Top Quark Pair Production Research Articles

Constraining the top-quark mass within the global MSHT PDF fit

We examine the ability of experimental measurements of top-quark pair production to constrain both the top-quark mass and the strong coupling within the global MSHT parton distribution function (PDF) fit. Specifically, we consider ATLAS and CMS measurements of differential distributions taken at a centre-of-mass energy of 8 TeV, as well as tbar{t} total cross section data taken at a variety of experiments, and compare to theoretical predictions including next-to-next-to-leading order corrections. We find that supplementing the global fit with this additional information results in relatively strong constraints on the top-quark mass, and is also able to bound the strong coupling in a limited fashion. Our final result is m_t=173.0pm 0.6~textrm{GeV} and is compatible with the world average pole mass extracted from cross section measurements of 172.5pm 0.7~textrm{GeV} by the Particle Data Group. We also study the effect of different top-quark masses on the gluon parton distribution function, finding changes at high x which nonetheless lie within the large PDF uncertainties in this region.

Search for a light charged Higgs boson in t → H±b decays, with H± → cb, in the lepton+jets final state in proton-proton collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV with the ATLAS detector

A search for a charged Higgs boson, H±, produced in top-quark decays, t → H±b, is presented. The search targets H± decays into a bottom and a charm quark, H± → cb. The analysis focuses on a selection enriched in top-quark pair production, where one top quark decays into a leptonically decaying W boson and a bottom quark, and the other top quark decays into a charged Higgs boson and a bottom quark. This topology leads to a lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of jets containing b-hadrons, and deploys a neural network classifier that uses the kinematic differences between the signal and the background. The search uses a dataset of proton-proton collisions collected at a centre-of-mass energy s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV between 2015 and 2018 with the ATLAS detector at CERN’s Large Hadron Collider, amounting to an integrated luminosity of 139 fb−1. Observed (expected) 95% confidence-level upper limits between 0.15% (0.09%) and 0.42% (0.25%) are derived for the product of branching fractions B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(t → H±b) × B(H± → cb) for charged Higgs boson masses between 60 and 160 GeV, assuming the SM production of the top-quark pairs.

First measurement of the top quark pair production cross section in proton-proton collisions at sqrt{s} = 13.6 TeV

The first measurement of the top quark pair ( textrm{t}overline{textrm{t}} ) production cross section in proton-proton collisions at sqrt{s} = 13.6 TeV is presented. Data recorded with the CMS detector at the CERN LHC in Summer 2022, corresponding to an integrated luminosity of 1.21 fb−1, are analyzed. Events are selected with one or two charged leptons (electrons or muons) and additional jets. A maximum likelihood fit is performed in event categories defined by the number and flavors of the leptons, the number of jets, and the number of jets identified as originating from b quarks. An inclusive textrm{t}overline{textrm{t}} production cross section of 881 ± 23 (stat + syst) ± 20 (lumi) pb is measured, in agreement with the standard model prediction of {924}_{-40}^{+32} pb.

Associated production of a top-quark pair with two isolated photons at the LHC through NLO in QCD

We report on the computation of NLO QCD corrections to top-quark pair production in association with two photons at the LHC. Higher-order effects and photon bremsstrahlung are taken into account in the production and decays of the top-quark pair. Top-quark and W-boson decays are treated in the Narrow Width Approximation conserving spin correlations up to NLO in QCD. This is the first time that the complete set of NLO QCD corrections to the ppto toverline{t}gamma gamma process including top-quark decays is calculated. We present results at the integrated and differential cross-section level in the di-lepton and lepton + jet channel. In addition, we investigate the effect of photon bremsstrahlung in toverline{t} production and top-quark decays, as well as the mixed contribution. The latter contribution, in which two photons occur simultaneously in the production and decay of the toverline{t} pair, proved to be significant at both the integrated and differential cross-section level.

Combination of inclusive top-quark pair production cross-section measurements using ATLAS and CMS data at sqrt{s} = 7 and 8 TeV

