Top Quark Pairs In Association Research Articles

Learning to increase matching efficiency in identifying additional b-jets in the text {t}bar{text {t}}text {b}bar{text {b}} process

The text {t}bar{text {t}}text {H}(text {b}bar{text {b}}) process is an essential channel in revealing the Higgs boson properties; however, its final state has an irreducible background from the text {t}bar{text {t}}text {b}bar{text {b}} process, which produces a top quark pair in association with a b quark pair. Therefore, understanding the text {t}bar{text {t}}text {b}bar{text {b}} process is crucial for improving the sensitivity of a search for the text {t}bar{text {t}}text {H}(text {b}bar{text {b}}) process. To this end, when measuring the differential cross section of the text {t}bar{text {t}}text {b}bar{text {b}} process, we need to distinguish the b-jets originating from top quark decays and additional b-jets originating from gluon splitting. In this paper, we train deep neural networks that identify the additional b-jets in the {text {t}}{bar{text {t}}}{text {b}}{bar{text {b}}} events under the supervision of a simulated text{t}bar{text{t}}text{b}bar{text{b}} event data set in which true additional b-jets are indicated. By exploiting the special structure of the text {t}bar{text {t}}text {b}bar{text {b}} event data, several loss functions are proposed and minimized to directly increase matching efficiency, i.e., the accuracy of identifying additional b-jets. We show that, via a proof-of-concept experiment using synthetic data, our method can be more advantageous for improving matching efficiency than the deep learning-based binary classification approach presented in [1]. Based on simulated text {t}bar{text {t}}text {b}bar{text {b}} event data in the lepton+jets channel from pp collision at sqrt{s} = 13 TeV, we then verify that our method can identify additional b-jets more accurately: compared with the approach in [1], the matching efficiency improves from 62.1% to 64.5% and from 59.9% to 61.7% for the leading order and the next-to-leading order simulations, respectively.

Hadronic production of top-quark pairs in association with a pair of leptons in the powheg box framework

We present an implementation of $t\bar t \ell^+\ell^-$ ($\ell=e,\mu$) hadronic production at next-to-leading order in QCD matched to parton-shower event generators in the POWHEG BOX framework. The program we developed includes all leading-order contributions of order $\alpha_s^2\alpha^2$ for the specified final state, as well as the corresponding first-order QCD corrections. Decays of the top quarks have been simulated retaining spin-correlations in all tree-level matrix elements. We consider the case of the Large Hadron Collider at $\sqrt{s}=13$ TeV and compare results for $t\bar t \ell^+\ell^-$ production in the fiducial volume where the invariant mass of the lepton pairs is centered around the $Z$-boson mass to corresponding predictions for $t\bar{t}Z$ on-shell production with $Z\rightarrow \ell^+\ell^-$. We find that off-shell effects in $t\bar t \ell^+\ell^-$ are in general small at the level of the total cross section, but can decrease the tail of the leptons' transverse momentum distributions by 10-20% and, in these regions, they are visible beyond the scale uncertainty due to renormalization and factorization scale variation. Moreover, we find that accounting for top-quark decays in the narrow-width approximation with tree-level spin correlations also gives origin to 10-20% effects in specific regions of the kinematic distributions of the $t\bar t \ell^+\ell^-$ decayed final state.

Measurement of the inclusive and differential t overline{t} γ cross sections in the dilepton channel and effective field theory interpretation in proton-proton collisions at sqrt{s} = 13 TeV

The production cross section of a top quark pair in association with a photon is measured in proton-proton collisions in the decay channel with two oppositely charged leptons (e±μ∓, e+e−, or μ+μ−). The measurement is performed using 138 fb−1 of proton-proton collision data recorded by the CMS experiment at sqrt{s} = 13 TeV during the 2016–2018 data-taking period of the CERN LHC. A fiducial phase space is defined such that photons radiated by initial-state particles, top quarks, or any of their decay products are included. An inclusive cross section of 175.2 ± 2.5(stat) ± 6.3(syst) fb is measured in a signal region with at least one jet coming from the hadronization of a bottom quark and exactly one photon with transverse momentum above 20 GeV. Differential cross sections are measured as functions of several kinematic observables of the photon, leptons, and jets, and compared to standard model predictions. The measurements are also interpreted in the standard model effective field theory framework, and limits are found on the relevant Wilson coefficients from these results alone and in combination with a previous CMS measurement of the t overline{t} γ production process using the lepton+jets final state.

