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Stefano Pozzorini

Publications and source records attributed to Stefano Pozzorini.

At least 37 records · Page 2Linked to original sources
arXiv2016

NLO QCD Predictions for off-shell $t \bar t$ and $t \bar t H$ Production and Decay at a Linear Collider

We present predictions for $t \bar t$ and $t \bar t H$ production and decay at future lepton colliders including non-resonant and interference contributions up to next-to-leading order (NLO) in perturbative QCD. The obtained precision predictions are necessary for a future precise determination of the top-quark Yukawa coupling, and allow for top-quark phenomenology in the continuum at an unprecedented level of accuracy. Simulations are performed with the automated NLO Monte-Carlo framework WHIZARD interfaced to the OpenLoops matrix element generator.

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arXiv2016

NLO QCD+EW for V+jets

In this contribution recent results regarding the NLO electroweak corrections for vector boson production in association with jets are presented. Besides discussing the phenomenology of the fixed-order results, their corporation in existing NLO QCD parton shower matched and merged calculations, which can directly be used in experimental analyses, will be shown.

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arXiv2016

Next-to-leading order QCD predictions for top-quark pair production with up to three jets

We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with Sherpa and OpenLoops. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale HT/2 at fixed order and the MINLO procedure. Analyzing various cross sections and distributions for tt+0,1,2,3 jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and MINLO results. The differences are typically below the respective factor-two scale variations, suggesting that for all considered jet multiplicities, missing higher-order effects should not exceed the ten percent level.

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arXiv2016

An NLO+PS generator for $t\bar{t}$ and $Wt$ production and decay including non-resonant and interference effects

We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+ν_{\scriptscriptstyle\ell}\, l^-\barν_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and $l$ belong to different families, and $b$ quarks are massive. Thus, the contributions from $t\bar{t}$ and $Wt$ single-top production as well as their quantum interference are fully included. Matrix elements are matched to the Pythia8 parton shower using a recently proposed method that allows for a consistent treatment of resonances in the POWHEG framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the $Wt$ channel, and for $t\bar{t}$ and $Wt$ backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects. The new generator is based on a process-independent interface of the OpenLoops amplitude generator with the POWHEGBOX framework.

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arXiv2016

$W^+W^-$ production at the LHC: fiducial cross sections and distributions in NNLO QCD

We consider QCD radiative corrections to $W^+W^-$ production at the LHC and present the first fully differential predictions for this process at next-to-next-to-leading order (NNLO) in perturbation theory. Our computation consistently includes the leptonic decays of the $W$ bosons, taking into account spin correlations, off-shell effects and non-resonant contributions. Detailed predictions are presented for the different-flavour channel $pp\toμ^+e^-ν_μ{\bar ν}_e+X$ at $\sqrt{s}=8$ and $13$ TeV. In particular, we discuss fiducial cross sections and distributions in the presence of standard selection cuts used in experimental $W^+W^-$ and $H\to W^+W^-$ analyses at the LHC. The inclusive $W^+W^-$ cross section receives large NNLO corrections, and, due to the presence of a jet veto, typical fiducial cuts have a sizeable influence on the behaviour of the perturbative expansion. The availability of differential NNLO predictions, both for inclusive and fiducial observables, will play an important role in the rich physics programme that is based on precision studies of $W^+W^-$ signatures at the LHC.

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arXiv2016

NLO QCD+EW predictions for V+jets including off-shell vector-boson decays and multijet merging

We present next-to-leading order (NLO) predictions including QCD and electroweak (EW) corrections for the production and decay of off-shell electroweak vector bosons in association with up to two jets at the 13 TeV LHC. All possible dilepton final states with zero, one or two charged leptons that can arise from off-shell W and Z bosons or photons are considered. All predictions are obtained using the automated implementation of NLO QCD+EW corrections in the OpenLoops matrix-element generator combined with the Munich and Sherpa Monte Carlo frameworks. Electroweak corrections play an especially important role in the context of BSM searches, due to the presence of large EW Sudakov logarithms at the TeV scale. In this kinematic regime, important observables such as the jet transverse momentum or the total transverse energy are strongly sensitive to multijet emissions. As a result, fixed-order NLO QCD+EW predictions are plagued by huge QCD corrections and poor theoretical precision. To remedy this problem we present an approximate method that allows for a simple and reliable implementation of NLO EW corrections in the MEPS@NLO multijet merging framework. Using this general approach we present an inclusive simulation of vector-boson production in association with jets that guarantees NLO QCD+EW accuracy in all phase space regions involving up to two resolved jets.

