Top Quark Width Research Articles

Analytic third-order QCD corrections to top-quark and semileptonic b→u decays

We present the first analytic results of next-to-next-to-next-to-leading-order (N3LO) QCD corrections to the top-quark decay width. We focus on the dominant leading color contribution, which includes light-quark loops. At next-to-next-to-leading order (NNLO), this dominant contribution accounts for 95% of the total correction. By utilizing the optical theorem, the N3LO corrections are related to the imaginary parts of the four-loop self-energy Feynman diagrams, which are calculated with differential equations. The results are expressed in terms of harmonic polylogarithms, enabling fast and accurate evaluation. The third-order QCD corrections decrease the leading-order decay width by 0.667%, and the scale uncertainty is reduced by half compared to the NNLO result. The most precise prediction for the top-quark width is now 1.321 GeV for mt=172.69 GeV. Additionally, we obtain the third-order QCD corrections to the dilepton invariant mass spectrum and decay width in the semileptonic b→u transition. The perturbative series in the on-shell mass scheme exhibits poor convergence behavior. In the MS¯ mass scheme, the scale dependence is greatly improved. A more precise determination of the CKM matrix element Vub could be obtained with such higher-order corrections. Published by the American Physical Society 2024

Properties of the Top Quark

Recent measurements of the properties of the top quark at the CERN Large Hadron Collider are discussed. The results were measured for single and top quark pair production in their final states, including jets with either one or two leptons or only in hadronic final states. Top quark properties include angular correlations, top quark spin correlations, mass, and width. When looking towards the future, top quark properties open new and even interdisciplinary avenues for probing quantum information science.

Analytic result for the top-quark width at next-to-next-to-leading order in QCD

Analytic result for the top-quark width at next-to-next-to-leading order in QCD

Top quark mass measurements at the tbar{t} threshold with CEPC

We present a study of top quark mass measurements at the tbar{t} threshold based on CEPC. A centre-of-mass energy scan near two times of the top quark mass is performed and the measurement precision of top quark mass, width and alpha _S are evaluated using the tbar{t} production rates. Realistic scan strategies at the threshold are discussed to maximise the sensitivity to the measurements individually and simultaneously in the CEPC scenarios assuming a total luminosity limited to 100 fb^{-1}. With the optimal scan for individual property measurements, the top quark mass precision is expected to be 9 MeV, the top quark width precision is expected to be 26 MeV, and alpha _S can be measured at a precision of 0.00039, considering only the statistical uncertainty. Taking into account the systematic uncertainties from theory, width, alpha _S, experimental efficiency, background subtraction, beam energy and luminosity spectrum, the top quark mass can be measured at a precision of 25 MeV optimistically and 59 MeV conservatively at CEPC.

Top-quark mass determination in the optimised threshold scan

One of the important goals at the future e^++e^-− colliders is to measure the top-quark mass and width in a scan of the pair production threshold. Presented in this work is the most general approach to the top-quark mass determination from the threshold scan at CLIC, with all relevant model parameters and selected systematic uncertainties included in the fit procedure. In the baseline scan scenario the top-quark mass can be extracted with precision of the order of 30 to 40,MeV, already for 100 fb^{-1}−1 of data collected at the threshold. We present the optimisation procedure based on the genetic algorithm with which the statistical uncertainty of the mass measurement can be reduced by about 20%.

Off-shell vs on-shell modelling of top quarks in photon associated production

We present a comparative study of various approaches for modelling of the {e}^{+}{v}_e{mu}^{-}{overline{v}}_{mu }boverline{b}gamma final state in toverline{t}gamma production at the LHC. Working at the NLO in QCD we compare the fully realistic description of the top quark decay chain with the one provided by the narrow-width-approximation. The former approach comprises all double, single and non-resonant diagrams, interferences, and off-shell effects of the top quarks. The latter incorporates only double resonant contributions and restricts the unstable top quarks to on-shell states. We confirm that for the integrated cross sections the finite top quark width effects are small and of the order of mathcal{O}left({Gamma}_t/{m}_tright) . We show, however, that they are strongly enhanced for more exclusive observables. In addition, we investigate fractions of events where the photon is radiated either in the production or in the decay stage. We find that large fraction of isolated photons comes from radiative decays of top quarks. Based on our findings, selection criteria might be developed to reduce such contributions, that constitute a background for the measurement of the anomalous couplings in the toverline{t}upgamma vertex.