Measurement of the inclusive and differential t overline{t} \u03b3 cross sections in the single-lepton channel and EFT interpretation at sqrt{s} = 13 TeV

The production cross section of a top quark pair in association with a photon is measured in proton-proton collisions at a center-of-mass energy of 13 TeV. The data set, corresponding to an integrated luminosity of 137 fb−1, was recorded by the CMS experiment during the 2016–2018 data taking of the LHC. The measurements are performed in a fiducial volume defined at the particle level. Events with an isolated, highly energetic lepton, at least three jets from the hadronization of quarks, among which at least one is b tagged, and one isolated photon are selected. The inclusive fiducial t overline{mathrm{t}} γ cross section, for a photon with transverse momentum greater than 20 GeV and pseudorapidity |η| < 1.4442, is measured to be 798 ± 7(stat) ± 48(syst) fb, in good agreement with the prediction from the standard model at next-to-leading order in quantum chromodynamics. The differential cross sections are also measured as a function of several kinematic observables and interpreted in the framework of the standard model effective field theory (EFT), leading to the most stringent direct limits to date on anomalous electromagnetic dipole moment interactions of the top quark and the photon.

Top-quark pair production in association with a W± gauge boson in the POWHEG-BOX

We present a new Monte Carlo event generator for the production of a top-quark pair in association with a $W^\pm$ boson at hadron colliders in the POWHEG-BOX framework. We consider the next-to-leading-order QCD corrections to the $pp\to t\bar{t} W^\pm$ cross section, corresponding to the $\mathcal{O}(\alpha_s^3\alpha)$ and $\mathcal{O}(\alpha_s\alpha^3)$ terms in the perturbative expansion of the parton-level cross section, and model the decays of $W$ and top quarks at leading order retaining spin correlations. The fixed-order QCD calculation is further interfaced with the Pythia8 parton-shower event generator via the POWHEG method as implemented in the POWHEG-BOX. The corresponding code is now part of the public repository of the POWHEG-BOX. We perform a comparison of different event generators for both the case of inclusive production and the case of the two same-sign leptons signature at the Large Hadron Collider operating at a center-of-mass energy of $13$ TeV. We investigate theoretical uncertainties in the modelling of the fiducial volume stemming from missing higher-order corrections, the different parton shower matching schemes, and the modelling of decays. We find that the subleading contribution at $\mathcal{O}(\alpha_s\alpha^3)$ is particularly sensitive to differences in the matching scheme and higher-order parton shower effects. We observe that in particular jet observables can differ quite visibly although these differences play only a subordinate role in the description of physical observables once all contributions are combined.

Recent highlights of top-quark physics with the ATLAS detector

Highlights of the most recent top-quark measurements obtained using the full data set collected with the ATLAS experiment at the LHC at a centre-of-mass energy of 13 TeV, corresponding to 139 fb−1 of integrated luminosity, are presented. The results include inclusive and differential cross-section measurements of the production of top-quark pairs in association with a photon or a Z boson, the first evidence of four-top-quark production and a novel measurement of lepton flavour universality in top-quark topologies.