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arXiv2015

Measuring the signal strength in ttH with H->bb

A precise measurement of the Higgs boson couplings to bottom and top quarks is of paramount importance during the upcoming LHC runs. We present a comprehensive analysis for the Higgs production process in association with a semi-leptonically decaying top-quark pair and subsequent Higgs boson decay into bottom quarks. Due to the highly complex final state and large Standard Model backgrounds, measuring the signal strength in this process is known to be challenging. To maximise the sensitivity, we analyse different, statistically independent, phase space regions, where one or more of the heavy resonances are boosted. This allows us to employ jet substructure techniques, which help to reduce large tt+X backgrounds. We find that combining several ttH(bb) phase space regions will allow one to measure deviations of the Standard Model signal strength of order 20% with 3 ab^{-1}.

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arXiv2015

Next-to-leading order QCD predictions for top-quark pair production with up to two jets merged with a parton shower

We present differential cross sections for the production of top-quark pairs in conjunction with up to two jets, computed at next-to leading order in perturbative QCD and consistently merged with a parton shower in the Sherpa+OpenLoops framework. Top quark decays including spin correlation effects are taken into account at leading order accuracy. The calculation yields a unified description of top-pair plus multi-jet production, and detailed results are presented for various key observables at the Large Hadron Collider. A large improvement with respect to the multi-jet merging approach at leading order is found for the total transverse energy spectrum, which plays a prominent role in searches for physics beyond the Standard Model.

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arXiv2015

NLO QCD+EW automation and precise predictions for V+multijet production

In this talk we present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in OpenLoops together with Sherpa and Munich. As a first application, we present NLO QCD+EW predictions for the production of positively charged W bosons in association with up to three jets and for the production of a Z boson or photon in association with one jet.

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arXiv2015

NLO electroweak automation and precise predictions for W+multijet production at the LHC

We present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in the OpenLoops matrix-element generator combined with the Sherpa and Munich Monte Carlo frameworks. The process-independent character of the implemented algorithms opens the door to NLO QCD+EW simulations for a vast range of Standard Model processes, up to high particle multiplicity, at current and future colliders. As a first application, we present NLO QCD+EW predictions for the production of positively charged on-shell W bosons in association with up to three jets at the Large Hadron Collider. At the TeV energy scale, due to the presence of large Sudakov logarithms, EW corrections reach the 20-40% level and play an important role for searches of physics beyond the Standard Model. The dependence of NLO EW effects on the jet multiplicity is investigated in detail, and we find that W+multijet final states feature genuinely different EW effects as compared to the case of W+1jet.

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arXiv2014

NNLO QCD subtraction for top-antitop production in the $q\bar{q}$ channel

We present the computation of the double real and real-virtual contributions to top-antitop pair production in the quark-antiquark channel at leading colour. The $q \bar q \to t \bar{t} g$ amplitudes contributing to the real-virtual part are computed with OpenLoops, and their numerical stability in the soft and collinear regions is found to be sufficiently high to perform a realistic NNLO calculation in double precision. The subtraction terms required at real-real and real-virtual levels are constructed within the antenna subtraction formalism extended to deal with the presence of coloured massive final state particles. We show that those subtraction terms approximate the real-real and real-virtual matrix elements in all their singular limits.

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arXiv2014

NLO matching for ttbb production with massive b-quarks

Theoretical uncertainties in the simulation of ttbb production represent one of the main obstacles that still hamper the observation of Higgs-boson production in association with top-quark pairs in the H->bb channel. In this letter we present a next-to-leading order (NLO) simulation of ttbb production with massive b-quarks matched to the Sherpa parton shower. This allows one to extend NLO predictions to arbitrary ttbb kinematics, including the case where one or both b-jets arise from collinear g->bb splittings. We find that this splitting mechanism plays an important role for the ttH(bb) analysis.