A combination of measurements of the inclusive top-quark pair production cross-section performed by ATLAS and CMS in proton–proton collisions at centre-of-mass energies of 7 and 8 TeV at the LHC is presented. The cross-sections are obtained using top-quark pair decays with an opposite-charge electron–muon pair in the final state and with data corresponding to an integrated luminosity of about 5 fb−1 at sqrt{s} = 7 TeV and about 20 fb−1 at sqrt{s} = 8 TeV for each experiment. The combined cross-sections are determined to be 178.5 ± 4.7 pb at sqrt{s} = 7 TeV and {243.3}_{-5.9}^{+6.0} pb at sqrt{s} = 8 TeV with a correlation of 0.41, using a reference top-quark mass value of 172.5 GeV. The ratio of the combined cross-sections is determined to be R8/7 = 1.363 ± 0.032. The combined measured cross-sections and their ratio agree well with theory calculations using several parton distribution function (PDF) sets. The values of the top-quark pole mass (with the strong coupling fixed at 0.118) and the strong coupling (with the top-quark pole mass fixed at 172.5 GeV) are extracted from the combined results by fitting a next-to-next-to-leading-order plus next-to-next-to-leading-log QCD prediction to the measurements. Using a version of the NNPDF3.1 PDF set containing no top-quark measurements, the results obtained are {m}_t^{textrm{pole}}={173.4}_{-2.0}^{+1.8} GeV and {alpha}_{textrm{s}}left({m}_Zright)={0.1170}_{-0.0018}^{+0.0021} .

Search for a new scalar resonance in flavour-changing neutral-current top-quark decays t → qX (q = u, c), with X → boverline{b} , in proton-proton collisions at sqrt{s} = 13 TeV with the ATLAS detector

A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle X decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to qX, with X → boverline{b} , and the other top quark decays according to the Standard Model, with the W boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from b-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb−1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction mathcal{B} (t → uX) and between 0.018% and 0.078% for the branching fraction mathcal{B} (t → cX), for masses of the scalar particle X between 20 and 160 GeV.

Inclusive and differential cross-sections for dilepton toverline{t} production measured in sqrt{s} = 13 TeV pp collisions with the ATLAS detector

Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs ( toverline{t} ) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of sqrt{s} = 13 TeV and correspond to an integrated luminosity of 140 fb−1. The measurements use events containing an oppositely charged eμ pair and b-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton pT distributions is obtained by reweighting the toverline{t} sample so as to reproduce the top-quark pT distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to beσtt¯=829±1stat±13syst±8lumi±2beampb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{t\\overline{t}}=829\\pm 1\\ \\left(\ extrm{stat}\\right)\\pm 13\\ \\left(\ extrm{syst}\\right)\\pm 8\\ \\left(\ extrm{lumi}\\right)\\pm 2\\ \\left(\ extrm{beam}\\right)\\ \ extrm{pb}, $$\\end{document}where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.

Search for CP violating top quark couplings in pp collisions at sqrt{s} = 13 TeV

Results are presented from a search for CP violation in top quark pair production, using proton-proton collisions at a center-of-mass energy of 13 TeV. The data used for this analysis consist of final states with two charged leptons collected by the CMS experiment, and correspond to an integrated luminosity of 35.9 fb−1. The search uses two observables, mathcal{O} 1 and mathcal{O} 3, which are Lorentz scalars. The observable mathcal{O} 1 is constructed from the four-momenta of the charged leptons and the reconstructed top quarks, while mathcal{O} 3 consists of the four-momenta of the charged leptons and the b quarks originating from the top quarks. Asymmetries in these observables are sensitive to CP violation, and their measurement is used to determine the chromoelectric dipole moment of the top quark. The results are consistent with the expectation from the standard model.

Measurement of the differential hbox {t}overline{hbox {t}} production cross section as a function of the jet mass and extraction of the top quark mass in hadronic decays of boosted top quarks

A measurement of the jet mass distribution in hadronic decays of Lorentz-boosted top quarks is presented. The measurement is performed in the lepton + jets channel of top quark pair production (hbox {t}overline{hbox {t}}) events, where the lepton is an electron or muon. The products of the hadronic top quark decay are reconstructed using a single large-radius jet with transverse momentum greater than 400,text {Ge}hspace{-.08em}text {V}. The data were collected with the CMS detector at the LHC in proton-proton collisions and correspond to an integrated luminosity of 138,text {fb}^{-1}. The differential hbox {t}overline{hbox {t}} production cross section as a function of the jet mass is unfolded to the particle level and is used to extract the top quark mass. The jet mass scale is calibrated using the hadronic W boson decay within the large-radius jet. The uncertainties in the modelling of the final state radiation are reduced by studying angular correlations in the jet substructure. These developments lead to a significant increase in precision, and a top quark mass of 173.06 pm 0.84,text {Ge}hspace{-.08em}text {V} .