Extracting the Top-Quark Width from Nonresonant Production.

In the context of the standard model of particle physics, the relationship between the top-quark mass and width (Γ_{t}) has been precisely calculated. However, the uncertainty from current direct measurements of the width is nearly 50%. A new approach for directly measuring the top-quark width using events away from the resonance peak is presented. By using an orthogonal dataset to traditional top-quark width extractions, this new method may enable significant improvements in the experimental sensitivity in a method combination. Recasting a recent ATLAS differential cross section measurement, we find Γ_{t}=1.28±0.30 GeV (1.33±0.29 GeV expected), providing the most precise direct measurement of the width.

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.

Model independent top quark width measurement using a combination of resonant and nonresonant cross sections

Though top quark was discovered more than twenty years ago, measurement of its width is still challenging task. Most measurements either have rather low precision or they are done in assumption of the SM top quark interactions. We consider model independent parametrization of the top quark width and provide estimations on achievable accuracy using a combination of fiducial cross sections in double, single and non-resonant regions.

Heavy quark-philic scalar dark matter with a vector-like fermion portal

The absence of confirmed signal in dark matter (DM) direct detection (DD) may suggest weak interaction strengths between DM and the abundant constituents inside nucleon, i.e. gluons and valence light quarks. In this work we consider a real scalar dark matter S interacting only with SU(2)L singlet Up-type quarks Ui = uR,cR,tR via a vector-like fermion ψ which has the same quantum number as Ui. The DM-nucleon scattering can proceed through both h-mediated Higgs portal (HP) and ψ-mediated vector-like portal (VLP), in which HP can receive sizable radiative corrections through the new fermions. We first study the separate constraints on the new Yukawa couplings yi and find that the constraints of XENON1T results are strong on y1 from VLP scattering and on y3 from its radiative contributions to HP scattering. Since both DM-light quark interactions and HP have been well studied in the existing literature, we move forward to focus on DM-heavy quark interactions. Since there is no valence c,t quark inside nucleons at μhad ∼ 1 GeV, y2,y3 interactions are manifested in DM-gluon scattering at loop level. We find that renormalization group equation (RGE) and heavy quark threshold effects are important if one calculates the DM-nucleon scattering rate σSIp at μhad ∼ 1 GeV while constructing the effective theory at μEFT ∼ mZ. For the benchmarks y3 = 0.5, y2 = 0.5, 1, 3, combined results from ΩDM h2 ≃ 0.12, XENON1T, Fermi-LAT, 13 TeV LHC data have almost excluded mS < mt/2 when only DM-{c,t} interactions are considered. FCNC of top quark can be generated at both tree level t → ψ(*)S → cSS and loop level t → c+γ/g/Z, of which the branching fractions are typically below 10−9 after passing the other constraints, which are still safe from the current top quark width measurements.

Probing the top-quark width using the charge identification of b jets

We propose a new method for measuring the top-quark width based on the on-/off-shell ratio of $b$-charge asymmetry in $pp\to Wbj$ production at the LHC. The charge asymmetry removes virtually all backgrounds and related uncertainties, while remaining systematic and theoretical uncertainties can be taken under control by the ratio of cross sections. Limited only by statistical error, in an optimistic scenario, we find that our approach leads to good precision at high integrated luminosity, at a few hundred MeV assuming 300-3000 fb$^{-1}$ at the LHC. The approach directly probes the total width, in such a way that model-dependence can be minimized. It is complementary to existing cross section measurements which always leave a degeneracy between the total rate and the branching ratio, and provides valuable information about the properties of the top quark. The proposal opens up new opportunities for precision top measurements using a $b$-charge identification algorithm.

On the top-quark width measurement using a combination of resonant and non resonant cross sections.

Top quark width measurement is important and complex task. Most measurements are done in assumption of the SM top quark interactions. We would like to discuss model independent way of measuring the top quark width. Experimental accuracies of the top quark fiducial cross section measurements from 2% to 10% may allow to put constrains on the top quark width with 6% to 29% precision.

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.

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.