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arXiv2014

Precise Higgs-background predictions: merging NLO QCD and squared quark-loop corrections to four-lepton + 0,1 jet production

We present precise predictions for four-lepton plus jets production at the LHC obtained within the fully automated Sherpa+OpenLoops framework. Off-shell intermediate vector bosons and related interferences are consistently included using the complex-mass scheme. Four-lepton plus 0- and 1-jet final states are described at NLO accuracy, and the precision of the simulation is further increased by squared quark-loop NNLO contributions in the gg -> 4l, gg -> 4l+g, gq -> 4l+q, and qq -> 4l+g channels. These NLO and NNLO contributions are matched to the Sherpa parton shower, and the 0- and 1-jet final states are consistently merged using the MEPS@NLO technique. Thanks to Sudakov resummation, the parton shower provides improved predictions and uncertainty estimates for exclusive observables. This is important when jet vetoes or jet bins are used to separate four-lepton final states arising from Higgs decays, diboson production, and top-pair production. Detailed predictions are presented for the ATLAS and CMS H->WW analyses at 8 TeV in the 0- and 1-jet bins. Assessing renormalisation-, factorisation- and resummation-scale uncertainties, which reflect also unknown subleading Sudakov logarithms in jet bins, we find that residual perturbative uncertainties are as small as a few percent.

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arXiv2012

NLO QCD corrections to off-shell top-antitop production with leptonic decays at hadron colliders

We present details of a calculation of the cross section for hadronic top-antitop production in next-to-leading order (NLO) QCD, including the decays of the top and antitop into bottom quarks and leptons. This calculation is based on matrix elements for νe e+ μ- \barν_μb\bar{b} production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Such contributions are formally suppressed by the top-quark width and turn out to be small in the inclusive cross section. However, they can be strongly enhanced in exclusive observables that play an important role in Higgs and new-physics searches. Also non-resonant and off-shell effects due to the finite W-boson width are investigated in detail, but their impact is much smaller than naively expected. We also introduce a matching approach to improve NLO calculations involving intermediate unstable particles. Using a fixed QCD scale leads to perturbative instabilities in the high-energy tails of distributions, but an appropriate dynamical scale stabilises NLO predictions. Numerical results for the total cross section, several distributions, and asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.

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arXiv2012

NLO QCD corrections to off-shell ttbar production at hadron colliders

The production of top-antitop-quark pairs at hadron colliders is interesting both in its own right as signal process, but also as background to many searches for new physics. The corresponding predictions aim at the precision level of few per cent, rendering not only the inclusion of radiative corrections of the strong and electroweak interactions relevant, but also of off-shell and finite-width effects originating from the top-quark decays t -> b W -> b l ν_l / qq'. We report on a calculation for the full process pp -> W^+ W^- b \bar{b} -> ν_e e^+ μ^- ν_μb \bar{b} at next-to-leading order QCD and discuss the effects of the finite widths of the top quarks and of the W bosons for selected observables. Generically it turns out that finite-top-width effects are at the per-cent level whenever the top-quark resonances dominate, but those effects can reach tens of per cent in off-shell tails. Finite-W-width effects, on the other hand, are suppressed to less than 0.5% whenever the top quarks can become resonant and only become sizeable in exceptional cases. One such case, however, is the invariant mass of a bottom quark and the corresponding charged lepton, which result from the same top-quark decay - an observable that is relevant for precision measurements of the top-quark mass.

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arXiv2012

Scattering Amplitudes with Open Loops

We introduce a new technique to generate scattering amplitudes at one loop. Traditional tree algorithms, which handle diagrams with fixed momenta, are promoted to generators of loop-momentum polynomials that we call open loops. Combining open loops with tensor-integral and OPP reduction results in a fully flexible, very fast, and numerically stable one-loop generator. As demonstrated with non-trivial applications, the open-loop approach will permit to obtain precise predictions for a very wide range of collider processes.

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arXiv2010

Production of t anti-t b anti-b at the LHC at NLO QCD

We summarise predictions for top anti-top bottom anti-bottom (tTbB) production at the LHC in next-to-leading order QCD. The precise description of this background process is a prerequisite to observe associated top anti-top Higgs production in the H -> bottom anti-bottom decay channel. The one-loop amplitudes are computed using Feynman diagrams and numerical tensor reduction. This approach provides very high numerical stability and CPU efficiency. We find that the scale choice adopted in ATLAS simulations underestimates the tTbB background by a factor two and introduce a new dynamical scale that stabilises the perturbative predictions. In the regime of highly boosted Higgs bosons, which offers better perspectives to observe the tTH(H-> bB) signal, the corrections induce significant distortions in the kinematic distributions.

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