Quark Pair Production In Association Research Articles

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.

Optimized Probes of the CP Nature of the Top Quark Yukawa Coupling at Hadron Colliders

We summarize our recent proposals for probing the CP-odd iκ˜t¯γ5th interaction at the LHC and its projected upgrades directly using associated on-shell Higgs boson and top quark or top quark pair production. We first recount how to construct a CP-odd observable based on top quark polarization in Wb→th scattering with optimal linear sensitivity to κ˜. For the corresponding hadronic process pp→thj we then present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to κ˜. For the case of top quark pair production in association with the Higgs boson, pp→tt¯h, with semileptonically decaying tops, we instead show how one can construct manifestly CP-odd observables that rely solely on measuring the momenta of the Higgs boson and the leptons and b-jets from the decaying tops without having to distinguish the charge of the b-jets. Finally, we introduce machine learning (ML) and non-ML techniques to study the phase-space optimization of such CP-odd observables. We emphasize a simple optimized linear combination α·ω that gives similar sensitivity as the studied fully fledged ML models. Using α·ω we review sensitivity projections to κ˜ at HL-LHC, HE-LHC, and FCC-hh.

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.

Constraining the top quark effective field theory using the top quark pair production in association with a jet at future lepton colliders

Our main aim in this paper is to constrain the effective field theory describing the top quark couplings through the e−e+→tt¯ + jet process. The analysis is carried out considering two different center-of-mass energies of 500 and 3000 GeV including a realistic simulation of the detector response and the main sources of background processes. The expected limits at 95% CL are derived on the new physics couplings such as tt¯γ, tt¯Z, and tt¯g for each benchmark scenario using the dileptonic tt¯ final state. We show that the 95% CL limits on dimensionless Wilson coefficients c¯i considered in this analysis could be probed down to 10−4.

Measurement of top quark pair production in association with a Z boson in proton-proton collisions at sqrt{mathrm{s}} = 13 TeV

A measurement of the inclusive cross section of top quark pair production in association with a Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC is performed. The data sample corresponds to an integrated luminosity of 77.5 fb−1, collected by the CMS experiment during 2016 and 2017. The measurement is performed using final states containing three or four charged leptons (electrons or muons), and the Z boson is detected through its decay to an oppositely charged lepton pair. The production cross section is measured to be σ( mathrm{t}overline{mathrm{t}}mathrm{Z} ) = 0.95 ± 0.05 (stat) ± 0.06 (syst) pb. For the first time, differential cross sections are measured as functions of the transverse momentum of the Z boson and the angular distribution of the negatively charged lepton from the Z boson decay. The most stringent direct limits to date on the anomalous couplings of the top quark to the Z boson are presented, including constraints on the Wilson coefficients in the framework of the standard model effective field theory.

Probing the C P nature of the top quark Yukawa at hadron colliders

We analyze the prospects of probing the C P -odd itilde{kappa}overline{t}gamma 5th interaction at the LHC and its projected upgrades, the high-luminosity and high-energy LHC, directly using associated on-shell Higgs boson and top quark or top quark pair production. To this end we first construct a C P -odd observable based on top quark polarization in Wb → th scattering with optimal linear sensitivity to tilde{kappa} . For the corresponding hadronic process pp → thj we present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to tilde{kappa} . We project future sensitivity to the signal in pp → t(→ ℓ\U0001d708b)h(→ b overline{b} )j. We also propose novel C P -odd observables for top quark pair production in association with the Higgs, pp → t overline{t} h, with semileptonically decaying tops and h → b overline{b} , that rely solely on measuring the momenta of leptons and b-jets from the decaying tops without having to distinguish the charge of the b-jets. Among the many possibilities we single out an observable that can potentially probe tilde{kappa} ∼ 0.1 at the high-energy LHC with 2\U0001d70e confidence.

