arXiv subjects

Stefan Kallweit

Publications and source records attributed to Stefan Kallweit.

At least 37 records · Page 2Linked to original sources

arXiv2021

Mixed QCD-EW corrections to $\boldsymbol{pp\!\to\!\ellν_\ell\!+\!X}$ at the LHC

We consider the hadroproduction of a massive charged lepton plus the corresponding neutrino through the Drell-Yan mechanism. We present a new computation of the mixed QCD-EW corrections to this process. The cancellation of soft and collinear singularities is achieved by using a formulation of the $q_T$ subtraction formalism derived from the next-to-next-to-leading order QCD calculation for heavy-quark production. For the first time, all the real and virtual contributions due to initial- and final-state radiation are consistently included without any approximation, except for the finite part of the two-loop virtual correction, which is computed in the pole approximation and suitably improved through a reweighting procedure. We demonstrate that our calculation is reliable in both on-shell and off-shell regions, thereby providing the first prediction of the mixed QCD-EW corrections in the entire region of the lepton transverse momentum. The computed corrections are in qualitative agreement with what we obtain in a factorised approach of QCD and EW corrections. At large values of the lepton $p_T$, the mixed QCD-EW corrections are negative and increase in size, to about $-20\%$ with respect to the next-to-leading order QCD result at $p_T=500\,$GeV.

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arXiv2021

Resummed predictions for hadronic Higgs boson decays

We present the NNLL$'$ resummed $2$-jettiness distribution for decays of the Standard Model Higgs boson to a $b\bar{b}$-quark pair and to gluons. The calculation exploits a factorisation formula derived using Soft-Collinear Effective Theory, in which large logarithms of the $2$-jettiness are resummed by renormalisation group evolution of the hard, soft and jet contributions to the differential decay rate. We match the resummed predictions to the fixed-order NNLO result using the GENEVA framework, extending the validity of the results to all values of the resolution variable and providing a fully exclusive NNLO event generator matched to the PYTHIA8 parton shower.

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arXiv2021

Next-to-next-to-leading order event generation for $Z$ boson pair production matched to parton shower

We present a novel next-to-next-to-leading order (NNLO) QCD calculation matched to parton shower for the production of a pair of $Z$ bosons decaying to four massless leptons, $p p \to \ell^+ \ell^- \ell'^+ \ell'^- + X$, at the LHC. Spin correlations, interferences and off-shell effects are included throughout. Our result is based on the resummed beam-thrust spectrum, which we evaluate at next-to-next-to-leading-logarithmic (NNLL$'_{\mathcal{T}_0}$) accuracy for the first time for this process, and makes use of the GENEVA Monte Carlo framework for the matching to PYTHIA8 shower and hadronisation models. We compare our predictions with data from the ATLAS and CMS experiments at 13 TeV, finding a good agreement.

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arXiv2021

Bottom-quark production at hadron colliders: fully differential predictions in NNLO QCD

We report on the first fully differential calculation of the next-to-next-to-leading-order (NNLO) QCD radiative corrections to the production of bottom-quark pairs at hadron colliders. The calculation is performed by using the $q_T$ subtraction formalism to handle and cancel infrared singularities in real and virtual contributions. The computation is implemented in the Matrix framework, thereby allowing us to efficiently compute arbitrary infrared-safe observables in the four-flavour scheme. We present selected predictions for bottom-quark production at the Tevatron and at the LHC at different collider energies, and we perform some comparisons with available experimental results. We find that the NNLO corrections are sizeable, typically of the order of $25$-$35\%$, and they lead to a significant reduction of the perturbative uncertainties. Therefore, their inclusion is crucial for an accurate theoretical description of this process.

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arXiv2021

Precise predictions for photon pair production matched to parton showers in GENEVA

We present a new calculation for the production of isolated photon pairs at the LHC with NNLL$'_{\mathcal{T}_0}$+NNLO accuracy. This is the first implementation within the GENEVA Monte Carlo framework of a process with a nontrivial Born-level definition which suffers from QED singularities. Throughout the computation we use a smooth-cone isolation algorithm to remove such divergences. The higher-order resummation of the 0-jettiness resolution variable $\mathcal{T}_0$ is based on a factorisation formula derived within Soft-Collinear Effective Theory which predicts all of the singular, virtual and real NNLO corrections. Starting from this precise parton-level prediction and by employing the GENEVA method, 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 compare our final predictions to LHC data at 7 TeV and find good agreement.

