Single Top Production Research Articles

Single-top and top-antitop cross sections

I present high-order calculations, including soft-gluon corrections, for single-top and top-antitop production cross sections and differential distributions. For single-top production, results are presented for the three different channels in the Standard Model, for associated production with a charged Higgs, and for processes involving anomalous couplings. For top-antitop pair production, total cross sections and top-quark transverse-momentum and rapidity distributions are presented for various LHC energies.

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.

Top quark measurements at ATLAS

The top quark is the heaviest known fundamental particle. As it is the only quark that decays before it hadronizes, it allows us to probe the properties of bare quarks at the Large Hadron Collider. Highlights of a few recent precision measurements by the ATLAS Collaboration of the top quark using 13 TeV and 8 TeV collision data will be presented: top-quark pair and single top production cross sections including differential distributions will be presented alongside measurements of top-quark properties, including results using boosted top quarks, probe our understanding of top-quark production in the TeV regime. Measurements of the top-quark mass and searches for rare top quark decays are also presented.

A general analysis of Wtb anomalous couplings**Supported by National Science Foundation of China (11275009, 11675002, 11635001), National Science Foundation (PHY-1315983, PHY-1316033) and DOE (DE- SC0011095)

We investigate new physics effects on the Wtb effective couplings in a model-independent framework. The new physics effects can be parametrized by four independent couplings, , , and . We further introduce a set of parameters x0, xm, xp and x5 which exhibit a linear relation to the single top production cross sections. Using recent data for the t-channel single top production cross section σt, tW associated production cross section σtW, s-channel single top production cross section σs, and W-helicity fractions F0, FL and FR collected at the 8 TeV LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our global fitting results show that the top quark effective couplings are strongly correlated. We show that (i) improving the measurements of σt and σtW is important in constraining the correlation of (,) and (,); (ii) and are anti-correlated, and are sensitive to all the four experiments; (iii) and are also anti-correlated, and are sensitive to the F0 and FL measurements; (iv) the correlation between and is sensitive to the precision of the σt, σtW and F0 measurements. The effective Wtb couplings are studied in three kinds of new physics models: the G(221) = SU(2)1 ⊗ SU(2)2 ⊗ U(1)X models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson’s mass (MW′) is less than several hundred GeV, but the constraint is loose if MW′ > 1 TeV. Furthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.

Higgs and Single Top Associated Production at the LHC in the Left-Right Twin Higgs Model**Supported by the National Natural Science Foundation of China under Grant No. U1404113, the Aid Project for the Mainstay Young Teachers in Henan Provincial Institutions of Higher Education of China under Grant Nos. 2016GGJS-135, 2014GGJS-283, and Colleges and Universities in Henan Province Key Scientific Research Project

In the framework of the left-right twin Higgs (LRTH) model, we study the Higgs boson and single top quark associated production at the LHC. We investigate the processes , and at the 8(14) TeV LHC and find that the production cross sections of the three processes can be enhanced at different levels. Among them, the production cross section of the process is enhanced significantly due to the contribution of the new particles predicted by the LRTH model.

Single production of the top partner in the T → tZ channel at the LHeC

In this paper, we study the possibility of detecting the heavy vector-like top partner T in a simplified model with the decay channel T→tZ at the Large Hadron Electron Collider (LHeC). We first investigate the single top partner production process including the intergenerational mixing and the results show that the mixing between the top partner with the first generation can largely enhance the production cross section. We further study the observability of the single top partner through the process e+p→T(→tZ)ν¯e→t(→bjj′)Z(→ℓ+ℓ−)ν¯e at the LHeC with the proposed 140 GeV electron beam and 7 TeV proton beam. For some typical heavy T quark masses, the 3σ exclusion limits as well as the 5σ discovery region are respectively presented in terms of parameter space regions.

Bounds on the Electromagnetic Dipole Moments through the Single Top Production at the CLIC

We obtain bounds on the anomalous magnetic and electric dipole moments of thet-quark from a future high-energy and high-luminosity linear electron positron collider, as the CLIC, with polarized and unpolarized electron beams which are powerful tools for determining new physics. We consider the processesγe-→t¯bνe(γis the Compton backscattering photon) ande+e-→e-γ⁎e+→t¯bνee+(γ⁎is the Weizsacker-Williams photon) as they are one of the most important sources of single top quark production. For systematic uncertainties ofδsys=0% (5%),b-tagging efficiency=0.8, center-of-mass energy ofs=3 TeV, and integrated luminosity ofL=2ab-1the futuree+e-collider may put bounds on the electromagnetic dipole momentsa^Vanda^Aof the top quark of the order ofO(10-2–10-1)at the2σ (3σ)level, which are competitive with those recently reported in previous studies at hadron colliders and the ILC.