Top quark pair production and top quark properties at CDF

We present the most recent measurements of top quark pairs production and top quark properties in proton-antiproton collisions with center-of-mass energy of 1.96 TeV using CDF II detector at the Tevatron. The combination of top pair production cross section measurements and the direct measurement of top quark width are reported. The test of Standard Model predictions for top quark decaying into b-quarks, performed by measuring the ratio R between the top quark branching fraction to b-quark and the branching fraction to any type of down quark is shown. The extraction of the CKM matrix element |Vtb| from the ratio R is discussed. We also present the latest measurements on the forward-backward asymmetry (AFB) in top anti-top quark production. With the full CDF Run II data set, the measurements are performed in top anti-top decaying to final states that contain one or two charged leptons (electrons or muons). In addition, we combine the results of the leptonic forward-backward asymmetry in tt‾ system between the two final states. All the results show deviations from the next-to-leading order (NLO) standard model (SM) calculation.

Probing the top-quark width through ratios of resonance contributions of e + e − → W + W − b b ¯ $$ {\mathrm{e}}^{+}{\mathrm{e}}^{-}\to {\mathrm{W}}^{+}{\mathrm{W}}^{-}\mathrm{b}\overline{\mathrm{b}} $$

We exploit offshell regions in the process $e^+e^-\rightarrow W^+W^-b\bar{b}$ to gain access to the top-quark width. Working at next-to-leading order in QCD we show that carefully selected ratios of offshell regions to onshell regions in the reconstructed top and antitop invariant mass spectra are, \emph{independently} of the coupling $g_{tbW}$, sensitive to the top-quark width. We explore this approach for different centre of mass energies and initial-state beam polarisations at $e^+e^-$ colliders and briefly comment on the applicability of this method for a measurement of the top-quark width at the LHC.

Next-to-Next-to-Next-to-Leading Order QCD Prediction for the Top Antitop S-Wave Pair Production Cross Section Near Threshold in e(+)e(-) Annihilation.

We present the third-order QCD prediction for the production of top antitop quark pairs in electron-positron collisions close to the threshold in the dominant S-wave state. We observe a significant reduction of the theoretical uncertainty and discuss the sensitivity to the top quark mass and width.

Direct Measurement of the Total Decay Width of the Top Quark

We present a measurement of the total decay width of the top quark using events with top-antitop quark pair candidates reconstructed in the final state with one charged lepton and four or more hadronic jets. We use the full Tevatron run II data set of sqrt[s]=1.96 TeV proton-antiproton collisions recorded by the CDF II detector. The top quark mass and the mass of the hadronically decaying W boson are reconstructed for each event and compared with distributions derived from simulated signal and background samples to extract the top quark width (Γtop) and the energy scale of the calorimeter jets with in situ calibration. For a top quark mass Mtop=172.5 GeV/c2, we find 1.10<Γtop<4.05 GeV at 68% confidence level, which is in agreement with the standard model expectation of 1.3 GeV and is the most precise direct measurement of the top quark width to date.

Measurements of top-quark properties at the Tevatron

Recent measurements of top-quark properties at the Tevatron are presented. CDF uses data corresponding up to 9.0 fb−1 to measure the ratio R of the branching fractions , the branching fraction for top-quarks decaying into τ leptons and the cross section for the production of an additional γ in tt̄ production. The results from all these measurements agree well with their respective Standard Model expectation. DØ uses 5.3 fb−1 of data to measure the tt̄ cross section as a function of the time. A time dependency would imply Lorentz invariance violation as implemented by the Standard Model extension. No time dependency is observed and DØ sets first limits in the top-quark sector for Lorentz invariance violation. DØ also determines indirectly the top quark width using the results of earlier measurements at DØ. The measured top quark width is in agreement with the SM expectation and does not show any hints for new physics contributions.

NLO QCD corrections to off-shell top-antitop production with leptonic decays at hadron colliders

We present details of a calculation of the cross section for hadronic top-antitop production in next-to-leading order (NLO) QCD, including the decays of the top and antitop into bottom quarks and leptons. This calculation is based on matrix elements for \nu e e+ \mu- \bar{\nu}_{\mu}b\bar{b} production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Such contributions are formally suppressed by the top-quark width and turn out to be small in the inclusive cross section. However, they can be strongly enhanced in exclusive observables that play an important role in Higgs and new-physics searches. Also non-resonant and off-shell effects due to the finite W-boson width are investigated in detail, but their impact is much smaller than naively expected. We also introduce a matching approach to improve NLO calculations involving intermediate unstable particles. Using a fixed QCD scale leads to perturbative instabilities in the high-energy tails of distributions, but an appropriate dynamical scale stabilises NLO predictions. Numerical results for the total cross section, several distributions, and asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.