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arXiv2021

Four lepton production in gluon fusion: off-shell Higgs effects in NLO QCD

We consider the production of four charged leptons in hadron collisions and compute the next-to-leading order (NLO) QCD corrections to the loop-induced gluon fusion contribution by consistently accounting for the Higgs boson signal, its corresponding background and their interference. The contribution from heavy-quark loops is exactly included in the calculation except for the two-loop $gg\to ZZ\to 4\ell$ continuum diagrams, for which the unknown heavy-quark effects are approximated through a reweighting procedure. Our calculation is combined with the next-to-next-to-leading order QCD and NLO electroweak corrections to the $q\bar{q}\to4\ell$ process, including all partonic channels and consistently accounting for spin correlations and off-shell effects. The computation is implemented in the MATRIX framework and allows us to separately study the Higgs boson signal, the background and the interference contributions, whose knowledge can be used to constrain the Higgs boson width through off-shell measurements. Our state-of-the-art predictions for the invariant-mass distribution of the four leptons are in good agreement with recent ATLAS data.

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arXiv2021

Accurate single- and double-differential resummation of colour-singlet processes with MATRIX+RadISH: $W^+W^-$ production at the LHC

We present the combination of fully differential cross sections for colour-singlet production processes at next-to-next-to-leading order (NNLO) QCD obtained with MATRIX and all-order resummation through RadISH. This interface allows us to achieve unprecedented accuracy for various transverse observables in $2 \rightarrow 2$ production processes. As an important application we consider $W^+ W^-$ production at the LHC, more precisely the full leptonic process $pp\to \ell^+\ell^{\prime\, -}ν_{\ell}{\barν}_{\ell^\prime}+X$ with $\ell'\neq\ell$, and we present resummed predictions for differential distributions in presence of fiducial selection cuts. In particular, we resum the transverse-momentum spectrum of the $W^+ W^-$ pair at next-to-next-to-next-to-leading logarithmic (N$^3$LL) accuracy and match it to the integrated NNLO cross section. The transverse-momentum spectrum of the leading jet in $W^+ W^-$ production is calculated at NNLO+NNLL accuracy. Finally, the joint resummation for the transverse-momentum spectrum of the $W^+ W^-$ pair in the presence of a jet veto is performed at NNLO+NNLL. Our phenomenological study highlights the importance of higher-order perturbative and logarithmic corrections for precision phenomenology at the LHC.

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arXiv2021

Triphoton production at hadron colliders in NNLO QCD

We present next-to-next-to-leading-order (NNLO) QCD corrections to the production of three isolated photons in hadronic collisions at the fully differential level. We employ qT subtraction within MATRIX and an efficient implementation of analytic two-loop amplitudes in the leading-colour approximation to achieve the first on-the-fly calculation for this process at NNLO accuracy. Numerical results are presented for proton-proton collisions at energies ranging from 7 TeV to 100 TeV. We find full agreement with the 8 TeV results of arXiv:1911.00479 and confirm that NNLO corrections are indispensable to describe ATLAS 8 TeV data. In addition, we demonstrate the significance of NNLO corrections for future precision studies of triphoton production at higher collision energies.

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arXiv2020

Top-quark pair hadroproduction at NNLO: differential predictions with the MSbar mass

We consider top-quark pair production at the LHC within the MSbar scheme for the renormalisation of the top-quark mass, and we present predictions for total and differential cross sections at next-to-next-to-leading order (NNLO) in QCD. Our state-of-the-art calculation extends the available differential results by one order in the perturbative expansion, and it is relevant for a precise determination of the top-quark mass, including possible effects of the running mass in the MSbar scheme. We consider variations of the scale at which the MSbar mass of the top quark is evaluated, extending the usual 7-point to a 15-point scale variation. This additional variation is crucial for a reliable estimate of the theoretical uncertainties, especially at low perturbative orders and close to the production threshold of the top-quark pair. We also compute, for the first time, the invariant-mass distribution of the top-quark pair by using a running mass, evaluated at a dynamic scale. Our predictions for the invariant-mass distribution in the MSbar scheme are compared with a recent measurement performed by the CMS Collaboration. We observe that the inclusion of the NNLO corrections improves the agreement with the data, and we discuss effects due to the QCD running of the MSbar mass of the top quark.