Identification of Additional Jets in the $${\rm{t}}{\rm\bar{t}b}{\rm\bar{b}}$$ Events by Using Deep Neural Network

In the top quark pair production in association with the Higgs boson decaying to a b quark pair t-tbar H (b-bbar), the final state has an irreducible nonresonant background from the production of a top quark pair in association with a b quark pair t-tbar b-bbar. Therefore, understanding of the t-tbar b-bbar process precisely in particular differential cross-section as functions of the properties of the additional b jets not from the top quark decay is essential for improving the sensitivity of a search for the t-tbar H b-bbar process. The two additional b jets can be identified by using various approaches. In this paper, the performances are compared quantitatively in the lepton+jets decay channel in terms of the matching efficiency of assigning two additional b jets as a figure of merit. This study provides valuable information towards the precise measurement of differential cross-sections as a function of properties of the additional b jets in the t-tbar b-bbar events. We showed that a matching efficiency of around 40% could be achieved using a deep neural network method. In the events with at least 4 b jets, this performance is 8% better than that achieved using minimum Delta R(b,bbar) method. This is consistent with the boosted decision tree method within its statistical uncertainty.

Measurement of the cross section for mathrm{t}overline{mathrm{t}} production with additional jets and b jets in pp collisions at sqrt{s} = 13 TeV

Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks left({sigma}_{mathrm{t}overline{mathrm{t}}mathrm{b}overline{mathrm{b}}}right) and in association with a pair of jets from quarks of any flavor or gluons left({sigma}_{mathrm{t}overline{mathrm{t}}mathrm{jj}}right) and their ratio are presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb−1. The measurements are performed in a fiducial phase space and extrapolated to the full phase space, separately for the dilepton and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results of the measurements in the fiducial phase space for the dilepton and lepton+jets channels, respectively, are {sigma}_{mathrm{t}overline{mathrm{t}}mathrm{jj}} = 2.36±0.02 (stat)±0.20 (syst) pb and 31.0±0.2 (stat)±2.9 (syst) pb, and for the cross section ratio 0.017 ± 0.001 (stat) ± 0.001 (syst) and 0.020 ± 0.001 (stat) ± 0.001 (syst). The values of {sigma}_{mathrm{t}overline{mathrm{t}}mathrm{b}overline{mathrm{b}}} are determined from the product of the {sigma}_{mathrm{t}overline{mathrm{t}}mathrm{jj}} and the cross section ratio, obtaining, respectively, 0.040±0.002 (stat)±0.005 (syst) pb and 0.62±0.03 (stat)±0.07 (syst) pb. These measurements are the most precise to date and are consistent, within the uncertainties, with the standard model expectations obtained using a matrix element calculation at next-to-leading order in quantum chromodynamics matched to a parton shower.

Measurement of the cross section for $\text{t}\bar{\text{t}}$ production with additional jets and b jets in pp collisions at $\sqrt{s}=$ 13 TeV

Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks $ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}}\right) $ and in association with a pair of jets from quarks of any flavor or gluons $ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}}\right) $ and their ratio are presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$^{−1}$. The measurements are performed in a fiducial phase space and extrapolated to the full phase space, separately for the dilepton and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results of the measurements in the fiducial phase space for the dilepton and lepton+jets channels, respectively, are $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $ = 2.36±0.02 (stat)±0.20 (syst) pb and 31.0±0.2 (stat)±2.9 (syst) pb, and for the cross section ratio 0.017 ± 0.001 (stat) ± 0.001 (syst) and 0.020 ± 0.001 (stat) ± 0.001 (syst). The values of $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}} $ are determined from the product of the $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $ and the cross section ratio, obtaining, respectively, 0.040±0.002 (stat)±0.005 (syst) pb and 0.62±0.03 (stat)±0.07 (syst) pb. These measurements are the most precise to date and are consistent, within the uncertainties, with the standard model expectations obtained using a matrix element calculation at next-to-leading order in quantum chromodynamics matched to a parton shower.[graphic not available: see fulltext]

