The study of dileptons in high-energy heavy-ion collisions provides critical insights into the properties of the quark-gluon plasma and the thermal radiation emitted throughout its evolution. In the low-mass region, dileptons originate from both direct photon conversion and hadronic decays, with the Kroll-Wada equation traditionally used to relate direct real and direct virtual photon production. In this work, we explore the possibility of using parton shower event generators to model this conversion process, leveraging their unitary treatment of internal photon conversions that naturally preserves normalisation, as well as their ability to incorporate higher-order corrections, recoil kinematics, and realistic experimental selection criteria. We compare the Kroll-Wada approach to simulations using the Pythia8 simple shower, the Vincia sector shower, and the POWHEG shower matched NLO event generator. Our results reveal that the parton shower approach offers improved accuracy in describing the dilepton spectrum, particularly towards larger invariant masses where phase-space suppression effects become relevant.

Germany

Tomáš Ježo

Michael Klasen

Italy

Alexander Neuwirth

Conversion of photons to dileptons in the Kroll-Wada and parton shower approaches

Prompt photon production is highly sensitive to the distribution of quarks and gluons in free protons and nuclei and an important baseline for phenomenological studies of the properties of the quark-gluon plasma. In this paper, we present a new calculation of the production of prompt photons in association with two jets at next-to-leading order in quantum chromodynamics matched to parton showers with the POWHEG method. This calculation extends our previous analysis of prompt photon production in association with one jet using POWHEG+PYTHIA. We investigate the role of the parton shower as an alternative description of the parton-to-photon fragmentation process and analyse correlations between the photon and the jets. In addition, we compare POWHEG+PYTHIA with POWHEG+HERWIG predictions, experimental ATLAS data for isolated photons, and a perturbative calculation at next-to-next-to-leading order. Both parton shower models bring the next-to-leading order prediction into good agreement with the experimental data and the next-to-next-to-leading order calculation.

Prompt photon production with two jets in POWHEG

The production of virtual photons is a very sensitive probe of the properties of the quark-gluon plasma. As they are experimentally detected by lepton pairs, they suffer from a large background arising from hadron decays. Light-flavour hadrons dominate at low invariant masses below m  ee  ~ 0.5 GeV and heavy flavours above. These contributions must therefore also be taken into account in experimental analyses at the LHC. In this paper, we calculate the direct contribution from virtual photons produced in the Drell-Yan process with an additional jet in POWHEG and find that it is significant at low invariant masses. We also simulate the background contributions from   <math> <mi>c</mi> <mover> <mi>c</mi> <mo>¯</mo> </mover> </math>  $$ c\overline{c} $$  and   <math> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ b\overline{b} $$  production with POWHEG and quantify the theoretical uncertainties due to variations of the perturbative scales and parton distribution functions. We find larger relative and absolute uncertainties for the lighter c quarks than for heavier b quarks.

Anton Andronic

Christian Klein-Bösing

Di-electron production at the LHC: unravelling virtual-photon and heavy-flavour contributions

The increasingly high accuracy of top-quark studies at the LHC calls for a theoretical description of $$ t\overline{t} $$ production and decay in terms of exact matrix elements for the full 2 → 6 process that includes the off-shell production and the chain decays of $$ t\overline{t} $$ and $\textit{tW}$ intermediate states, together with their quantum interference. Corresponding NLO QCD calculations matched to parton showers are available for the case of dileptonic channels and are implemented in the bb4l Monte Carlo generator, which is based on the resonance-aware POWHEG method. In this paper, we present the first NLOPS predictions of this kind for the case of semileptonic channels. In this context, the interplay of off-shell $$ t\overline{t} $$ + $\textit{tW}$ production with various other QCD and electroweak subprocesses that yield the same semileptonic final state is discussed in detail. On the technical side, we improve the resonance-aware POWHEG procedure by means of new resonance histories based on matrix elements, which enable a realistic separation of $$ t\overline{t} $$ and $\textit{tW}$ contributions. Moreover, we introduce a general approach which makes it possible to avoid certain spurious terms that arise from the perturbative expansion of decay widths in any off-shell higher-order calculation, and which are large enough to jeopardise physical finite-width effects. These methods are implemented in a new version of the bb4l Monte Carlo generator, which is applicable to all dileptonic and semileptonic channels, and can be extended to fully hadronic channels. The presented results include a NLOPS comparison of off-shell against on-shell $$ t\overline{t} $$ + $\textit{tW}$ production and decay, where we highlight various non-trivial aspects related to NLO and parton-shower radiation in leptonic and hadronic top decays.