Measurements of differential cross sections of top quark pair production in association with jets in pp collisions at sqrt{s}=13 TeV using the ATLAS detector

Measurements of differential cross sections of top quark pair production in association with jets by the ATLAS experiment at the LHC are presented. The measurements are performed as functions of the top quark transverse momentum, the transverse momentum of the top quark-antitop quark system and the out-of-plane transverse momentum using data from pp collisions at sqrt{s}=13 TeV collected by the ATLAS detector at the LHC in 2015 and corresponding to an integrated luminosity of 3.2 fb−1. The top quark pair events are selected in the lepton (electron or muon) + jets channel. The measured cross sections, which are compared to several predictions, allow a detailed study of top quark production.

Hard photons in hadroproduction of top quarks with realistic final states

We present a complete description of top quark pair production in association with a hard photon in the dilepton channel. Our calculation is accurate to NLO in QCD. It is based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}upgamma production and includes all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. This calculation constitutes the first full computation for top quark pair production with a final state photon in hadronic collisions at NLO in QCD. Numerical results for total and differential cross sections are presented for the LHC at a centre-of-mass energy of sqrt{s}=13 TeV. For a few observables relevant for new physics searches, beyond some kinematic bounds, we observe shape distortions of more than 100%. In addition, we confirm that the size of the top quark off-shell effects for the total cross section is consistent with the expected uncertainties of the narrow width approximation. Results presented here are not only relevant for beyond the Standard Model physics searches but also important for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

Measurement of the cross section for top quark pair production in association with a W or Z boson in proton-proton collisions at sqrt{s}=13 TeV

A measurement is performed of the cross section of top quark pair production in association with a W or Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC. The data sample corresponds to an integrated luminosity of 35.9 fb−1, collected by the CMS experiment in 2016. The measurement is performed in the same-sign dilepton, three- and four-lepton final states. The production cross sections are measured to be sigma left(mathrm{t}overline{mathrm{t}}mathrm{W}right)={0.77}_{-0.11}^{+0.12}{left(mathrm{stat}right)}_{-0.12}^{+0.13}left(mathrm{syst}right)mathrm{p}mathrm{b} and sigma left(mathrm{t}overline{mathrm{t}}mathrm{Z}right)={0.99}_{-0.08}^{+0.09}{left(mathrm{stat}right)}_{-0.10}^{+0.12}left(mathrm{syst}right)mathrm{p}mathrm{b} . The expected (observed) signal significance for the mathrm{t}overline{mathrm{t}}mathrm{W} production in same-sign dilepton channel is found to be 4.5 (5.3) standard deviations, while for the mathrm{t}overline{mathrm{t}}mathrm{Z} production in three- and four-lepton channels both the expected and the observed significances are found to be in excess of 5 standard deviations. The results are in agreement with the standard model predictions and are used to constrain the Wilson coefficients for eight dimension-six operators describing new interactions that would modify mathrm{t}overline{mathrm{t}}mathrm{W} and mathrm{t}overline{mathrm{t}}mathrm{Z} production.

Top Quark Mass and Properties Measurements with the ATLAS Detector

The top quark is the heaviest elementary particle and unique among the known quarks since it decays before forming hadronic bound states. This makes measurements of the top quark itself particularly interesting as one can access directly the properties of a bare quark. The latest measurements of these properties with the ATLAS detector are reported using data from 8 TeV and 13 TeV proton-proton collisions at the Large Hadron Collider at CERN. The latest measurements of the top quark mass using template methods, as well as others aiming to measure the mass in a well-defined scheme, are presented. In addition, measurements for the top quark decay width, top quark spin observables and [Formula: see text]-boson helicity in events with top quark pairs ([Formula: see text]) are presented and compared to the Standard Model predictions. The cross-section measurements of top quark pair production in association with a photon, [Formula: see text]- or [Formula: see text]-boson are also reported and compared to the most accurate theoretical calculations. These measurements probe the top quark electroweak couplings. Limits on the rate of flavour-changing neutral current processes in the production or decay of the top quark are also shown.

Electroweak corrections to top quark pair production in association with a hard photon at hadron colliders

We present the next-to-leading order (NLO) electroweak (EW) corrections to the top quark pair production associated with a hard photon at the current and future hadron colliders. The dependence of the leading order (LO) and NLO EW corrected cross sections on the photon transverse momentum cut are investigated. We also provide the LO and NLO EW corrected distributions of the transverse momentum of final top quark and photon and the invariant mass of top quark pair and top-antitop-photon system. The results show that the NLO EW corrections are significant in high energy regions due to the EW Sudakov effect.

