Single Top Quark Research Articles

The Impact of Single Top Data on CT14nnlo PDFs

In this paper we study the impact of ATLAS 8 TeV t-channel single top quark production data on CT14nnlo PDFs. First, we calculate the total cross section, cross section ratio, differential cross section, and normalized differential cross section as a function of transverse momentum pT and rapidity y, at next-to-leading order QCD by MadGraph using CT14nnlo PDFs. And then we update CT14nnlo PDFs and observables by applying Error PDF Updating method. We find that down quark PDF increased roughly about 3% in 10− 4 < x < 4 × 10− 3 range at Q = 1.3 GeV, when weight factor equals to six. Furthermore, for data and theory comparison, we get slightly better agreement after updating observables.

CP Properties of Higgs Boson Interactions with Top Quarks in the tt¯H and tH Processes Using H→γγ with the ATLAS Detector

A study of the $CP$ properties of the interaction between the Higgs boson and top quarks is presented. Higgs bosons are identified via the diphoton decay channel ($H \rightarrow \gamma\gamma$) and their production in association with a top quark pair ($t\bar{t}H$) or single top quark ($tH$) is studied. The analysis uses 139 fb$^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider. Assuming a $CP$-even coupling, the $t\bar{t}H$ process is observed with a significance of 5.2 standard deviations. The measured cross section times $H \to \gamma \gamma$ branching ratio is $1.64\;^{+0.38}_{-0.36}\;\text{(stat.)}\;^{+0.17}_{-0.14}\;(\text{sys.})$ fb, and the measured rate for $t\bar{t}H$ is $1.43\;^{+0.33}_{-0.31}\;\text{(stat.)}\;^{+0.21}_{-0.15}\;(\text{sys.})$ times the Standard Model expectation. The $tH$ production process is not observed and an upper limit on its rate of 12 times the Standard Model expectation is set. A $CP$-mixing angle greater (less) than 43 (-43)$^\circ$ is excluded at 95% confidence level.

NLO QCD+EW predictions for tHj and tZj production at the LHC

In this work we calculate the cross sections for the hadroproduction of a single top quark or antiquark in association with a Higgs (tHj) or Z boson (tZj) at NLO QCD+EW accuracy. In the case of tZj production we consider both the case of the Z boson undecayed and the complete final state tℓ+ℓ−j, including off-shell and non-resonant effects. We perform our calculation in the five-flavour-scheme (5FS), without selecting any specific production channel (s-, t- or tW associated). Moreover, we provide a more realistic estimate of the theory uncertainty by carefully including the differences between the four-flavour-scheme (4FS) and 5FS predictions. The difficulties underlying this procedure in the presence of EW corrections are discussed in detail. We find that NLO EW corrections are in general within the NLO QCD theory uncertainties only if the flavour scheme uncertainty (4FS vs. 5FS) is taken into account. For the case of tℓ+ℓ−j production we also investigate differences between NLO QCD+EW predictions and NLO QCD predictions matched with a parton shower simulation including multiple photon emissions.

Measurement of CKM matrix elements in single top quark t-channel production in proton-proton collisions at [formula omitted

The first direct, model-independent measurement is presented of the modulus of the Cabibbo–Kobayashi–Maskawa (CKM) matrix elements |η|Vtb, |η|Vtd, and |η|Vts, in final states enriched in single top quark t-channel events. The analysis uses proton-proton collision data from the LHC, collected during 2016 by the CMS experiment, at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb−1. Processes directly sensitive to these matrix elements are considered at both the production and decay vertices of the top quark. In the standard model hypothesis of CKM unitarity, a lower limit of |η|Vtb>0.970 is measured at the 95% confidence level. Several theories beyond the standard model are considered, and by releasing all constraints among the involved parameters, the values |η|Vtb=0.988±0.024, and |η|Vtd2+|η|Vts2=0.06±0.06, where the uncertainties include both statistical and systematic components, are measured.

Observation of the associated production of a top quark and a Z boson in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

Single top-quark production in association with a Z boson, where the Z boson decays to a pair of charged leptons, is measured in the trilepton channel. The proton-proton collision data collected by the ATLAS experiment from 2015 to 2018 at a centre-of-mass energy of 13 TeV are used, corresponding to an integrated luminosity of 139 fb−1. Events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a b-hadron, are selected. The main backgrounds are from toverline{t}Z and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for tℓ+ℓ−q production, including non-resonant dilepton pairs with {m}_{{mathrm{ell}}^{+}{mathrm{ell}}^{-}} > 30 GeV, is 97 ± 13 (stat.) ± 7 (syst.) fb, consistent with the Standard Model prediction.