Top quark mass measurement in radiative events at electron-positron colliders

In this letter, we evaluate the potential of linear e+e− colliders to measure the top quark mass in radiative events and in a suitable short-distance scheme. We present a calculation of the differential cross section for production of a top quark pair in association with an energetic photon from initial state radiation, as a function of the invariant mass of the tt¯ system. This matched calculation includes the QCD enhancement of the cross section around the tt¯ production threshold and remains valid in the continuum well above the threshold. The uncertainty in the top mass determination is evaluated in realistic operating scenarios for the Compact Linear Collider (CLIC) and the International Linear Collider (ILC), including the statistical uncertainty and the theoretical and experimental systematic uncertainties. With this method, the top quark mass can be determined with a precision of 110 MeV in the initial stage of CLIC, with 1 ab−1 at s=380 GeV, and with a precision of approximately 150 MeV at the ILC, with L=4 ab−1 at s=500 GeV. Radiative events allow measurements of the top quark mass at different renormalization scales, and we demonstrate that such a measurement can yield a statistically significant test of the evolution of the MSR mass mtMSR(R) for scales R<mt.

Measurement of the [formula omitted] production cross section in the all-jet final state in pp collisions at [formula omitted

A measurement of the production cross section of top quark pairs in association with two b jets (tt‾bb‾) is presented using data collected in proton-proton collisions at s=13 TeV by the CMS detector at the LHC corresponding to an integrated luminosity of 35.9 fb−1. The cross section is measured in the all-jet decay channel of the top quark pair by selecting events containing at least eight jets, of which at least two are identified as originating from the hadronization of b quarks. A combination of multivariate analysis techniques is used to reduce the large background from multijet events not containing a top quark pair, and to help discriminate between jets originating from top quark decays and other additional jets. The cross section is determined for the total phase space to be 5.5±0.3(stat)−1.3+1.6(syst)pb and also measured for two fiducial tt‾bb‾ definitions. The measured cross sections are found to be larger than theoretical predictions by a factor of 1.5–2.4, corresponding to 1–2 standard deviations.

Measurements of inclusive and differential fiducial cross-sections of tbar{t}gamma production in leptonic final states at sqrt{s}=13~text {TeV} in ATLAS

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 text{ fb }^{-1}, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 text {TeV}. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521 pm 9text {(stat.)} pm 41text {(sys.)}~text {fb} and 69 pm 3text {(stat.)} pm 4text {(sys.)}~text {fb} for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.

Multi-jet merged top-pair production including electroweak corrections

We present theoretical predictions for the production of top-quark pairs in association with jets at the LHC including electroweak (EW) corrections. First, we present and compare differential predictions at the fixed-order level for tbar{t} and tbar{t}+text {jet} production at the LHC considering the dominant NLO EW corrections of order {{mathcal {O}}}(alpha _mathrm {s}^2 alpha ) and {{mathcal {O}}}(alpha _mathrm {s}^3 alpha ) respectively together with all additional subleading Born and one-loop contributions. The NLO EW corrections are enhanced at large energies and in particular alter the shape of the top transverse momentum distribution, whose reliable modelling is crucial for many searches for new physics at the energy frontier. Based on the fixed-order results we motivate an approximation of the EW corrections valid at the percent level, that allows us to readily incorporate the EW corrections in the MePs@Nlo framework of Sherpa combined with OpenLoops. Subsequently, we present multi-jet merged parton-level predictions for inclusive top-pair production incorporating NLO QCD + EW corrections to tbar{t} and tbar{t}+text {jet}. Finally, we compare at the particle-level against a recent 8 TeV measurement of the top transverse momentum distribution performed by ATLAS in the lepton + jet channel. We find very good agreement between the Monte Carlo prediction and the data when the EW corrections are included.

Top-quark properties and mass measurements with the ATLAS detector

The top quark is unique among the known quarks in that it decays before it has an opportunity to form hadronic bound states. This makes measurements of its properties particularly interesting as one can access directly the properties of a bare quark. The latest measurements of these properties with the ATLAS detector at the LHC [1] are presented. Measurements of top-quark spin observables in top-antitop events, each sensitive to a different coefficient of the spin density matrix, are presented and compared to the Standard Model predictions. The helicity of the W boson from the top decays and the production angles of the top quark are further discussed. Limits on the rate of flavour changing neutral currents in the production or decay of the top quark are reported. The production of top-quark pairs in association with W and Z bosons is also presented. The measurement probes the coupling between the top quark and the Z boson. The cross-section measurement of photons produced in association with top-quark pairs is also discussed. These process are all compared to the best available theoretical calculations. The latest ATLAS measurements of the top-quark mass in lepton+jets, dilepton, and all-hadronic final states are also reported. In addition, measurements aiming to measure the mass in a well-defined scheme are presented.

