Single Top Quark In Association Research Articles

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.

$tW$ and $tZ'$ production at hadron colliders

We present theoretical results with soft-gluon corrections for two separate processes: (1) the production of a single top quark in association with a W boson in the Standard Model; and (2) the production of a single top quark in association with a heavy Z' boson in new physics models with or without anomalous couplings. We show that the higher-order corrections from soft-gluon emission are dominant for a wide range of collider energies. Results are shown for the total cross sections and top-quark transverse-momentum and rapidity distributions for tW and tZ' production at LHC and future collider energies up to 100 TeV. The uncertainties from scale dependence and parton distribution functions are also analyzed.

Single top production via FCNC couplings at an $$e^{-}e^{+}$$ collider with center-of-mass energy $$\sqrt{s}=240$$ GeV

Aspects of top quark physics can be studied at a future Circular Electron-Positron Collider. In particular, flavor-changing neutral current interactions (FCNC) of the top quark can be studied even with a center-of-mass energy from \(\sqrt{s}\gtrsim m_t\) to \(\sqrt{s}=240\) GeV, through the production a single top quark in association with a charm quark, which can arise in several scenarios of new physics. We explore here this signal within the Left-Right Mirror Model, which is a Left-Right model that includes mirror fermions; this model is motivated as a possible solution to the strong CP problem and by its rich phenomenology. We find that for particular scenarios of the model parameters, which are in agreement with the most up-to-date experimental constraints, the signal coming from the process \(e^+e^-\rightarrow V \rightarrow tc\) (\(V=Z,\,Z^{\prime }\)) provides a detectable signal and thus, it can give evidence for FCNC interactions; in these scenarios, the mass of a new neutral gauge boson can be as high as \(m_{Z'}=7\) TeV.

TZ' production at hadron colliders

We study the production of a single top quark in association with a heavy extra Z' at hadron colliders in new physics models with and without flavor-changing neutral-current (FCNC) couplings. We use QCD soft-gluon resummation and threshold expansions to calculate higher-order corrections for the total cross section and transverse-momentum distributions for t Z' production. The impact of the uncertainties due to the structure of the proton and scale dependence is also analyzed.

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.

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.

Top Quark Production Cross Section Measurements

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 are presented at centre-of-mass energies of 8 TeV and 13 TeV.The inclusive measurements reach high precision and are compared to the best available theoretical calculations. These measurements, including results using boosted tops, probe our understanding of top-pair production in the TeV regime. The results are compared to Monte Carlo generators implementing LO and NLO matrix elements matched with parton showers and NLO QCD calculations.For the t-channel single top measurement, the single top-quark and anti-top-quark total production cross-sections, their ratio, as well as differential cross sections are also presented. A measurement of the production cross section of a single top quark in association with a W boson, the second largest single-top production mode, is also presented. Finally, measurements of the properties of the Wtb vertex allow to set limits on anomalous couplings. All measurements are compared to state-of-the-art theoretical calculations.

Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at sqrt{s}=8 TeV

A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 fb−1 recorded by the CMS experiment at the LHC in proton-proton collisions at sqrt{s}=8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be σ(pp → tZq → ℓνbℓ+ℓ−q) = 10− 7+ 8 fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be ℬ(t → Zu) < 0.022% and ℬ(t → Zc) < 0.049%.

Anomalous tqg and tqH couplings effects in a Top-Higgs Final State

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor-changing neutral current interactions in tqg and tqH vertices. We derive the discovery potentials and 68% C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a bb‾ pair with 10 fb−1 integrated luminosity of data at 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton pT and η as a strong tool to observe the signal.

Measurement of the Wt Production Cross-Section in Dilepton Events with the ATLAS Detector

Single top-quark production, accounting for ∼1/3 of the overall top-quark production at the LHC, constitutes an excellent probe of the W-t-b coupling, which is sensitive to certain types of new physics. One important production mode in the Standard Model is the creation of a single top quark in association with a W boson. The close similarity of its final state to that of top-quark pair production, which has a more than 10 times larger production cross-section, makes the measurement a challenging endeavour. Hence multivariate techniques are appropriate to identify the Wt signal. The results of the latest Wt cross-section measurement with the ATLAS detector, based on 20 fb−1 of pp-collision data at s=8 TeV, are presented.

Measurement of the production cross-section of a single top quark in association with a W boson at 8 TeV with the ATLAS experiment

The cross-section for the production of a single top quark in association with a $W$ boson in proton--proton collisions at $\sqrt{s}= 8$ TeV is measured. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central $b$-jet are selected. The $Wt$ signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of $Wt$ events is observed with a significance of $7.7 \sigma$. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The $Wt$ cross-section, inclusive of decay modes, is measured to be $23.0 \pm 1.3$(stat.)$^{+3.2}_{-3.5}$(syst.)$\pm 1.1$(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element $|V_{tb}|$ of $1.01 \pm 0.10$ and a lower limit of 0.80 at the 95\% confidence level. The cross-section for the production of a top quark and a $W$ boson is also measured in a fiducial acceptance requiring two leptons with $p_{\textrm{T}}>25$ GeV and $|\eta|<2.5$, one jet with $p_{\textrm{T}}>20$ GeV and $|\eta|<2.5$, and $E_{\textrm{T}}^{\textrm{miss}}>20$ GeV, including both $Wt$ and top-quark pair events as signal. The measured value of the fiducial cross-section is $0.85 \pm 0.01$(stat.)$^{+0.06}_{-0.07}$(syst.)$\pm 0.03$(lumi.) pb.

Probing the anomalous FCNC interactions in a top-Higgs boson final state and the charge ratio approach

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor changing neutral current (FCNC) interactions in $tqg$ and $tqH$ vertices. We derive the discovery potentials and $68\%$ C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a $b\bar{b}$ pair with 10 fb$^{-1}$ integrated luminosity of data in proton-proton collisions at the center-of-mass energy of 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton $p_{T}$ and $\eta$ as a strong tool to observe the signal. In particular, we show that the charge ratio increases significantly at large $p_{T}$ of the charged lepton. While the main background from $t\bar{t}$ is nearly charge symmetric and $W+jets$ background has much smaller charge ratio with respect to the signal. We show that this feature can also be used in the probe of anomalous single top production with a $Z-$boson or a photon which are under the attention of the experimental collaborations.

Associated production of single top quark and W boson through anomalous couplings at LHeC based γp colliders

We consider the production of a single top quark in association with a W boson at LHeC based γp collider. We compute the cross-section for the process γp→WtX with the anomalous Wtb and Wtbγ couplings. We find that the sensitivities to anomalous couplings of top quark are shown to be comparable, even better than the ones obtained from direct searches at hadron colliders.

Effect of anomalous couplings on the associated production of a single top quark and a Higgs boson at the LHC

Abstract We consider the production of a single top quark in association with a Higgs boson at the LHC. In particular, we compute the cross sections for the processes pp → thj, thb, thW , thjj, thjb, thW j, thW b in the presence of the anomalous W tb, W W h and tth couplings. We find that the anomalous W tb and tth couplings can enhance the cross sections significantly. We also analyze a few signatures and show that, if these couplings are indeed anomalous, then with enough data, one should be able to observe the production of the Higgs boson in association with single top quark.

Probing top quark electromagnetic dipole moments in single-top-plus-photon production

The production of a single top quark in association with an isolated photon probes the electromagnetic coupling structure of the top quark. We investigate the sensitivity of kinematical distributions at the LHC in single-top-plus-photon production in view of a detection of anomalous electric and magnetic dipole moments of the top quark.