Top Quark Production Research Articles

HEP-Frame: an efficient tool for big data applications at the LHC

HEP-Frame is a new C++ package designed to efficiently perform analyses of datasets from a very large number of events, like those available at the Large Hadron Collider (LHC) at CERN, Geneva. It mainly targets high-performance servers and mini-clusters, and it was designed for natural science researchers with a user-friendly interface to access structured databases. HEP-Frame automatically evaluates the underlying computing resources and builds an adequate code skeleton when creating a data analysis application. At run-time, HEP-Frame analyses a sequence of datasets exploring the available parallelism in the code and hardware resources: it concurrently reads inputs from a user-defined data structure and processes them, following the user-specific sequence of requirements to select relevant data; it manages the efficient execution of that sequence; and it outputs results in user-defined objects (e.g., ROOT structures), stored together with the used input dataset. This paper shows how a domain expert software development can benefit from HEP-Frame, and how it significantly improved the performance of analyses of large datasets produced in proton-proton collisions at the LHC. Two case studies are discussed: the associated production of top quarks together with a Higgs boson (tbar{t}H ) at the LHC, and a double- and single-top quark productions at the high-luminosity phase of the LHC (HL-LHC). Results show that the HEP-Frame awareness of the analysis code behaviour and structure, and the underlying hardware system, provides powerful and transparent parallelization mechanisms that largely improve the execution time of data analysis applications.

Collinear approach for top quark production at ep colliders

An approximately analysis study of the collinear approach for $t\overline{t}$ production at the Future Circular Collider hadron-electron (FCC-he) and the Large Hadron electron Collider (LHeC) is performed. To study heavy quark production processes, the collinear generalized double asymptotic scaling (DAS) approach in a wide rang of $Q^{2}$ is used. The reduced cross sections for top quark pair production at the FCC-he (for $Q^{2}{\geq}m^{2}_{t}$) and LHeC (for $Q^{2}{\leq}m^{2}_{t}$) center-of-mass energies at the high inelasticity are derived. These results may be considering for both the future collider experiments at center-of-mass energy frontier and the improvement of the phenomenological models on kinematics of low values of the Bjorken variable $x$. The evolution of the gluon density for heavy-quark production in DIS is obtained at low $x$ to include effects of heavy-quark masses. A nonlinear modification of these results shown that the nonlinear corrections, with respect to the top quark mass, are very small for $Q^{2}{\propto}m^{2}_{t}$.

Observability of triple top quark signal at future hadron colliders

Abstract The Standard Model (SM) cross-sections for the production of single top quarks, top-quark pairs, triple top quarks, and four top quarks at three different center-of-mass energies, i.e. $\sqrt{s}=$ 7, 10, and 14 TeV, at existing particle colliders as well as at future hadron colliders, are studied. A fully kinematic analysis with the optimized preselection cuts along with invariant mass reconstruction of top quarks is performed for triple-top production processes pp→tttW, tttd, and tttb in the presence of the SM background. All three signal processes are forced to decay in fully hadronic mode. The studies are performed for the High-Luminosity Large Hadron Collider (HL−LHC) at $\sqrt{s}=$ 14 TeV and for the High-Energy LHC (HE−LHC) at $\sqrt{s}=$ 27 TeV. The signal to background ratio and signal significance of all signal and background processes are estimated for both the scenarios. It is found that the chances of signal observability at the HE−LHC are higher than those at the HL−LHC. PACS numbers: 12.60.Fr, 14.80.Fd

On soft gluon resummation for associated single top production with a Higgs boson at the LHC

We investigate threshold resummation of soft gluon corrections for the s-channel process of a single top quark production in association with a Higgs boson in the Standard Model. We choose to work in the three-particle invariant mass kinematics in direct QCD, i.e. in the space of Mellin moments. Our results take into account terms of up to next-to-leading logarithmic (NLL) precision, as well as {mathcal {O}} (alpha _{textrm{s}}) non-logarithmic terms which do not vanish at threshold. After presenting analytical expressions, we discuss the corresponding numerical results for the total cross section and the invariant mass distribution at the LHC.

