In the present work the Z ′ tc coupling is bounded by using the current experimental data on the D 0 − D 0 meson mixing system. It is found that the strength associated to this coupling is less than 5.75 × 10 −2 . The single top production through the e + e − → Z ′ → tc process at the Z ′ boson resonance is studied and we found that around 10 7 tc events will be expected at the International Linear Collider. For the Z ′ → tc decay, we predict a branching ratio of 10 −2 .

The electromagnetic dipole moments of the tau lepton and the chromoelectromagnetic dipole moments of the top quark are estimated via flavor-changing neutral currents, mediated by a new neutral massive gauge boson. We predict them in the context of models beyond the Standard Model with extended current sectors, in which simple analytic expressions for the dipole moments are presented. For the different Z 0 gauge boson considered, the best prediction for the magnetic dipole moment of the tau lepton, ja τ j, is of the order of 10 −8 , while the highest value for the electric one, jd τ j, corresponds to 10 −24 e cm; our main result for the chromomagnetic dipole moment of the top quark, jμ t j, is 10 −6 , and the value for the chromoelectric one, jd t j, can be as high as 10 −22 e cm. We compare our results, revisiting the corresponding Standard Model predictions, in which the chromomagnetic dipole moment of the top quark is carefully evaluated, finding explicit imaginary contributions.

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Low-energy Lorentz-invariant quantities could receive contributions from a fundamental theory producing small Lorentz-violating effects. Within the Lorentz-violating extension of quantum electrodynamics, we investigate, perturbatively, the contributions to the one-loop f f γ vertex from the CP T -violating axial coupling of a vector background field to fermions. We find that the resulting vertex function has a larger set of Lorentz structures than the one characterizing the usual, Lorentz invariant, parametrization of the f f γ vertex. We prove gauge invariance of the resulting one-loop expression through a set of gauge invariant nonrenormalizable operators introducing new-physics effects at the first and second orders in Lorentz violation, and which generate tree-level contributions to the f f γ vertex. Whereas loop contributions involving parameters that violate Lorentz invariance at the first order are CP T -odd, those arising at the second order are CP T -even, so that contributions to low-energy physics are restricted to emerge for the first time at the second order. In this context, we derive a contribution to anomalous magnetic moment of fermions, which we use to set a bound on Lorentz violation.

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