Probing top quark neutral couplings in the Standard Model Effective Field Theory at NLO in QCD

Top quark pair production in association with a Z-boson or a photon at the LHC directly probes neutral top-quark couplings. We present predictions for these two processes in the Standard Model (SM) Effective Field Theory (EFT) at next-to-leading order (NLO) in QCD. We include the full set of CP-even dimension-six operators that enter the top-quark interactions with the SM gauge bosons. For comparison, we also present predictions in the SMEFT for top loop-induced HZ production at the LHC and for $$ t\overline{t} $$ production at the ILC at NLO in QCD. Results for total cross sections and differential distributions are obtained and uncertainties coming from missing higher orders in the strong coupling and in the EFT expansions are discussed. NLO results matched to the parton shower are available, allowing for event generation to be directly employed in an experimental analyses. Our framework provides a solid basis for the interpretation of current and future measurements in the SMEFT, with improved accuracy and precision.

Top Quark Pair Production in Association with a Jet with Next-to-Leading-Order QCD Off-Shell Effects at the Large Hadron Collider

We present a complete description of top quark pair production in association with a jet in the dilepton channel. Our calculation is accurate to next-to-leading order (NLO) in QCD and includes all nonresonant diagrams, interferences, and off-shell effects of the top quark. Moreover, nonresonant and off-shell effects due to the finite W gauge boson width are taken into account. This calculation constitutes the first fully realistic NLO computation for top quark pair production with a final state jet in hadronic collisions. Numerical results for differential distributions as well as total cross sections are presented for the Large Hadron Collider at 8TeV. With our inclusive cuts, NLO predictions reduce the unphysical scale dependence by more than a factor of 3 and lower the total rate by about 13% compared to leading-order QCD predictions. In addition, the size of the top quark off-shell effects is estimated to be below 2%.

Erratum to: Constraining couplings of top quarks to the Z boson in t t ¯ + Z $$ \mathrm{t}\overline{\mathrm{t}}+\mathrm{Z} $$ production at the LHC

We study top quark pair production in association with a Z boson at the Large Hadron Collider (LHC) and investigate the prospects of measuring the couplings of top quarks to the Z boson. To date these couplings have not been constrained in direct measurements. Such a determination will be possible for the first time at the LHC. Our calculation improves previous coupling studies through the inclusion of next-to-leading order (NLO) QCD corrections in production and decays of all unstable particles. We treat top quarks in the narrow-width approximation and retain all NLO spin correlations. To determine the sensitivity of a coupling measurement we perform a binned log-likelihood ratio test based on normalization and shape information of the angle between the leptons from the Z boson decay. The obtained limits account for statistical uncertainties as well as leading theoretical systematics from residual scale dependence and parton distribution functions. We use current CMS data to place the first direct constraints on the ttbZ couplings. We also consider the upcoming high-energy LHC run and find that with 300 inverse fb of data at an energy of 13 TeV the vector and axial ttbZ couplings can be constrained at the 95% confidence level tomore » C_V=0.24^{+0.39}_{-0.85} and C_A=-0.60^{+0.14}_{-0.18}, where the central values are the Standard Model predictions. This is a reduction of uncertainties by 25% and 42%, respectively, compared to an analysis based on leading-order predictions. We also translate these results into limits on dimension-six operators contributing to the ttbZ interactions beyond the Standard Model.« less

SUSY-QCD Effects in Top Quark Pair Production in Association with a Gluon at the ILC* *Supported by the National Natural Science Foundation of China (NNSFC) under Grant Nos. 11305049, 11275057, and 11405047, by Specialized Research Fund for the Doctoral Program of Higher Education under Grant No. 20134104120002 and by the Startup Foundation for Doctors of Henan Normal University under Grant

Given the null results of searches for new physics at the LHC, we investigate the one-loop effects SUSY QCD in the process at the ILC in Minimal Supersymmetric Standard Model (MSSM). We find that the relative SUSY-QCD corrections to the cross section of can maximally reach 6.5%(3.2%) at the ILC with GeV when GeV and GeV.

