Using the CLEO data for b → sγ, we constrain the chromomagnetic dipole moment of the top quark within the context of a decoupling effective Lagrangian approach. Our results are in agreement with the results obtained for the e + e − and hadron colliders.

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We consider the two Higgs doublet model ͑2HDM͒ of type III which leads to flavor changing neutral currents ͑FCNC͒ at the tree level. In the framework of this model we can have, in principle, two situations: the case ͑a͒ when both doublets acquire a vacuum expectation value different from zero and the case ͑b͒ when one of them is zero. In addition, we show that we can make two types of rotations for the flavor mixing matrices which generates four types of different Lagrangians. Two of the four possible Lagrangians correspond to the 2HDM type I and type II plus flavor changing ͑FC͒ interactions. The analytical expressions of the partial lepton number violating widths ⌫(→eee) and ⌫(→e␥) are derived for the cases ͑a͒ and ͑b͒ and both types of rotations. In all cases these widths go asymptotically to zero in the limit when all Higgs boson masses go to infinity. We present from our analysis upper bounds for the flavor changing transition →e, and we show that such bounds are sensitive to the VEV structure and the type of rotation utilized.

Processes τ → lγ, τ → lll with l = e, µ and µ(τ ) → e(µ)γ are evaluated in the framework of a model based on the extended symmetry gauge SU (3) c ⊗ SU (3) L ⊗ U (1) Y with a leptonic sector consistent of five triplets. Lepton flavor violating processes are allowed at tree level in this model through the new Z gauge boson. We obtained bounds for the mixing angles in the leptonic sector of the model, considering the experimental measurements of the processes from the BELLE and the BABAR collaborations.

The flavor changing neutral top quark decay t → cX is computed, where X is a neutral standard model particle, in a 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 originated 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 T eV . For the decay t → cH, we have found Br(t → cH) ≃ 10