Properties of the top quark

The top quark was discovered at the CDF and D0 experiments in 1995. As the partner of the bottom quark its properties within the Standard Model are fully defined. Only the mass is a free parameter. The measurement of the top quark mass and the verification of the expected properties have been an important topic of experimental top quark physics since. In this review the recent results on top quark properties obtained by the Tevatron experiments CDF and D0 are summarised. At the advent of the LHC special emphasis is given to the basic measurement methods and the dominating systematic uncertainties.     

Top quark forward-backward asymmetry and same-sign top quark pairs

The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a nonuniversal Z' model is of particular interest as it naturally predicts a top quark in the forward region of large rapidity. To reproduce the size of the asymmetry, the couplings of the Z' to standard model quarks must be large, inevitably leading to copious production of same-sign top quark pairs at the energies of the Large Hadron Collider (LHC). We explore the discovery potential for tt and ttj production in early LHC experiments at 7-8 TeV and conclude that if no tt signal is observed with 1 fb?1 of integrated luminosity, then a nonuniversal Z' alone cannot explain the Tevatron forward-backward asymmetry.     

A spin-1 top quark superpartner

We construct a supersymmetric model where the left-handed top and bottom quarks are mainly the gauginos of a vector supermultiplet and hence their superpartners are spin 1. The right-handed top quark is unified with the Higgs field; the top Yukawa coupling arises from the gaugino coupling.     

Model-independent measurement of the top quark polarisation

We introduce a new asymmetry in the decay t→Wb→?νb$t\to Wb\to ?\nu b$, which is shown to be directly proportional to the polarisation of the top quark along a chosen axis, times a sum of W helicity fractions. The latter have already been precisely measured at the Tevatron and the Large Hadron Collider. Therefore, this new asymmetry can be used to obtain a model-independent measurement of the polarisation of top quarks produced in any process at hadron or lepton colliders.     

Unparticle effects on top quark rare decays

In this work we study the flavor changing neutral current (FCNC) decays of the top quark, t→cγ and t→cg, in the framework of the unparticle physics. The Standard Model predictions for the branching ratios of these decays are about 5×10⁻¹⁴, and 1×10⁻¹², respectively. The parameter space of λ, Λ, and d is obtained by taking into account the SM predictions and the results of the simulation performed by the ATLAS Collaboration for the branching ratios of t→cγ and t→cg decays.     

FCNC top quark decays in extra dimensions

The flavor changing neutral top quark decay t→cX is computed, where X is a neutral standard model particle, in an extended model with a single extra dimension. The cases for the photon, X=γ, and a standard model Higgs boson, X=H, are analyzed in detail in a non-linear R_ξ gauge. We find that the branching ratios can be enhanced by the dynamics originating in the extra dimension. In the limit where 1/R≫m_t, we have found Br(t→cγ)≃10^-10 for 1/R = 0.5 TeV. For the decay t→cH, we have found Br(t→cH)≃10^-10 for a low Higgs mass value. The branching ratios go to zero when 1/R→∞.     

Determination of the width of the top quark

We extract the total width of the top quark, Γ(t), from the partial decay width Γ(t → Wb) measured using the t-channel cross section for single top-quark production and from the branching fraction B(t → Wb) measured in tt events using up to 2.3 fb(-1) of integrated luminosity collected by the D0 Collaboration at the Tevatron pp Collider. The result is Γ(t) = 1.99(-0.55)(+0.69) GeV, which translates to a top-quark lifetime of τ(t) = (3.3(-0.9)(+1.3)) × 10(-25) s. Assuming a high mass fourth generation b' quark and unitarity of the four-generation quark-mixing matrix, we set the first upper limit on |V(tb')| < 0.63 at 95% C.L.     

Measurement of helicity parameters in top quark decay

It is important to be able to quantitatively assay future measurements of competing observables consistent with the coupling predictions for decay, so plots of the values of these helicity parameters are given in terms of a “ + Additional Lorentz Structure”. Three phase-type ambiguities are shown to exist, but measurement of the sign of the large interference between the W longitudinal/transverse amplitudes will resolve two of them. The large effects in both and amplitudes from some couplings demonstrate that it is less model dependent to measure helicity parameters, instead of setting limits on some arbitrary subset of coupling constants. Received: 12 November 1998 / Published online: 27 April 1999     

Chromomagnetic dipole moment of the top quark revisited

We study the complete one-loop contributions to the chromagnetic dipole moment Δκ of the top quark in the standard model, two Higgs doublet models, topcolor assisted technicolor models, 331 models and extended models with a single extra dimension. We find that the SM predicts Δκ=-0.056 and the predictions of the other models are also consistent with the constraints imposed on Δκ by low-energy precision measurements.     

Probing the top quark flavor-changing couplings at CEPC

We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider(CEPC) in China.We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark,which can be tested at CEPC in the single top production channel.Compared with the current bounds from the LEP2 data and the projected limits at the high-luminosity LHC,we find that CEPC could improve the limits of the four-fermion flavor-changing coefficients by one to two orders of magnitude,and would also provide similar sensitivity for the two-fermion flavor-changing coefficients.Overall,CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the LHC upgrade.We show that the c-jet tagging capacity at CEPC could further improve its sensitivity to top-charm flavor-changing couplings.If a signal is observed,the kinematic distribution as well as the c-jet tagging could be exploited to pinpoint the various flavor-changing couplings,providing valuable information about the flavor properties of the top quark.