Electroweak radiative corrections to polarized top quark pair production

Electroweak radiative corrections to polarized top quark pair production

Measurement of single top-quark production in the s-channel in proton–proton collisions at sqrt{s} = 13 TeV with the ATLAS detector

A measurement of single top-quark production in the s-channel is performed in proton–proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb−1. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two b-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is sigma ={8.2}_{-2.9}^{+3.5} pb, consistent with the Standard Model prediction of {sigma}^{textrm{SM}}={10.32}_{-0.36}^{+0.40} pb.

Search for flavour-changing neutral current interactions of the top quark and the Higgs boson in events with a pair of τ-leptons in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

A search for flavour-changing neutral current (FCNC) tqH interactions involving a top quark, another up-type quark (q = u, c), and a Standard Model (SM) Higgs boson decaying into a τ-lepton pair (H → τ+τ−) is presented. The search is based on a dataset of pp collisions at sqrt{s} = 13 TeV that corresponds to an integrated luminosity of 139 fb−1 recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson (pp → tH), and top quark pair production in which one of top quarks decays into Wb and the other decays into qH through the FCNC interactions. The search selects events with two hadronically decaying τ-lepton candidates (τhad) or at least one τhad with an additional lepton (e, μ), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3σ above the expected SM background, and 95% CL upper limits on the t → qH branching ratios are derived. The observed (expected) 95% CL upper limits set on the t → cH and t → uH branching ratios are 9.4times {10}^{-4}left({4.8}_{-1.4}^{+2.2}times {10}^{-4}right) and 6.9times {10}^{-4}left({3.5}_{-1.0}^{+1.5}times {10}^{-4}right) , respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective tqH couplings are Ccϕ< 1.35 (0.97) and Cuϕ< 1.16 (0.82).

Measurement of the toverline{t} production cross-section in pp collisions at sqrt{s} = 5.02 TeV with the ATLAS detector

The inclusive top-quark pair ( toverline{t} ) production cross-section {sigma}_{toverline{t}} is measured in proton–proton collisions at a centre-of-mass energy sqrt{s} = 5.02 TeV, using 257 pb−1 of data collected in 2017 by the ATLAS experiment at the LHC. The toverline{t} cross-section is measured in both the dilepton and single-lepton final states of the toverline{t} system and then combined. The combination of the two measurements yieldsσtt¯=67.5±0.9stat.±2.3syst.±1.1lumi.±0.2beampb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{t\\overline{t}}=67.5\\pm 0.9\\left(\ extrm{stat}.\\right)\\pm 2.3\\left(\ extrm{syst}.\\right)\\pm 1.1\\left(\ extrm{lumi}.\\right)\\pm 0.2\\left(\ extrm{beam}\\right)\ extrm{pb}, $$\\end{document}where the four uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, and imperfect knowledge of both the integrated luminosity and the LHC beam energy, giving a total uncertainty of 3.9%. The result is in agreement with theoretical quantum chromodynamic calculations at next-to-next-to-leading order in the strong coupling constant, including the resummation of next-to-next-to-leading logarithmic soft-gluon terms, and constrains the parton distribution functions of the proton at large Bjorken-x.

Measurement of the charge asymmetry in top-quark pair production in association with a photon with the ATLAS experiment

A measurement of the charge asymmetry in top-quark pair (tt¯) production in association with a photon is presented. The measurement is performed in the single-lepton tt¯ decay channel using proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 2015–2018, corresponding to an integrated luminosity of 139 fb−1. The charge asymmetry is obtained from the distribution of the difference of the absolute rapidities of the top quark and antiquark using a profile likelihood unfolding approach. It is measured to be AC=−0.003±0.029 in agreement with the Standard Model expectation.

Study of additional jet activity in top quark pair production and decay at the LHC

We report on the calculation of the NLO QCD corrections to top quark pair production in association with two hard jets at the LHC. We take into account higher-order effects in both the production and decays of the top-quark pair. The latter are treated in the narrow width approximation and $t\bar{t}$ spin correlations are preserved throughout the calculation. This is the first time that such a complete study for this process is conducted at NLO in QCD. We present results for fiducial cross sections at the integrated and differential level. Furthermore, we investigate kinematic properties of the additional light jets and their distribution in the $pp\to t\bar{t}jj$ process. We examine jets in production and top-quark decays as well as the mixed contribution, where the two hardest non-$b$ jets are present simultaneously in the production and decay processes.

