Single Top Quark Research Articles

Model-independent constraints on the CKM matrix elements |Vtb|, |Vts| and |Vtd|

Single top quark production cross sections at hadron colliders are traditionally used to extract the modulus of the Vtb element of the Cabibbo-Kobayashi-Maskawa matrix under the following assumption: |Vtb| ≫ |Vtd|, |Vts|. For the first time, direct limits on |Vtd| and |Vts| are obtained using experimental data without the assumption of the unitarity of the CKM matrix. Limits on the |Vtd|, |Vts| and |Vtb| are extracted from differential measurements of single top quark cross sections in t-channel as a function of the rapidity and transverse momentum of the top quark and the light jet recoiling against the top quark. We have shown that the pseudorapidity of the forward jet in the single top production is one of the most powerful observables for discriminating between the |Vtd| and |Vtb| events. We perform a global fit of top quark related CKM elements to experimental data from the LHC Runs I and II and Tevatron. Experimental data include inclusive and differential single top cross sections in t-channel, inclusive tW production cross section, and top quark branching ratio to b quark and W boson. We present bounds on |Vtb|, |Vts| and |Vtd| using current data and project the results for future LHC data sets corresponding to luminosities of 300 and 3000 fb−1.

Transverse momentum resummation for s -channel single top quark production at the LHC

We study the soft gluon radiation effects for the $s$-channel single top quark production at the LHC. By applying the transverse momentum dependent factorization formalism, the large logarithms about the small total transverse momentum ($q_\perp$) of the single-top plus one-jet final state system, are resummed to all orders in the expansion of the strong interaction coupling at the accuracy of Next-to-Leading Logarithm (NLL). We compare our numerical results with PYTHIA and find that both the $q_\perp$ and $\phi^*$ observables from PYTHIA are consistent with our prediction. Furthermore, we point out the soft gluon radiation effects from the final state become significant in this process, especially for the boosted kinematical region.

LHC constraints and potential on resonant monotop production

We discuss the phenomenology associated with a resonant monotop collider signal, i.e. a signal in which a single top quark is resonantly produced in association with missing energy through an s-channel scalar exchange. We study both the bounds originating from dedicated monotop searches performed by the ATLAS and CMS experiments, and the constraints associated with other processes that could be induced by a new physics context favouring monotop production at colliders. The latter class of constraints includes, in particular, the recasting of analyses from the LHC and the TeVatron. All theoretical calculations are performed at the next-to-leading order accuracy in QCD, and we finally combine all results to establish the present limits on the parameter space and test the relevance of the monotop signal at the LHC Run 2.

Single-Top Quark Production at CMS

An overview of recent results of single-top quark production at the LHC using data collected with the CMS detector is presented. The CMS experiment has measured the electroweak production of the top quark in three production modes, namely t-channel, tW-channel, and s-channel. Measurements of the rare processes involving a single-top quark with a Z boson and a single-top quark with a γ are also discussed. All measurements are in agreement with the standard model prediction, and no sign of physics beyond the standard model is observed.

Higgs boson production in association with a single top quark at the LHC

Higgs boson production in association with a single top quark is the only process sensitive to the sign of the Top Yukawa coupling. We present a Monte-Carlo study of the pp → tHqb process and discuss the esperimental signatures that can help to discover it at the LHC. Two scenarios have been considered, the Standard Model case and the Inverted Top Coupling scenario.

The top quark width constrains from the single top quark production channel.

Using model independent and gauge invariant method proposed in previous studies we provide estimations on the top quark width measurement from the single top quark production channel and discuss the results of combining measurements from single and pair top quark production.

Differences in fully differential cross section of single top quark production and its subsequent decay process depending on various anomalous Wtb couplings.

The fully differential cross sections of t-channel production of a single top quark with its subsequent decay are compared for the case of contributions involving anomalous Wtb couplings. It is shown that the most noticeable differences, for cases corresponding to different anomalous couplings, appear in the shape of two-dimensional distributions, where one of the variables is the energy of a charged lepton and the other is one of the t-quark spin orientation angles.

Probing anomalous off-shell electroweak dipole moment of the top quark in single top quark production associated with Z or W bosons

An extension of the previous studies of anomalous Wtb interaction with offshell W boson was considered in associated single top quark production with W or Z bosons. More general variant with covariant derivative DµW−µ b−(fLWPL + fRWPR)t for the same processes is considered in this paper.

Evidence for the Associated Production of a Single Top Quark and a Photon in Proton-Proton Collisions at sqrt[s]=13 TeV.