Model-independent sensitivity estimates for the electromagnetic dipole moments of the top quark at the Large Hadron Collider and beyond

As the heaviest known fundamental particle, the top quark ensures testing of the Standard Model and occupies a significant role in a lot of theories of new physics beyond the Standard Model. Up to now, the top quark has been only generated at the Tevatron and the Large Hadron Collider. However, one of the most important tasks of the physics program at the Large Hadron Collider is the investigation of the anomalous top quark interactions. In addition, the production of single top quarks, though rarer than production in pairs, presents us a different way to study the top quark produced via the electroweak interaction. This makes single top quark production an important signature for studying the properties of the top quark. Hence, we examine the anomalous tt¯γ interactions to investigate limits on the anomalous aA and aV couplings through single top quark production of the process pp→pγ⁎p→ptWX at the Large Hadron Collider, the High Luminosity Large Hadron Collider and High Energy Large Hadron Collider. The best limits obtained on the anomalous aA and aV couplings through the subprocess γ⁎b→Wt at the High Energy Large Hadron Collider with Lint=15 ab−1 at 68% Confidence Level are found to be |aA|=0.0509 and aV=[−0.3171;0.0077]. Therefore, we understand that γ⁎p collisions at the High Energy Large Hadron Collider improves the sensitivity limits of the anomalous coupling parameters according to previous studies.

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.

Measurement of differential cross sections and charge ratios for t-channel single top quark production in proton\u2013proton collisions at sqrt{s}=13,text {Te}text {V}

A measurement is presented of differential cross sections for t-channel single top quark and antiquark production in proton–proton collisions at a centre-of-mass energy of 13,text {Te}text {V} by the CMS experiment at the LHC. From a data set corresponding to an integrated luminosity of 35.9,text {fb}^{-1}, events containing one muon or electron and two or three jets are analysed. The cross section is measured as a function of the top quark transverse momentum (p_{mathrm{T}} ), rapidity, and polarisation angle, the charged lepton p_{mathrm{T}} and rapidity, and the p_{mathrm{T}} of the text {W}{}{} boson from the top quark decay. In addition, the charge ratio is measured differentially as a function of the top quark, charged lepton, and text {W}{}{} boson kinematic observables. The results are found to be in agreement with standard model predictions using various next-to-leading-order event generators and sets of parton distribution functions. Additionally, the spin asymmetry, sensitive to the top quark polarisation, is determined from the differential distribution of the polarisation angle at parton level to be 0.440 pm 0.070, in agreement with the standard model prediction.

Single top production in PDF fits

We study the impact of recent LHC t-channel single top-quark and top-antiquark measurements at centre-of-mass energies of 7, 8 and 13 TeV on the parton distribution functions (PDFs) of the proton. We consider, namely, total cross sections, top-antitop cross section ratios, and differential distributions. We present a critical appraisal of the data, studying in particular how their description is affected by the theoretical details that enter the computation of the corresponding observables: QCD and electroweak higher-order corrections, the flavour scheme, and the value of the bottom-quark threshold. We perform a series of fits to the data within the NNPDF3.1 framework, whereby next-to-next-to-leading order QCD corrections are applied to single top measurements in a systematic way. We find that there exists an optimal combination of data that maximises consistency with the rest of the dataset, and efficiency in constraining the up, down and, partially, gluon PDFs.

Probing the top-Higgs boson FCNC couplings via the h→γγ channel at the HE-LHC and FCC-hh

We investigate the sensitivity of future searches for the top-Higgs boson Flavour Changing Neutral Current (FCNC) couplings $tqh$~($q= u, c$) at the proposed High Energy Large Hadron Collider~(HE-LHC) and Future Circular Collider in hadron-hadron mode (FCC-hh). We perform a full simulation for two processes in the $h\to \gamma\gamma$ decay channel (where $h$ is the discovered Higgs state): single top quark FCNC production in association with the Higgs boson (plus a jet) and top quark pair production with FCNC decays $t\to qh$. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the Branching Ratios (BRs) of $t\to uh$ and $t\to ch$. It is shown that, at the HE-LHC with an integrated luminosity of 15 ab$^{-1}$ and at the FCC-hh with an integrated luminosity of 30 ab$^{-1}$, {{the BR($t\to uh$) (BR($t\to ch$)) can be probed, respectively, to $7.0~(8.5)\times 10^{-5}$ and $2.3~(3.0) \times 10^{-5}$ at the 95\% Confidence Level (CL) (assuming a 10\% systematic uncertainty on the background), which is almost two orders of magnitude better than the current 13 TeV LHC experimental results.