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arXiv2020

NNLO QCD+NLO EW with MATRIX+OpenLoops: precise predictions for vector-boson pair production

We present the first combination of NNLO QCD and NLO EW corrections for vector-boson pair production at the LHC. We consider all final states with two, three and four charged leptons, including resonant and non-resonant diagrams, spin correlations and off-shell effects. Detailed predictions are discussed for three representative channels corresponding to $W^+W^-$, $W^{\pm}Z$ and $ZZ$ production. Both QCD and EW corrections are very significant, and the details of their combination can play a crucial role to achieve the level of precision demanded by experimental analyses. In this context we point out nontrivial issues that arise at large transverse momenta, where the EW corrections are strongly enhanced by Sudakov logarithms and the QCD corrections can feature so-called giant K-factors. Our calculations have been carried out in the MATRIX+OpenLoops framework and can be extended to the production of an arbitrary colour singlet in hadronic collisions, provided that the required two-loop QCD amplitudes are available. Combined NNLO QCD and NLO EW predictions for the full set of massive diboson processes will be made publicly available in the next release of MATRIX and will be instrumental in advancing precision diboson studies and new-physics searches at the LHC and future hadron colliders.

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arXiv2020

$ZZ$ production at the LHC: NLO QCD corrections to the loop-induced gluon fusion channel

We consider QCD radiative corrections to the production of four charged leptons in hadron collisions. We present the computation of the next-to-leading order QCD corrections to the loop-induced gluon fusion contribution. Our predictions include, for the first time, also the quark-gluon partonic channels. The computed corrections, which are formally of ${\cal O}(α_{\rm s}^3)$, turn out to increase the loop-induced Born-level result by an amount ranging from 75% to 71% as $\sqrt{s}$ ranges from 8 to 13 TeV. We combine our result with state-of-the-art NNLO corrections to the quark annihilation channel, and present updated predictions for fiducial cross sections and distributions for this process.

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arXiv2020

$W^+W^-$ production at the LHC: NLO QCD corrections to the loop-induced gluon fusion channel

We compute the NLO QCD corrections to the loop-induced gluon fusion contribution in $W^+W^-$ production at the LHC. We consider the full leptonic process $pp\to \ell^+\ell^{\prime\, -}ν_{\ell}{\barν}_{\ell^\prime}+X$, by including resonant and non-resonant diagrams, spin correlations and off-shell effects. Quark-gluon partonic channels are included for the first time in the calculation, and our results are combined with NNLO predictions to the quark annihilation channel at the fully differential level. The computed corrections, which are formally of ${\cal O}(α_{\mathrm{S}}^3)$, increase the NNLO cross section by only about 2%, but have an impact on the shapes of kinematical distributions, in part due to the jet veto, which is usually applied to reduce the top-quark background. Our results, supplemented with NLO EW effects, provide the most advanced fixed-order predictions available to date for this process, and are compared with differential ATLAS data at $\sqrt{s}=$ 13 TeV.

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arXiv2019

NLO QCD predictions for $t\bar{t}b\bar{b}$ production in association with a light jet at the LHC

Theoretical predictions for ttbb production are of crucial importance for ttH measurements in the H->bb channel at the LHC. To address the large uncertainties associated with the modelling of extra QCD radiation in ttbb events, in this paper we present a calculation of pp->ttbbj at NLO QCD. The behaviour of NLO corrections is analysed in a variety of observables, and to assess theoretical uncertainties we use factor-two rescalings as well as different dynamic scales. In this context, we propose a systematic alignment of dynamic scales that makes it possible to disentangle normalisation and shape uncertainties in a transparent way. Scale uncertainties at NLO are typically at the level of 20-30% in fiducial cross sections, and below 10% for the shapes of distributions. The kinematics of QCD radiation is investigated in detail, including the effects of its recoil on the objects of the ttbb system. In particular, we discuss various azimuthal correlations that allow one to charaterise the QCD recoil pattern in a precise and transparent way. In general, the calculation at hand provides a variety of precise benchmarks that can be used to validate the modelling of QCD radiation in ttbb generators. Moreover, as we will argue, pp->ttbbj at NLO entails information that can be used to gain insights into the perturbative convergence of the inclusive ttbb cross section beyond NLO. Based on this idea, we address the issue of the large NLO K-factor observed in the ttbb cross section, and we provide evidence that supports the reduction of this K-factor through a mild adjustment of the QCD scales that are conventionally used for this process. The presented 2->5 NLO calculations have been carried out using OpenLoops 2 in combination with Sherpa and Munich.