The first evidence of events consistent with the production of a single top quark in association with a photon is reported. The analysis is based on proton-proton collisions at sqrt[s]=13 TeV and recorded by the CMS experiment in 2016, corresponding to an integrated luminosity of 35.9 fb^{-1}. Events are selected by requiring the presence of a muon (μ), a photon (γ), an imbalance in transverse momentum from an undetected neutrino (ν), and at least two jets (j) of which exactly one is identified as associated with the hadronization of a b quark. A multivariate discriminant based on topological and kinematic event properties is employed to separate signal from background processes. An excess above the background-only hypothesis is observed, with a significance of 4.4 standard deviations. A fiducial cross section is measured for isolated photons with transverse momentum greater than 25GeV in the central region of the detector. The measured product of the cross section and branching fraction is σ(pp→tγj)B(t→μνb)=115±17(stat)±30(syst) fb, which is consistent with the standard model prediction.

Effective operators in t-channel single top production and decay

The production of a single top quark in the t-channel and its subsequent decay is studied at NLO accuracy in QCD, augmented with the relevant dimension-6 effective operators from the Standard Model Effective Theory. We examine various kinematic and angular distributions for proton-proton collisions at the LHC at 13 TeV, in order to assess the sensitivity to these operators, both with and without the top quark narrow width approximation. Our results will be helpful when devising strategies to establish bounds on their coefficients, including the amount of CP violation of the weak dipole operator.

Probing the Quantum Interference between Singly and Doubly Resonant Top-Quark Production in pp Collisions at sqrt[s]=13 TeV with the ATLAS Detector.

This Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a W boson and a b-quark are significant. Events with exactly two leptons (ee, μμ, or eμ) and two b-tagged jets that satisfy a multiparticle invariant mass requirement are selected from 36.1 fb^{-1} of proton-proton collision data taken at sqrt[s]=13 TeV with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within 2σ of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.

S -channel single top quark production and decay at next-to-next-to-leading-order in QCD

We report on a fully differential next-to-next-to-leading order (NNLO) calculation of s-channel single top (anti-)quark production with a semi-leptonic decay at the LHC, neglecting the color correlation between the light and heavy quark lines and in the narrow width approximation. The NNLO corrections can increase the cross section by about 10% in the low transverse momentum region of the top quark and reduce scale variation uncertainty. In order to compare with experimental results without unfolding procedures, we also present theoretical predictions with fiducial cuts, including total cross sections and distributions of observables used in the experimental multivariate analysis. The NNLO corrections are found to be about -8% for fiducial cross sections.

Measurement of the production cross section for single top quarks in association with W bosons in proton-proton collisions at sqrt{s}=13 TeV

A measurement is presented of the associated production of a single top quark and a W boson in proton-proton collisions at sqrt{s}=13 TeV by the CMS Collaboration at the CERN LHC. The data collected corresponds to an integrated luminosity of 35.9 fb−1. The measurement is performed using events with one electron and one muon in the final state along with at least one jet originated from a bottom quark. A multivariate discriminant, exploiting the kinematic properties of the events, is used to separate the signal from the dominant mathrm{t}overline{mathrm{t}} background. The measured cross section of 63.1 ± 1.8(stat) ± 6.4(syst) ± 2.1 (lumi) pb is in agreement with the standard model expectation.

Transverse momentum resummation for t -channel single top quark production at the LHC

We study the effect of multiple soft gluon radiation on the kinematical distributions of the $t$-channel single top quark production at the LHC. By applying the transverse momentum dependent factorization formalism, large logarithms (of the ratio of large invariant mass $Q$ and small total transverse momentum $q_\perp$ of the single-top plus one-jet final state system) are resummed to all orders in the expansion of the strong interaction coupling at the accuracy of next-to-leading logarithm, including the complete next-to-leading order corrections. We show that the main difference from PYTHIA prediction lies on the inclusion of the exact color coherence effect between the initial and final states in our resummation calculation, which becomes more important when the final state jet is required to be in the forward region. We further propose a new experimental observable $\phi^*$ to test the effect of multiple gluon radiation in the single-top events. The effect of bottom quark mass is also discussed.

Dark matter production in association with a single top-quark at the LHC in a two-Higgs-doublet model with a pseudoscalar mediator

The sensitivity of the LHC experiments to the associated production of dark matter with a single top is studied in the framework of an extension of the standard model featuring two Higgs doublets and an additional pseudoscalar mediator. It is found that the experimental sensitivity is dominated by the on-shell production of a charged Higgs boson, when this assumes a mass below 1 TeV. Dedicated selections considering one and two lepton final states are developed to assess the coverage in parameter space for this signature at a centre-of-mass energy of 14 TeV assuming an integrated luminosity of 300 fb$^{-1}$. For a pseudoscalar mediator with mass 150 GeV and maximally mixed with the pseudoscalar of the two Higgs doublets, values of $tan\beta$ up to 3 and down to 15 can be excluded at 95% CL, if the $H^{\pm}$ mass is in the range 300 GeV-1 TeV. This novel signature complements the parameter space coverage of the mono-Higgs, mono-Z and $t{\bar t}$+$E_{\mathrm T}^{\mathrm miss}$ signatures considered in previous publications for this model.