Top quark couplings and polarization

Precise measurements of top quark couplings are presented. The measurements cover the single top tW cross section, the top-quark branching-fraction ratio R=B(t → Wb)/B(t → Wq) and the CKM matrix element Vtb from the single top cross sections. Top quark polarisation in the t-channel single-top quark production and dilepton tt, and W-helicity fraction measurements along with searches of top quark anomalous couplings in lepton plus jet channel are also presented. Finally measurements of top quark pair production in association with a W or Z boson or a photon are presented.

Constraining couplings of top quarks to the Z boson in t t ¯ $$ t\overline{t} $$ + Z production at the LHC

We study top quark pair production in association with a Z boson at the Large Hadron Collider (LHC) and investigate the prospects of measuring the couplings of top quarks to the Z boson. To date these couplings have not been constrained in direct measurements. Such a determination will be possible for the first time at the LHC. Our calculation improves previous coupling studies through the inclusion of next-to-leading order (NLO) QCD corrections in production and decays of all unstable particles. We treat top quarks in the narrow-width approximation and retain all NLO spin correlations. To determine the sensitivity of a coupling measurement we perform a binned log-likelihood ratio test based on normalization and shape information of the angle between the leptons from the Z boson decay. The obtained limits account for statistical uncertainties as well as leading theoretical systematics from residual scale dependence and parton distribution functions. We use current CMS data to place the first direct constraints on the ttbZ couplings. We also consider the upcoming high-energy LHC run and find that with 300 inverse fb of data at an energy of 13 TeV the vector and axial ttbZ couplings can be constrained at the 95% confidence level to C_V=0.24^{+0.39}_{-0.85} and C_A=-0.60^{+0.14}_{-0.18}, where the central values are the Standard Model predictions. This is a reduction of uncertainties by 25% and 42%, respectively, compared to an analysis based on leading-order predictions. We also translate these results into limits on dimension-six operators contributing to the ttbZ interactions beyond the Standard Model.

Automatic spin-entangled decays of heavy resonances in Monte Carlo simulations

Abstract We present a general method that allows one to decay narrow resonances in Les Houches Monte Carlo events in an efficient and accurate way. The procedure preserves both spin correlation and finite width effects to a very good accuracy, and is therefore particularly suited for the decay of resonances in production events generated at next-to-leading-order accuracy. The method is implemented as a generic tool in the Mad-Graph5 framework, giving access to a very large set of possible applications. We illustrate the validity of the method and the code by applying it to the case of single top and top quark pair production, and show its capabilities on the case of top quark pair production in association with a Higgs boson.

Top quark pair production in association with a jet: QCD corrections and jet radiation in top quark decays

We consider top quark pair production in association with a hard jet through next-to-leading order in perturbative QCD. Top quark decays are treated in the narrow width approximation and spin correlations are retained throughout the computation. We include hard jet radiation by top quark decay products and explore their importance for basic kinematic distributions at the Tevatron and the LHC. Our results suggest that QCD corrections and jet radiation in decays can lead to significant changes in shapes of basic distributions and, therefore, need to be included for the description of ttbar+jet production. We compare the shape of the transverse momentum distribution of a top quark pair recently measured by the D0 collaboration with the result of our computation and find reasonable agreement.

Next-to-Leading Order QCD Corrections to tt̄γ Production at the 7 TeV LHC

We present theoretical predictions including the complete next-to-leading order (NLO) QCD corrections to the top quark pair production in association with a photon at the LHC with the center-of-mass system energy of 7 TeV. The uncertainties of the leading order (LO) and NLO QCD corrected cross sections due to the renormalization/factorization scale, and the distributions of the transverse momenta of the final top quark and the photon are studied. Moreover, we provide the numerical results of the LO, NLO QCD corrected cross sections and the corresponding K-factors with different photon transverse momentum cuts. We also discuss the impact of QCD corrections to the pp → tt̄γ + X in the case that there exists an exotic top quark with an electric charge of −4e/3 at the 7 TeV LHC.