We update the constraints on anomalous dimension four t-b-W and t-t-Z couplings by using CLEO b → sγ and LEP/SLC precision Z-pole data. It is found that the data imposes very stringent bounds on them. Moreover, the 2σ pull from SM predictions of A LR (hadrons), A b and A F B (b) have little chance of being explained by the strongly constrained anomalous couplings.

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We study the complete one-loop contributions to the chromagnetic dipole moment ∆κ of the top quark in the Standard Model, two Higgs doublet models, topcolor assited technicolor models (TC2), 331 models and extended models with a single extra dimension. We find that the SM predicts ∆κ = −0.056 and the predictions of the other models are also consitent with the constraints imposed on ∆κ by low-energy precision measurements.

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We survey the flavor-changing neutral couplings (FCNC) of the top quark predicted by some extensions of the Standard Model: THDM, SUSY, L-R symmetric, TC2, 331, and models with extra quarks. Since the expected sensitivity of the LHC and ILC for the tcV (V = γ, g, Z) and tcH couplings is of order of a few percent, we emphasize the importance of any new physics effect that gives a prediction for these FCNC couplings within this limit. We also review the constraints imposed on these couplings from lowenergy precision measurements.

We study the one-loop contributions of the effective flavor changing neutral couplings (FCNC) tcZ and tcH on the electroweak precision observables Γ Z , R c , R b , R ℓ , A c and A F B c . Using the known experimental limits on these observables, we may place 95% CL bounds on these FCNC couplings which in turn translate into the following limits for the branching ratios BR(t → cZ) ≤ 6.7 × 10 −2 and BR(t → cH) ≤ (0.09 − 2.9) × 10 −3 for 114 ≤ m H ≤ 170GeV . PACS numbers 14.65.Ha;12.60.C;12.15.M;12.15.L As soon as it was confirmed that the flavor changing neutral couplings (FCNC) of the top quark are highly suppressed in the standard model (SM)[1], it was realized that some of its FCNC decay modes can be enhanced by several orders of magnitude in scenarios beyond the SM, and some of them

A new evaluation of the charged Higgs boson decay H ϩ →W ϩ ␥ is presented in the context of the general two-Higgs doublet model. A nonlinear R gauge which considerably simplifies the calculation is introduced and simple expressions are obtained for the fermionic and bosonic contributions. The H ϩ →W ϩ ␥ branching ratio is analyzed for several values of the parameters of the model. Although this decay can have a branching fraction as large as 10 Ϫ4 in a certain region of the parameter space, it is found that such a region is disfavored by the most recent constraints on b→s␥, gϪ2 of the muon, Z→bb , and the parameter, along with the exclusions from direct searches at the CERN e Ϫ e ϩ LEP collider. The possibility of detecting this decay at future colliders is discussed.