United Kingdom

Jonas Lindert

Switzerland

Stefano Pozzorini

Resonance-aware NLOPS matching for off-shell $$ t\overline{t} $$ + $\textit{tW}$ production with semileptonic decays

We present a new Monte Carlo event generator for the hadronic production of four top quarks in the powheg box framework. Besides the dominant next-to-leading order QCD corrections at <math><mi>O</mi><mo>(</mo><msubsup><mi>α</mi><mi>s</mi><mn>5</mn></msubsup><mo>)</mo></math> we also include all subleading electroweak productions channels at leading-order accuracy. We validate our theoretical predictions by comparing to parton-shower matched predictions obtained within the mc@nlo framework for stable top quarks. Furthermore, we investigate in detail the various sources of theoretical uncertainties. Finally, we investigate a single lepton plus jets signature to study for the first time the impact of the electroweak production modes as well as spin-correlation effects at the fiducial level.

United States

Manfred Kraus

Hadroproduction of four top quarks in the powheg box

We present a new calculation of next-to-leading order QCD corrections to electroweak top-quark pair hadroproduction in extensions of the Standard Model (SM) with extra heavy neutral and charged spin-1 resonances which considerably extends and improves an earlier calculation performed by some of us. In particular, we allow for flavor-non-diagonal <math><msup><mi>Z</mi><mo>′</mo></msup></math> couplings and take into account nonresonant production in the SM and beyond including the contributions with <math><mi>t</mi></math>-channel <math><mi>W</mi></math> and <math><msup><mi>W</mi><mo>′</mo></msup></math> bosons. As a result, models with a more complicated flavor structure which have been proposed to explain the flavor anomalies in <math><mi>B</mi></math> decays can now be accommodated in our code. Moreover, the new <math><mi>t</mi></math>-channel contributions lead to improved cross sections predictions at higher energies beyond the LHC reach. All amplitudes are generated using the recola2 package. As in our previous work, we include next-to-leading order QCD corrections and consistently match to parton showers with the POWHEG method fully taking into account the interference effects between the SM and new physics amplitudes. We consider the sequential Standard Model, the topcolor model, as well as the third family hypercharge model featuring non-flavor-diagonal <math><msup><mi>Z</mi><mo>′</mo></msup></math> couplings and present numerical results for <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math> cross sections at hadron colliders with a center-of-mass energy up to 100 TeV.

France

Mohammad Mahdi Altakach

Jean-Nicolas Lang

Ingo Schienbein

Electroweak <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math> hadroproduction in the presence of heavy <math><msup><mi>Z</mi><mo>′</mo></msup></math> and <math><msup><mi>W</mi><mo>′</mo></msup></math> bosons at NLO QCD in POWHEG

This paper is a follow-up of Ref. Ferrario Ravasio et al. (Eur Phys J C 78:458, 2018. arXiv:1801.03944), where we studied the impact of next-to-leading order calculations merged with parton shower generators (NLO+PS) of increasing accuracy in the extraction of the top mass at hadron colliders. Here we examined results obtained with the older (fortran-based) shower generators Pythia6.4 and Herwig6.5. Our findings are in line with what we found in Ref. Ferrario Ravasio et al. (2018) with the new, c++-based, generators Pythia8.2 and Herwig7.1.