Symbolic expressions for fully differential single top quark production cross section and decay width of polarized top quark in the presence of anomalous Wtb couplings

Spin correlations in the t-channel single top quark production and its subsequent decay are investigated for the case of contributions involving anomalous Wtb couplings. We obtain analytical expressions for the differential widths for the three-particle decay of a polarized t quark in its rest frame and also expressions for the differential cross sections of the full process of production and decay of the t quark ($2\to 4$) as a function of the energy of a charged lepton and two angles of orientation of the quantization axis of the t-quark spin. The expression is presented in the most general form for the case of real and imaginary vector and tensor anomalous Wtb couplings. We show that shapes of certain multidimensional kinematic distributions of final state particles are significantly different for the contributions proportional to different combinations of the anomalous couplings. The most noticeable differences appear in the shape of the surfaces 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. Observed properties are confirmed by two methods of computations either from the obtained symbolic expression for the differential cross sections of the full process of the polarized single top quark production with its subsequent decay or by means of the CompHEP program for the complete process involving the t-channel single top. In addition, using the obtained analytical expressions, we estimate the statistical accuracy of extracting values of the anomalous Wtb couplings for different levels of the expected integral luminosity at the LHC.

Next-to-leading order QCD corrections for single top-quark production in association with two jets

In this article we calculate the next-to-leading order (NLO) QCD corrections for single on-shell top-quark production in association with two jets at proton-proton colliders. The tW channel is assumed to be measured independently. The QCD corrections to the inclusive cross section are about 28 (22)\% for top (anti-top) quark production at the 13 TeV LHC. Theoretical errors are dominated by scale uncertainties, which are found to be around 5\% at NLO. Results for various kinematical distributions are also provided using a well-motivated dynamical scale. The QCD corrections are found to have a non-trivial dependence on the phase-space.

Search for new physics in top quark production in dilepton final states in proton-proton collisions at sqrt{s} = 13,text {TeV}

A search for new physics in top quark production is performed in proton-proton collisions at 13,text {TeV} . The data set corresponds to an integrated luminosity of 35.9{,text {fb}^{-1}} collected in 2016 with the CMS detector. Events with two opposite-sign isolated leptons (electrons or muons), and mathrm{b} quark jets in the final state are selected. The search is sensitive to new physics in top quark pair production and in single top quark production in association with a mathrm{W} boson. No significant deviation from the standard model expectation is observed. Results are interpreted in the framework of effective field theory and constraints on the relevant effective couplings are set, one at a time, using a dedicated multivariate analysis. This analysis differs from previous searches for new physics in the top quark sector by explicitly separating mathrm{t}mathrm{W} from mathrm{t}{bar{mathrm{t}}} events and exploiting the specific sensitivity of the mathrm{t}mathrm{W} process to new physics.

Measurement of the single top quark and antiquark production cross sections in the t channel and their ratio in proton-proton collisions at [formula omitted

Measurements of the cross sections for the production of single top quarks and antiquarks in the t channel, and their ratio, are presented for proton-proton collisions at a center-of-mass energy of 13 TeV. The data set used was recorded in 2016 by the CMS detector at the LHC and corresponds to an integrated luminosity of 35.9 fb−1. Events with one muon or electron are selected, and different categories of jet and b jet multiplicity and multivariate discriminators are applied to separate the signal from the background. The cross sections for the t-channel production of single top quarks and antiquarks are measured to be 130±1(stat)±19(syst)pb and 77±1(stat)±12(syst)pb, respectively, and their ratio is 1.68±0.02(stat)±0.05(syst). The results are in agreement with the predictions from the standard model.