Measurements of observables sensitive to colour reconnection in t{bar{t}} events with the ATLAS detector at sqrt{s} = 13 TeV

A measurement of observables sensitive to effects of colour reconnection in top-quark pair-production events is presented using 139 hbox {fb}^{-1} of 13 TeV proton–proton collision data collected by the ATLAS detector at the LHC. Events are selected by requiring exactly one isolated electron and one isolated muon with opposite charge and two or three jets, where exactly two jets are required to be b-tagged. For the selected events, measurements are presented for the charged-particle multiplicity, the scalar sum of the transverse momenta of the charged particles, and the same scalar sum in bins of charged-particle multiplicity. These observables are unfolded to the stable-particle level, thereby correcting for migration effects due to finite detector resolution, acceptance and efficiency effects. The particle-level measurements are compared with different colour reconnection models in Monte Carlo generators. These measurements disfavour some of the colour reconnection models and provide inputs to future optimisation of the parameters in Monte Carlo generators.

Top and bottom quark forward-backward asymmetries at next-to-next-to-leading order QCD in (un)polarized electron positron collisions

We consider, at order {alpha}_s^2 in the QCD coupling, top-quark pair production in the continuum at various center-of-mass energies and b-quark pair production at the Z resonance by (un)polarized electron and positron beams. For top quarks we compute the forward-backward asymmetry with respect to the top-quark direction of flight, the associated polar angle distribution, and we analyze the effect of beam polarization on the QCD corrections to the leading-order asymmetry. We calculate also the polarized forward-backward asymmetry. For b-quark production at the Z peak we explore different definitions of AFB. In particular, we analyze b jets defined by the Durham and the flavor-kT clustering algorithms. We compute the inclusive b-jet and two-jet asymmetry with respect to the b-jet direction. For the latter asymmetry the QCD corrections to order {alpha}_s^2 are small. That predestines it to act as a precision observable.

NLO QCD corrections to off-shell top–antitop production with semi-leptonic decays at lepton colliders

The study of top-quark properties will be a central aspect of the physics programme of any future lepton collider. In this article, we investigate the production of top-quark pairs in the semi-leptonic decay channel in {textrm{e}}^{+}textrm{e}^{-} collisions, whose experimental signature is one charged lepton, jets, and missing energy. We present for the first time fiducial cross sections and differential distributions at next-to-leading-order accuracy in QCD for the full off-shell process. We find that the QCD corrections for the considered process are strongly dependent on the beam energies and range from few per cent up to more than 100% (near threshold and above 1,text {TeV} ). We focus, in particular, on two scenarios: one close to threshold (365,text {GeV} ), dominated by top-pair production, and one at the TeV scale (1.5,text {TeV} ), for which irreducible-background contributions become relevant. An assessment of polarised-beam effects is also provided.

Simultaneous CTEQ-TEA extraction of PDFs and SMEFT parameters from jet and toverline{t} data

Recasting phenomenological Lagrangians in terms of SM effective field theory (SMEFT) provides a valuable means of connecting potential BSM physics at momenta well above the electroweak scale to experimental signatures at lower energies. In this work we jointly fit the Wilson coefficients of SMEFT operators as well as the PDFs in an extension of the CT18 global analysis framework, obtaining self-consistent constraints to possible BSM physics effects. Global fits are boosted with machine-learning techniques in the form of neural networks to ensure efficient scans of the full PDF+SMEFT parameter space. We focus on several operators relevant for top-quark pair and jet production at hadron colliders and obtain constraints on the Wilson coefficients with Lagrange Multiplier scans. We find mild correlations between the extracted Wilson coefficients, PDFs, and other QCD parameters, and see indications that these correlations may become more prominent in future analyses based on data of higher precision. This work serves as a new platform for joint analyses of SM and BSM physics based on the CTEQ-TEA framework.

Determination of the top-quark mass from top-quark pair events with the matrix element method at next-to-leading order: Potential and prospects

In 2004 the matrix element method was used in a pioneering work by the Tevatron experiment D0 to determine the top-quark mass from a handful of events. Since then the method has been matured into a powerful analysis tool. While the first applications were restricted to leading-order accuracy, in the meantime also the extension to next-to-leading order (NLO) accuracy has been studied. In this article we explore the potential of the matrix element method at NLO to determine the top-quark mass using events with pair-produced top quarks. We simulate a toy experiment by generating unweighted events with a fixed input mass and apply the matrix element method to construct an estimator for the top-quark mass. Two different setups are investigated: unweighted events obtained from the fixed-order cross section at NLO accuracy as well as events obtained using POWHEG matched to a parton shower. The latter lead to a more realistic simulation and allow to study the impact of higher-order corrections as well as the robustness of the approach. We find that the matrix element method in NLO accuracy leads to a significant reduction of the theoretical uncertainties compared to leading order. In view of the high luminosity phase of the LHC, this observation is especially relevant in analyses which are no longer dominated by statistical uncertainties.