Single top-quark production at the Tevatron and the LHC

This paper provides a review of the experimental studies of processes with a single top quark at the Tevatron proton-antiproton collider and the LHC proton-proton collider. Single top-quark production in the t-channel process has been measured at both colliders. The s-channel process has been observed at the Tevatron, and its rate has been also measured at the center-of-mass energy of 8 TeV at the LHC in spite of the comparatively harsher background contamination. LHC data also brought the observation of the associated production of a single top quark with a W boson as well as with a Z boson. The Cabibbo-Kobayashi-Maskawa matrix element |Vtb| is extracted from the single-top-quark production cross sections, and t-channel events are used to measure several properties of the top quark and set constraints on models of physics beyond the Standard Model. Rare final states with a single top quark are searched for, as enhancements in their production rates, if observed, would be clear signs of new physics.

Measurements of top quark production cross-section at ATLAS

Measurements of the inclusive and differential cross-sections for top quark pair and single top production cross-sections in proton-proton collisions with the ATLAS detector at the Large Hadron Collider (LHC) are presented at centre-of-mass energies of 8 TeV and 13 TeV. These measurements reach high precision and are compared to the best available theoretical calculations. The differential measurements, including results using boosted topology, probe our understanding of top quark pair production in the TeV regime. Measurements of top-quark pair with additional jets enable the study of effect of gluon radiation, especially the measurement with additional heavy flavour jets provides test of multi-scale QCD calculation. The first evidence of single top quark production in the s-channel is quoted. Interference of top quark pair and single top productions is investigated, and the single top production cross-section in association with a Z boson is measured. Measurements of the properties of the Wtb vertex in single top quark production allow to set limits on anomalous couplings.

Search for the flavor-changing neutral current interactions of the top quark and the Higgs boson which decays into a pair of b quarks at sqrt{s}=13 TeV

A search for flavor-changing neutral currents (FCNC) in events with the top quark and the Higgs boson is presented. The Higgs boson decay to a pair of b quarks is considered. The data sample corresponds to an integrated luminosity of 35.9 fb−1 recorded by the CMS experiment at the LHC in proton-proton collisions at sqrt{s}=13 TeV. Two channels are considered: single top quark FCNC production in association with the Higgs boson (pp → tH), and top quark pair production with FCNC decay of the top quark (t → qH). Final states with one isolated lepton and at least three reconstructed jets, among which at least two are associated with b quarks, are studied. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95% confidence level are set on the branching fractions of top quark decays, ℬ(t → uH) < 0.47% (0.34%) and ℬ(t → cH) < 0.47% (0.44%), assuming a single nonzero FCNC coupling.

The Matrix Element Method at next-to-leading order QCD for hadronic collisions: single top-quark production at the LHC as an example application

Recently, a general algorithm to extend the Matrix Element Method (MEM) by taking into account next-to-leading-order (NLO) corrections in quantum chromodynamics (QCD) has been presented. In this article, the algorithm is applied to the most general case that coloured partons are encountered in the initial as well as the final state. This represents a substantial extension compared to previous work. As a concrete example, the production of single top quarks at the LHC is studied. We present in detail the generation of unweighted events following the NLO predictions. By treating these events as the result of a toy experiment, we show the first proof-of-principle application of the Matrix Element Method at NLO QCD for hadronic jet production. As an illustration, we study the determination of the top-quark mass. We find that — apart from elevating the powerful MEM to a sound theoretical foundation at NLO — the inclusion of the NLO corrections can lead to sizeable effects compared to the Matrix Element Method relying on leading-order predictions only. Furthermore, we find that the incorporation of the NLO corrections is mandatory to obtain reliable estimates of the theoretical uncertainties. In addition, this work shows that measuring the top-quark mass using the MEM in single top-quark production offers an interesting alternative to mass measurements in top-quark pair production.

Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with the ATLAS detector

The production of a top quark in association with a Z boson is investigated. The proton–proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of s=13TeV are used, corresponding to an integrated luminosity of 36.1fb−1. Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, tt¯ and Z+jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2σ in the data and the expected significance is 5.4σ. The measured cross-section for tZq production is 600±170(stat.)±140(syst.)fb.