Modeling of associated tW single top production including the interference with diagrams

The associated tW single top production channel is one of the main mechanism of the electroweak production of top quarks at the LHC. Correct simulation of the SM prediction for this process is one of the important tasks in the SM measurements and BSM searches. One of the difficulties to simulate it correctly is a significant contribution of the interference terms with the pair top quark production. In this article authors compare most popular simulation schemes for tW production with simulation of the complete set of diagrams and propose new scheme to simulate interference terms without negative weights of the events.

Precision Measurements of Top Quark Production with the ATLAS Detector

The top quark is the heaviest known fundamental particle. It is the only quark that decays before it hadronises which gives us the unique opportunity to probe the properties of bare quarks and to test perturbative QCD. This overview will focus on a few recent precision top quark measurements by the ATLAS Collaboration at the LHC: Fiducial top pair and single top production cross-sections including differential distributions will be presented and compared with QCD predictions. The results include the first top quark measurements at 13 TeV using data from LHC Run 2.

An NLO+PS generator for $${\varvec{t \bar{t}}}$$ t t ¯ and $${\varvec{W t}}$$ W t production and decay including non-resonant and interference effects

We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+\nu_{\scriptscriptstyle\ell}\, l^-\bar{\nu}_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and $l$ belong to different families, and $b$ quarks are massive. Thus, the contributions from $t\bar{t}$ and $Wt$ single-top production as well as their quantum interference are fully included. Matrix elements are matched to the Pythia8 parton shower using a recently proposed method that allows for a consistent treatment of resonances in the POWHEG framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the $Wt$ channel, and for $t\bar{t}$ and $Wt$ backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects. The new generator is based on a process-independent interface of the OpenLoops amplitude generator with the POWHEGBOX framework.

Searches for the FCNC couplings from top-Higgs associated production signal with h→γγ at the LHC

In this paper, we study the observability of the top-Higgs flavor changing neutral current (FCNC) $tqh$ coupling through the process $pp\to qg\to t(\to \ell^{+}b\nu)h(\to \gamma\gamma)$ at the Large Hadron Collider~(LHC), where $\ell=e, \mu$. Our numerical results show that, in some parameter regions, the LHC may observe the above signals at the $5\sigma$ level. Otherwise, the branching ratios $Br(t\to uh)$ and $Br(t\to ch)$ can be respectively probed to $0.036\%$ and $0.13\%$ at $3\sigma$ level at 14 TeV LHC with the high integrated luminosity of 3000 fb$^{-1}$. On the other hand, studying the charge ratio for the lepton in top quark decay can be not only used to discriminate between signal and backgrounds, but also used to discriminate between $tuh$ and $tch$ couplings, for which anomalous single top production comes from the up initiated channel and charm initiated channel.

Search for anomalous couplings via single top quark production in association with a photon at LHC

Considering the experimental constraints given by CMS collaboration at $\sqrt{s} = 8$ TeV on the strength of top quark flavour-changing neutral-current (FCNC) couplings $tq\gamma$ and $tqg$, we investigate the production of top quark in association with a photon and carry out a full simulation for the signals $\ell\nu b\gamma$ and $jjb\gamma$ at 14 TeV LHC. In our numerical analysis, the contributions of single top production with a photon radiation off the top decay products are also included. The discovery potential for anomalous couplings $tq\gamma$ and $tqg$ with an integrated luminosity of 100 fb$^{-1}$ are examined in detail.

Deconvolving the detector in Fourier space

We discuss algorithms for the analysis of hadronic final states, with application to single top t-channel production, heavy Higgs decaying as H → W+W- in the lepton plus jets mode, and others. In our examples, nature has arranged for the triple differential decay rates in angles we call θ1, θ2, ϕ to be a short finite series in orthogonal functions, √2πamk,lYmk (θ1, ϕ)Yml (θ2, 0) (summation implied), where the amk,l are complex constants. This observation can be exploited in two ways; first, a technique called orthogonal series density estimation may be employed to extract coefficients amk,l of the decay and physics parameters related to these coefficients; second, an angular analog of the convolution theorem may be employed to analytically deconvolve detector resolution effects from an observed signal.