New probes for axionlike particles at hadron colliders

Axion-like particles (ALPs) appear from spontaneous global symmetry breaking in many extensions of the Standard Model (SM). In this paper, we find bounds on ALP ($a$) model parameters at the LHC from the ALP production associated with a photon and a jet ($j+\gamma+a$) as well as single top and top quark pairs ($t+j+a$, $t\bar{t}+a$) in a model independent approach. In particular, it is shown that the ALP production associated with a photon plus a jet at the LHC is a promising channel with significant sensitivity to probe the ALP couplings to gluons and electroweak gauge bosons. The prospects are presented at the High Luminosity LHC including a realistic detector simulation and pile up effects. Furthermore, the ALP model is examined through its contributions to the top quark (chromo)magnetic dipole moments. It is shown that the top quark magnetic and chromomagnetic dipole moments enable us to probe the ALP couplings to top quark and gauge bosons at a time. The constraints are complementary to those obtained from direct searches, as they are sensitive to light ALPs.

The s-Channel Single Top Quark Production as a Constraint for W ' Boson Contribution

An analysis is performed to constrain the W' boson production using the measurement of the s‑channel single top quark production cross section. Both phenomenological and statistical approaches are examined and results are presented. In the best case, W' bosons that interact only to the right-handed fermions are excluded below 1390 GeV. To our knowledge, it is the first time that the measured cross section of the s-channel single top quark production from the colliders is used to rule out a part of the phase space of the W' boson.

Search for associated production of a Higgs boson and a single top quark in proton-proton collisions at s=13 TeV

A search is presented for the production of a Higgs boson in association with a single top quark, based on data collected in 2016 by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The production cross section for this process is highly sensitive to the absolute values of the top quark Yukawa coupling, $y_t$, the Higgs boson coupling to vector bosons, $g_\mathrm{HVV}$, and, uniquely, to their relative sign. Analyses using multilepton signatures, targeting H $\to$ WW, H $\to$ $\tau\tau$, and H $\to$ ZZ decay modes, and signatures with a single lepton and a $\mathrm{b\overline{b}}$ pair, targeting the H $\to$ $\mathrm{b\overline{b}}$ decay, are combined with a reinterpretation of a measurement in the H $\to$ $\gamma\gamma$ channel to constrain $y_\mathrm{t}$. For a standard model-like value of $g_\mathrm{HVV}$, the data favor positive values of $y_\mathrm{t}$ and exclude values of $y_\mathrm{t}$ below about $-$0.9 $y_\mathrm{t}^\mathrm{SM}$.

Soft anomalous dimensions for single-top production at three loops

I present results for soft anomalous dimensions in single top-quark production processes through three loops. I first discuss the cusp anomalous dimension with one massive and one massless line at three loops. I then calculate the three-loop soft anomalous dimension for $tW$ production and for related processes with new physics such as $tZ$, $t\gamma$, and $tH^-$ production. This is followed by results at two and three loops for the soft anomalous dimension matrix in $s$-channel single-top production and, finally, in $t$-channel single-top production. These new results are needed for soft-gluon resummation at N$^3$LL accuracy and for calculations of soft-gluon corrections at N$^3$LO.

Observation of Single Top Quark Production in Association with a Z Boson in Proton-Proton Collisions at sqrt[s]=13 TeV.

The observation of single top quark production in association with a Z boson and a quark (tZq) is reported. Events from proton-proton collisions at a center-of-mass energy of 13TeV containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data were collected with the CMS detector in 2016 and 2017 and correspond to an integrated luminosity of 77.4fb^{-1}. The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous searches for tZq production. The tZq signal is observed with a significance well over 5 standard deviations. The measured tZq production cross section is σ(pp→tZq→tℓ^{+}ℓ^{-}q)=111±13(stat)_{-9}^{+11}(syst) fb, for dilepton invariant masses above 30GeV, in agreement with the standard model expectation.

Search for dark matter produced in association with a single top quark or a top quark pair in proton-proton collisions at sqrt{s}=13 TeV

A search for dark matter produced in association with top quarks in proton-proton collisions at a center-of-mass energy of 13 TeV is presented. The data set used corresponds to an integrated luminosity of 35.9 fb−1 recorded with the CMS detector at the LHC. Whereas previous searches for neutral scalar or pseudoscalar mediators considered dark matter production in association with a top quark pair only, this analysis also includes production modes with a single top quark. The results are derived from the combination of multiple selection categories that are defined to target either the single top quark or the top quark pair signature. No significant deviations with respect to the standard model predictions are observed. The results are interpreted in the context of a simplified model in which a scalar or pseudoscalar mediator particle couples to a top quark and subsequently decays into dark matter particles. Scalar and pseudoscalar mediator particles with masses below 290 and 300 GeV, respectively, are excluded at 95% confidence level, assuming a dark matter particle mass of 1 GeV and mediator couplings to fermions and dark matter particles equal to unity.