Automation of NLO QCD and EW corrections with Sherpa and Recola

This publication presents the combination of the one-loop matrix-element generator Recola with the multipurpose Monte Carlo program Sherpa. Since both programs are highly automated, the resulting Sherpa +Recola framework allows for the computation of – in principle – any Standard Model process at both NLO QCD and EW accuracy. To illustrate this, three representative LHC processes have been computed at NLO QCD and EW: vector-boson production in association with jets, off-shell mathrm{Z}-boson pair production, and the production of a top-quark pair in association with a Higgs boson. In addition to fixed-order computations, when considering QCD corrections, all functionalities of Sherpa, i.e. particle decays, QCD parton showers, hadronisation, underlying events, etc. can be used in combination with Recola. This is demonstrated by the merging and matching of one-loop QCD matrix elements for Drell–Yan production in association with jets to the parton shower. The implementation is fully automatised, thus making it a perfect tool for both experimentalists and theorists who want to use state-of-the-art predictions at NLO accuracy.

Measurements of [formula omitted] using the ATLAS detector

The large centre-of-mass energy available at the LHC proton-proton collider allows for copious production of top-quark pairs in association with other final state particles at high transverse momentum. Results on the top-quark pair production in association with W and Z bosons at both 8 and 13 TeV are presented as well as the measurement of the cross section for production with an associated isolated photon at 7 TeV. The ATLAS experiment has measured several final state observables that are sensitive to additional radiation in top anti-top quark final states. Analyses probing the top-quark pair production with additional QCD radiation include the multiplicity of jets for various transverse momentum thresholds in the 13 TeV data. These measurements are compared to some of the most recent Monte Carlo event generators based on NLO QCD or LO multi-leg matrix element calculations.

Top Physics at CMS/LHC

Recent results on the inclusive and differential production cross sections of top-quark pair and single top-quark processes are presented, obtained using data from proton-proton collisions collected with the CMS detector at the LHC. The large centre-of-mass energies available at LHC allow for the copious production of top-quark pairs in association with other final state particles at high transverse momentum. Measurements of such processes as well as of the top-quark mass and other properties will be discussed. The results are compared with the most up-to-date standard model theory predictions.

Off-shell top quarks with one jet at the LHC: a comprehensive analysis at NLO QCD

We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order ${\cal O}(\alpha_s^4 \alpha^4)$ are taken into account, including irreducible backgrounds to $t\bar{t}j$ production, interferences and off-shell effects of the top quark and the $W$ gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the $t\bar{t}$ threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

Associated production of a top-quark pair with vector bosons at NLO in QCD: impact on t t ¯ H $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{H} $$ searches at the LHC

We study the production of a top-quark pair in association with one and two vector bosons, $t \bar t V$ and $t \bar t VV$ with $V=\gamma, Z, W^\pm$, at the LHC. We provide predictions at next-to-leading order in QCD for total cross sections and top-quark charge asymmetries as well as for differential distributions. A thorough discussion of the residual theoretical uncertainties related to missing higher orders and to parton distribution functions is presented. As an application, we calculate the total cross sections for this class of processes (together with $t \bar t H$ and $t \bar t t \bar t$ production) at hadron colliders for energies up to 100 TeV. In addition, by matching the NLO calculation to a parton shower, we determine the contribution of $t \bar t V$ and $t \bar t VV$ to final state signatures (two-photon and two-same-sign-, three- and four-lepton) relevant for $t \bar t H$ analyses at the Run II of the LHC.

Measurement of the t t ¯ W $$ t\overline{t}W $$ and t t ¯ Z $$ t\overline{t}Z $$ production cross sections in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All measurements are consistent with next-to-leading-order calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.