Use of \Lambda_b polarimetry in top quark spin-correlation functions

Due to the absence of hadronization effects and the large mass, top quark decay will be uniquely sensitive to fundamental electroweak physics at the Tevatron, at the LHC, and at a future linear collider. A “complete measurement” of the four helicity amplitudes in decay is possible by the combined use of andW polarimetry in stage-two spin-correlation functions (S2SC). In this paper, the most general Lorentz-invariant decay density matrix is obtained for the decay sequence where and [or ], and likewise for . These density matrices are expressed in terms of b-polarimetry helicity parameters which enable a unique determination of the relative phases among the amplitudes. Thereby, S2SC distributions and single-sided b-W-interference distributions are expressed in terms of these parameters. The four b-polarimetry helicity parameters involving the amplitude are considered in detail. polarimetry signatures will not be suppressed in top quark analyses when final angles-and-energy variables are used for . Received: 26 January 2001 / Published online: 15 March 2001     

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.     

CP violation and top quark decays

We investigate how to testCP invariance in the weak decay of top quarks. A prediction ofCP violation from the Standard Model is made by an explicit calculation. The effect is very small as expected. We then use a form factor approach to parametrize possible new interactions ofCP violation. The differences between the form factor approach and an effective Lagrangian approach are discussed. The sensitivity of ourCP-odd observables to the form factors is estimated.     

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.     

Forward-backward asymmetry in top quark semileptonic decay

The semileptonic decay of the top quark t → bW⁺ → bl⁺ν_l is analyzed in the rest system of the W. The forward-backward asymmetry of the lepton l⁺ with respect to the heavy quark direction is defined and computed. It is argued that this observable will be an ideal tool to study top quark properties at Tevatron and LHC. Higher order QCD corrections are calculated and their structure is elucidated in some detail.     

Higgs boson bremsstrahlung in top quark decay

In top quark decay a neutral Higgs boson may be emitted in bremsstrahlung from the top quark or from the W-boson. We evaluate the branching fraction for this hitherto overlooked t→H⁰bW⁺ decay mode, where the W⁺ can be virtual or real. It has the standard model values of 0.02%, 0.2%, 0.8% for m_t=50, 100, 200 GeVand m_H=10 GeV.     

Two-loop QCD corrections to top quark width

We present O(α_s²) corrections to the decay t → bW in the limit of a very large top quark mass, m_t ⪢ m_W. We find that the O(α_s²) effects decrease the top quark decay width Γ_t by about 2%: Γ_t = Γ₀ (1 − 0.8α_t (m_t) − 1.7α_s²). The complete corrections are smaller by about 24% than their estimate based on the BLM effects O(α_s²). We explain how to compute a new type of diagrams which contribute to Γ(t → bW) at the O(α_s²) level.     

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→∞.     

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.     

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.