best of our understanding, could be used in experimental analyses. We show that our approach can be successfully applied also to on-shell $ZZ$ production. We finally study diphoton production and verify that our approach is insufficient to capture the linear power corrections introduced by the isolation procedure. The recoil prescription is available in version 2.1 of MATRIX.

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arXiv2021

$t\bar{t}H$ production at NNLO: the flavour off-diagonal channels

We consider QCD radiative corrections to the associated production of a heavy-quark pair ($Q{\bar Q}$) with a generic colourless system $F$ at hadron colliders. We discuss the resummation formalism for the production of the $Q{\bar Q}F$ system at small values of its total transverse momentum $q_T$. The perturbative expansion of the resummation formula leads to the explicit ingredients that can be used to apply the $q_T$ subtraction formalism to fixed-order calculations for this class of processes. We use the $q_T$ subtraction formalism to perform a fully differential perturbative computation for the production of a top-antitop quark pair and a Higgs boson. At next-to-leading order we compare our results with those obtained with established subtraction methods and we find complete agreement. We present, for the first time, the results for the flavour off-diagonal partonic channels at the next-to-next-to-leading order.

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arXiv2021

Mixed strong$-$electroweak corrections to the Drell$-$Yan process

We report on the first complete computation of the mixed QCD$-$electroweak (EW) corrections to the neutral-current Drell$-$Yan process. Superseding previously applied approximations, our calculation provides the first result at this order that is valid in the entire range of dilepton invariant masses. The two-loop virtual contribution is computed by using semi-analytical techniques, overcoming the technical problems in the evaluation of the relevant master integrals. The cancellation of soft and collinear singularities is achieved by a formulation of the $q_T$ subtraction formalism valid in presence of charged massive particles in the final state. We present numerical results for the fiducial cross section and selected kinematical distributions. At large values of the lepton $p_T$ the mixed QCD$-$EW corrections are negative and increase in size, to about $-15\%$ with respect to the next-to-leading-order QCD result at $p_T=500\,$GeV. Up to dilepton invariant masses of 1 TeV the computed corrections amount to about $-1.5\%$ with respect to the next-to-leading-order QCD result.

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