Minimal dilaton model and the diphoton excess

In light of the recent 750 GeV diphoton excesses reported by the ATLAS and CMS collaborations, we investigate the possibility of explaining this excess using the Minimal Dilaton Model. We find that this model is able to explain the observed excess with the presence of additional top partner(s), with same charge as the top quark, but with mass in the TeV region. First, we constrain model parameters using in addition to the 750 GeV diphoton signal strength, precision electroweak tests, single top production measurements, as well as Higgs signal strength data collected in the earlier runs of the LHC. In addition we discuss interesting phenomenolgy that could arise in this model, relevant for future runs of the LHC.

TbW anomalous couplings in the Two Higgs Doublet Model

We make a complete one loop calculation of the $tbW$ couplings in the Two Higgs Doublet Model. We evaluate both the anomalous couplings $g_L$ and $g_R$ as well as left handed and right handed component of $tbW$. The computation is done in the Feynman gauge using the on-shell scheme renormalization for the Standard Model wave functions and parameters. We first show that the relative corrections to these anomalous couplings are rather small in most regions of the parameter space. We then analyze the effects of these anomalous couplings on certain observables such as top quark polarization in single top production through $t-$channel as well as $W^\pm$ boson helicity fractions in top decay.

Off-shell single-top production at NLO matched to parton showers

We study the hadroproduction of a $Wb$ pair in association with a light jet, focusing on the dominant $t$-channel contribution and including exactly at the matrix-element level all non-resonant and off-shell effects induced by the finite top-quark width. Our simulations are accurate to the next-to-leading order in QCD, and are matched to the HERWIG6 and PYTHIA8 parton showers through the MC@NLO method. We present phenomenological results relevant to the 8 TeV LHC, and carry out a thorough comparison to the case of on-shell $t$-channel single-top production. We formulate our approach so that it can be applied to the general case of matrix elements that feature coloured intermediate resonances and are matched to parton showers.

Single Top Production at Next-to-Leading Order in the Standard Model Effective Field Theory.

Single top production processes at hadron colliders provide information on the relation between the top quark and the electroweak sector of the standard model. We compute the next-to-leading order QCD corrections to the three main production channels: t-channel, s-channel, and tW associated production, in the standard model including operators up to dimension six. The calculation can be matched to parton shower programs and can therefore be directly used in experimental analyses. The QCD corrections are found to significantly impact the extraction of the current limits on the operators, because both of an improved accuracy and a better precision of the theoretical predictions. In addition, the distributions of some of the key discriminating observables are modified in a nontrivial way, which could change the interpretation of measurements in terms of UV complete models.

Measurements of single top quark cross sections in pp collisions at 8 TeV with the CMS detector

Measurements are presented of the single top quark production in proton-proton collisions at the LHC at a centre-of-mass energy of 8 TeV, using data collected with the CMS experiment during 2012. The first analysis, in the t-channel single top production mode, considers decay channels where the W comes from the top decays into electron-neutrino or muon-neutrino. It makes use of a template fit exploiting the pseudorapidity distribution of the recoil jet and the reconstructed top quark mass, using background estimates determined from control samples in data. The measurement of top/antitop cross section ratio is also presented.The second analysis, measuring the associated production cross section of single top quark and W boson, considers final states in which the associated W boson, as well as the one originating from the top quark, decay leptonically. Multivariate methods are used to separate the signal from the topologically similar top pair production background. Multivariate techniques are also adopted in the search for s-channel single top quark production finally reported, in order to discriminate the very small signal from the huge background processes.

Constraining top quark effective theory in the LHC Run II era

We perform an up-to-date global fit of top quark effective theory to experimental data from the Tevatron, and from LHC Runs I and II. Experimental data includes total cross-sections up to 13 TeV, as well as differential distributions, for both single top and pair production. We also include the top quark width, charge asymmetries, and polarisation information from top decay products. We present bounds on the coefficients of dimension six operators, and examine the interplay between inclusive and differential measurements, and Tevatron / LHC data. All results are currently in good agreement with the Standard Model.

Recent top physics highlights at the LHC

Recent highlights of measurements by the ATLAS and CMS collaborations involving top quarks produced at the LHC are presented. Emphasis is put on precise inclusive and differential top pair production cross section measurements and comparisons to QCD calculations. Special attention is given to fiducial measurements. Results on electroweak single top quarks production and the recent evidence of top pair produced in associated with vector bosons are summarised. The experimental status of top quark mass measurements is critically reviewed.