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arXiv2019

Higgsstrahlung at NNLL$'+$NNLO Matched to Parton Showers in GENEVA

We present results for the Higgsstrahlung process within the GENEVA Monte Carlo framework. We combine the fully differential NNLO calculation with the higher-order resummation in the 0-jettiness resolution variable (beam-thrust). The resulting parton-level events are further showered and hadronised by Pythia8. The beam-thrust resummation is carried out to NNLL$'$ accuracy, which consistently incorporates all singular virtual and real NNLO corrections. It thus provides a natural perturbative connection between the NNLO calculation and the parton shower regime, including a systematic assessment of perturbative uncertainties. In this way, observables which are inclusive over the additional radiation are correct to NNLO, while the description of 0-jet-like resummation variables is improved beyond the parton shower approximation. We provide predictions for the 13 TeV LHC.

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arXiv2019

Top-quark pair production at the LHC: Fully differential QCD predictions at NNLO

We report on a new fully differential calculation of the next-to-next-to-leading-order (NNLO) QCD radiative corrections to the production of top-quark pairs at hadron colliders. The calculation is performed by using the $q_T$ subtraction formalism to handle and cancel infrared singularities in real and virtual contributions. The computation is implemented in the Matrix framework, thereby allowing us to efficiently compute arbitrary infrared-safe observables for stable top quarks. We present NNLO predictions for several single- and double-differential kinematical distributions in $pp$ collisions at the centre-of-mass energy $\sqrt{s}=13$ TeV, and we compare them with recent LHC data by the CMS collaboration.

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arXiv2019

Top-quark pair hadroproduction at next-to-next-to-leading order in QCD

We report on a new calculation of the next-to-next-to-leading order (NNLO) QCD radiative corrections to the inclusive production of top-quark pairs at hadron colliders. The calculation is performed by using the $q_T$ subtraction formalism to handle and cancel infrared singular contributions at intermediate stages of the computation. We present numerical results for the total cross section in $pp$ collisions at $\sqrt{s}=8$ TeV and $13$ TeV, and we compare them with those obtained by using the publicly available numerical program Top++. Our computation represents the first complete application of the $q_T$ subtraction formalism to the hadroproduction of a colourful high-mass system at NNLO.

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arXiv2018

$ZZ$ production at the LHC: NNLO predictions for $2\ell2ν$ and $4\ell$ signatures

We consider QCD radiative corrections to $ZZ$ production for all experimentally relevant leptonic processes. We report on a novel computation of next-to-next-to-leading-order (NNLO) corrections to the diboson signature with two charged leptons and missing transverse energy ($\ell\ell$+$E_T^{\rm miss}$). All relevant final states are considered: $\ell\ellν_\ellν_\ell$, $\ell\ellν_{\ell'}ν_{\ell'}$ and $\ellν_{\ell}\ell'ν_{\ell'}$. We also study processes with four charged leptons: $\ell\ell\ell\ell$ and $\ell\ell\ell'\ell'$. For the first time NNLO accuracy is achieved for a process mixing two double-resonant diboson topologies ($ZZ/W^+W^-\to\ell\ellν_\ellν_\ell$). We find good agreement with ATLAS data at 8 TeV. NNLO corrections are large (5-20% and more), and interference effects between $ZZ$ and $W^+W^-$ resonances turn out to be negligible in most cases.

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arXiv2018

Higgs boson pair production at NNLO with top quark mass effects

We consider QCD radiative corrections to Higgs boson pair production through gluon fusion in proton collisions. We combine the exact next-to-leading order (NLO) contribution, which features two-loop virtual amplitudes with the full dependence on the top quark mass $M_t$, with the next-to-next-to-leading order (NNLO) corrections computed in the large-$M_t$ approximation. The latter are improved with different reweighting techniques in order to account for finite-$M_t$ effects beyond NLO. Our reference NNLO result is obtained by combining one-loop double-real corrections with full $M_t$ dependence with suitably reweighted real--virtual and double-virtual contributions evaluated in the large-$M_t$ approximation. We present predictions for inclusive cross sections in $pp$ collisions at $\sqrt{s}$=13, 14, 27 and 100TeV and we discuss their uncertainties due to missing $M_t$ effects. Our approximated NNLO corrections increase the NLO result by an amount ranging from +12% at $\sqrt{s}$=13TeV to +7% at $\sqrt{s}$=100TeV, and the residual uncertainty from missing $M_t$ effects is estimated to be at the few percent level. Our calculation is fully differential in the Higgs boson pair and the associated jet activity: we also present predictions for various differential distributions at $\sqrt{s}$=14 and 100TeV. Our results represent the most advanced perturbative prediction available to date for this process.

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