Silvia Ferrario Ravasio

Paolo Nason

Carlo Oleari

Addendum to: A theoretical study of top-mass measurements at the LHC using NLO+PS generators of increasing accuracy

In the context of the standard model of particle physics, the relationship between the top-quark mass and width (<math><mrow><msub><mrow><mi>Γ</mi></mrow><mrow><mi>t</mi></mrow></msub></mrow></math>) has been precisely calculated. However, the uncertainty from current direct measurements of the width is nearly 50%. A new approach for directly measuring the top-quark width using events away from the resonance peak is presented. By using an orthogonal dataset to traditional top-quark width extractions, this new method may enable significant improvements in the experimental sensitivity in a method combination. Recasting a recent ATLAS differential cross section measurement, we find <math><msub><mi>Γ</mi><mi>t</mi></msub><mo>=</mo><mn>1.28</mn><mo>±</mo><mn>0.30</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> (<math><mn>1.33</mn><mo>±</mo><mn>0.29</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> expected), providing the most precise direct measurement of the width.

Christian Herwig

Benjamin Nachman

Extracting the Top-Quark Width from Nonresonant Production

In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-to-leading-order (NLO) generators interfaced to parton showers (PS) that have different levels of accuracy. Specifically we consider three generators: one that implements NLO corrections in the production dynamics, one that includes also NLO corrections in top decay in the narrow width approximation, and one that implements NLO corrections for both production and decay including finite-width and interference effects. Since our aim is to provide an assessment of the uncertainties of purely theoretical origin, we consider simplified top-mass related observables that are broadly related to those effectively used by experiments, eventually modelling experimental resolution effects with simple smearing procedures. We estimate the differences in the value of the extracted top mass that would occur due to the use of the three different NLO generators, to the variation of scales, to the choice of parton distribution functions and to the matching procedure. Furthermore, we also consider differences due to the shower and to the modelling of non-perturbative effects by interfacing our NLO generators to both Pythia8.2 and Herwig7.1, with various settings. We find very different results depending upon the adopted shower model. While with Pythia8.2 we find moderate differences between the different NLO+PS generators, with Herwig7.1 we find very large ones. Furthermore, the differences between Pythia8.2 and Herwig7.1 generators are also remarkably large.

CERN

A theoretical study of top-mass measurements at the LHC using NLO+PS generators of increasing accuracy

Measurements of $$t \bar{t} H$$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mi>H</mi></mrow></math>  production in the $$H\rightarrow b \bar{b} $$ <math><mrow><mi>H</mi><mo>→</mo><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  channel depend in a critical way on the theoretical uncertainty associated with the irreducible $$t \bar{t} +b$$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mo>+</mo><mi>b</mi></mrow></math> -jet background. In this paper, analysing the various topologies that account for b-jet production in association with a $$t \bar{t} $$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  pair, we demonstrate that the process at hand is largely driven by final-state $$g\rightarrow b \bar{b} $$ <math><mrow><mi>g</mi><mo>→</mo><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  splittings. We also show that in five-flavour simulations based on $$t \bar{t} +$$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mo>+</mo></mrow></math> multi-jet merging, b-jet production is mostly driven by the parton shower, while matrix elements play only a marginal role in the description of $$g\rightarrow b \bar{b} $$ <math><mrow><mi>g</mi><mo>→</mo><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  splittings. Based on these observations we advocate the use of NLOPS simulations of $$pp\rightarrow t \bar{t} b\bar{b} $$ <math><mrow><mi>p</mi><mi>p</mi><mo>→</mo><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for $$t \bar{t} +b$$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mo>+</mo><mi>b</mi></mrow></math> -jet observables at the 13 TeV LHC are presented both for the case of stable top quarks and with spin-correlated top decays. Besides QCD scale variations we consider also theoretical uncertainties related to the Powheg matching method and to the parton shower modelling, with emphasis on $$g\rightarrow b \bar{b} $$ <math><mrow><mi>g</mi><mo>→</mo><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.

Niccolo Moretti

New NLOPS predictions for $$\varvec{t \bar{t} +b}$$ <math><mrow><mi>t</mi><mover><mrow><mi>t</mi></mrow><mrow><mo>¯</mo></mrow></mover><mo>+</mo><mi>b</mi></mrow></math> -jet production at the LHC