Two-loop master integrals for a planar topology contributing to pp → toverline{t}j

We consider the case of a two-loop five-point pentagon-box integral configuration with one internal massive propagator that contributes to top-quark pair production in association with a jet at hadron colliders. We construct the system of differential equations for all the master integrals in a canonical form where the analytic form is reconstructed from numerical evaluations over finite fields. We find that the system can be represented as a sum of d-logarithmic forms using an alphabet of 71 letters. Using high precision boundary values obtained via the auxiliary mass flow method, a numerical solution to the master integrals is provided using generalised power series expansions.

QCD improved top-quark decay at next-to-next-to-leading order

We analyse the top-quark decay at the next-to-next-to-leading order (NNLO) in QCD by using the Principle of Maximum Conformality (PMC) which provides a systematic way to eliminate renormalization scheme and scale ambiguities in perturbative QCD predictions. The PMC renormalization scales of the coupling constant alpha _s are determined by absorbing the non-conformal beta terms that govern the behavior of the running coupling by using the Renormalization Group Equation (RGE). We obtain the PMC scale Q_star =15.5 GeV for the top-quark decay, which is an order of magnitude smaller than the conventional choice mu _r=m_t, reflecting the small virtuality of the QCD dynamics of the top-quark decay process. Moreover, due to the non-conformal beta terms disappear in the pQCD series, there is no renormalon divergence and the NLO QCD correction term is greatly increased while the NNLO QCD correction term is suppressed compared to the conventional results obtained at mu _r=m_t. By further including the next-to-leading (NLO) electroweak corrections, the finite W boson width and the finite bottom quark mass, we obtain the top-quark total decay width Gamma ^{textrm{tot}}_t=1.3112^{+0.0190}_{-0.0189} GeV, where the error is the squared averages of the top-quark mass Delta m_t=pm 0.7 GeV, the coupling constant Delta alpha _s(M_Z)=pm 0.0009 and the estimation of unknown higher-order terms using the PAA method with [N/M]=[1/1]. The PMC improved predictions for the top-quark decay are complementary to the previous PMC calculations for top-quark pair production and helpful for detailed studies of properties of the top-quark.

Soft-gluon corrections for tqZ production

We study soft-gluon corrections for the associated production of a single top quark and a Z boson (tqZ production) at hadron colliders. We find that the radiative corrections are dominated by soft-gluon emission. We derive an approximate NNLO (aNNLO) cross section by adding second-order soft-gluon corrections to the exact NLO result. We calculate the aNNLO cross section at LHC energies, including uncertainties from scale dependence and from parton distributions. We also calculate differential distributions in top-quark rapidity. We show that the aNNLO corrections are significant and they enhance the NLO cross section while decreasing theoretical uncertainties.

Top-quark pair production by electron–positron annihilation in a circularly polarized laser field

In the framework of the electroweak standard model, we have investigated the top quarks pair production from annihilation at the energies of a future compact linear collider in the presence of an intense circularly polarized laser field. We have analytically generated the detailed expression for the laser-assisted differential cross section using the lowest-order Dirac-Volkov formalism, including the and interactions, in the center-of-mass frame. A wide range of center-of-mass energies corresponding to the future collider has been covered to study the behavior of the total cross section (TCS); this future compact linear collider constitutes an important direction in the study of top quark physics in order to improve the measurements, and the limits of some physical parameters obtained so far at the LHC and the Tevatron. We have analyzed the dependence of the laser-assisted total and partial cross sections on the energy of the center-of-mass of the colliding system and also on the parameters of the laser field, such as the number of exchanged photons, the laser field strength, and its frequency. The results indicate that the laser field decreases the order of magnitude of the TCS as much as the laser field strength increases or decreases its frequency.

Evidence for Top-quark Production in Nucleus–Nucleus Collisions with the CMS Experiment

Evidence for Top-quark Production in Nucleus–Nucleus Collisions with the CMS Experiment

Parton distribution functions and QCD coupling constant from LHC and non-LHC data

We present a new QCD analysis of parton distribution functions (PDFs) at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO). In the present paper, the role of special types of experimental measurements for extracting PDFs is investigated, and simultaneously the values of the strong coupling constant are determined. Essential elements of this QCD analysis are the HERA combined data as a base, heavy-quark cross-section measurement together with Tevatron data on jet production and H1 and ZEUS jet cross sections as ``non-LHC'' data, and especially the ``LHC'' dataset for top-quark and jet cross sections. These different datasets have allowed detailed information at low $x$, providing worthwhile information on the nucleon's flavor, and have played an important role for some PDFs at large-$x$ and strong coupling constant. Since the large-$x$ gluon PDF benefits from an accurate determination of quark PDFs, we have enough motivation to focus on the top-quark production and jet cross-section measurements from LHC to find the impact of these data on the gluon PDF and strong coupling constant. These experimental data have an impact on the relative uncertainties of PDF. The gluon PDF at large $x$ and the values of ${\ensuremath{\alpha}}_{s}({M}_{Z}^{2})$ are affected significantly. Although the main motivation of this paper is to focus on the gluon PDF and its uncertainty at large-$x$, giving notice to heavy PDF behavior in this region is also very important. We study the intrinsic charm (IC) using the BHPS model together with our extracted extrinsic charm PDF at large $x$, which can be very worthwhile for future experiments at the LHC.

Dark Matter Searches with Top Quarks

Collider signatures with top quarks provide sensitive probes of dark matter (DM) production at the Large Hadron Collider (LHC). In this article, we review the results of DM searches in final states with top quarks conducted by the ATLAS and CMS Collaborations at the LHC, including the most recent results on the full LHC Run 2 dataset. We highlight the complementarity of DM searches in final states with top quarks with searches in other final states in the framework of various simplified models of DM. A reinterpretation of a DM search with top quarks in the context of an effective field theory description of scalar dark energy is also discussed. Finally, we give an outlook on the potential of DM searches with top quarks in LHC Run 3, at the high-luminosity LHC, and possible future colliders. In this context, we highlight new benchmark models that could be probed by existing and future searches as well as those that predict still-uncovered signatures of anomalous top-quark production and decays at the LHC.

Infrared Singularities of Multileg QCD Amplitudes with a Massive Parton at Three Loops

We derive the structure of three-loop anomalous dimensions governing infrared singularities of QCD amplitudes with one massive and an arbitrary number of massless external partons. The contributions of tripole and quadrupole correlations involving a massive parton are studied in detail. The analytical expression of tripole correlations between one massive and two massless partons is obtained at three loops for the first time. We regularize the infrared divergences in the soft matrix element in a novel approach, where no extra scale dependence is involved, and the calculation can be performed in momentum space. Our results are essential to improve the theoretical predictions of single top and top quark pair productions at hadron colliders.

Studying Same-Sign Top Pair Production through Top-Higgs FCNC Interactions at the HL-LHC

We investigate the potential of the HL-LHC for discovering new physics effects via the same-sign top pair signatures. We focus on the semileptonic (electron and muon) decay of the top quarks and study the reach for a simplified model approach where top quark flavor changing could occur through a neutral scalar exchange. A relatively smaller background contribution and clean signature are the advantages of the leptonic decay mode of the same-sign W bosons in the same-sign production processes of top quark pairs. Assuming the FCNC between top quark, up-type quark, and scalar boson from the new physics interactions, the branchings could be excluded of the order O 1 0 − 4 . We use angular observables of the same-sign lepton pairs and the top quark kinematics in the process which provide the possibility of separation of new physics signal from the SM backgrounds using machine-learning techniques. We find that the same-sign top quark pair production is quite capable of testing the top-Higgs FCNCs at the HL-LHC.

Complete SMEFT predictions for four top quark production at hadron colliders

We study four top quark production at hadron colliders in the Standard Model Effective Field Theory (SMEFT). We perform an analysis at the tree-level, including all possible QCD- and EW-coupling orders and relevant dimension-six operators. We find several cases where formally subleading terms give rise to significant contributions, potentially providing sensitivity to a broad class of operators. Inclusive and differential predictions are presented for the LHC and a future pp circular collider operating at 100 TeV. We estimate the sensitivity of different operators and perform a simplified chi-square fit to set limits on SMEFT Wilson coefficients. In so doing, we assess the importance of including subleading terms and differential information in constraining new physics contributions. Finally, we compute the SMEFT predictions for the double insertion of dimension-six operators and scrutinise the possible enhancements to the sensitivity induced by a specific class of higher order terms in the EFT series.

NLO QCD and EW corrections to off-shell tZj production at the LHC

The production of a single top quark in association with a Z boson (tZj production) at the LHC is a relevant probe of the electroweak sector of the Standard Model as well as a window to possible new-physics effects. The growing experimental interest in performing differential measurements for this process demands an improved theoretical modelling in realistic fiducial regions. In this article we present an NLO-accurate tZj calculation that includes complete off-shell effects and spin correlations, combining QCD and electroweak radiative corrections to the LO signal. Integrated and differential cross sections are shown for a fiducial setup characterised by three charged leptons, two jets, and missing energy.

Search for flavour-changing neutral-current couplings between the top quark and the photon with the ATLAS detector at [formula omitted] TeV

This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly suppressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in pp collisions at s=13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb−1. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant excess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left- and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t→uγ)<0.85(0.88−0.25+0.37)×10−5 and B(t→cγ)<4.2(3.40−0.95+1.35)×10−5 for a left-handed tqγ coupling, and B(t→uγ)<1.2(1.20−0.33+0.50)×10−5 and B(t→cγ)<4.5(3.70−1.03+1.47)×10−5 for a right-handed coupling.

Bounds on the absorptive parts of the chromomagnetic and chromoelectric dipole moments of the top quark from LHC data

Bounds on the absorptive (imaginary) parts of the top quark chromomagnetic \({\hat{\mu }}_t\) and chromoelectric \({\hat{d}}_t\) dipole moments are obtained by reinterpreting the most recent LHC data in top quark pair production. It is found that both limits are of the order of \(10^{-1}-10^{-2}\), which are consistent with the standard model prediction of \(\mathrm{Im}\big [{\hat{\mu }}_t\big ]\). The effects of the absorptive parts of the top quark dipole moments are also studied via some kinematic distributions of \({\overline{t}}t\) production, though no significant deviation from the standard model leading-order contribution is observed. Our bounds can be useful to constrain the parameter space of standard model extensions.

QCD corrections in tqgamma production at hadron colliders

We study QCD corrections for the associated production of a single top quark and a photon (tqgamma production) at hadron colliders. We calculate the NLO cross section at LHC and future collider energies for a variety of kinematical cuts, and we estimate uncertainties from scale dependence and from parton distributions. We also calculate differential distributions in top-quark transverse-momentum and rapidity as well as photon energy. Finally, we study higher-order corrections from soft-gluon emission for this process, and we provide approximate NNLO (aNNLO) results for the cross section and top-quark differential distributions. We also compare our calculations with recent measurements from CMS and ATLAS at the LHC and find that the aNNLO corrections improve the comparison between the data and the Standard Model predictions.

Three-loop soft anomalous dimensions for top-quark processes

I present results for soft anomalous dimensions through three loops for several processes involving the production of top quarks. In particular, I discuss single-top and top-pair production. I also present some numerical results for double-differential distributions in t{\bar t}tt‾ production through approximate N^33LO.

Top quark pair and single top cross sections and ttX (CMS)

Latest results on inclusive and differential top quark pair and single top quark production cross sections are presented using proton-proton collision data collected by the CMS experiment. The differential cross sections are measured as a function of various kinematic observables of the top quarks and the jets and leptons of the event final state. The results are confronted with precise theory calculations and used to constrain Standard Model parameters. Measurements are performed also with high p_TpT top quarks, i.e., in boosted regimes.