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.

The rare top quark decays mediated by a new neutral massive gauge boson that is predicted in models with extended gauge symmetries are studied. We focus on the processes t → cV, uV induced at the one loop level, where V = γ, g, by considering different extended models. It is found that, within a broad range of mass of the new neutral gauge boson, the models predict branching ratios for the decays in study that are competitive with respect to the corresponding branching ratios in the standard model. In order to establish bound on our branching ratios, we consider the recent experimental bounds as m Z ′ ≥ 3.8-4.5 TeV, depending on the model, which also impose restrictions on our calculation. Even in this case, the resulting branching ratios are of the same order of magnitude as that predicted by the standard model. It should be noted that for the case of two models studied here, since no experimental bound exists to compare with, the results could be important, as they are, in the best of cases, two orders of magnitude larger than the predicted by the standard model. PACS numbers:

We study two-body decays of a new neutral pseudoscalar into gauge bosons within the context of the Littlest Higgs model. Concretely, the Φ P → W W, V V, gg processes induced at the one-loop level, with V = γ, Z, are considered. Since the branching ratios of the Φ P → V V decays result very suppressed, only the Φ P → W W, gg processes are thoroughly studied. The branching ratios for the Φ P → gg and Φ P → W W decays are of the order of 10 −4 and 10 −6 , respectively, for f around 2 TeV, which represents the global symmetry breaking scale of the theory. The production cross section of the Φ P boson via gluon fusion at LHC is estimated.

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Abstract. The Z one-loop effects on the flavor-violating b → sγ process are analyzed. We employed the sequential Z model in order to establish a bound for the flavor-violating Z bs coupling, |Ω bs | 2 . We found a bound of the order of 10 −1 for the mentioned coupling in the mass range 2 TeV< m Z <3 TeV. IntroductionThe presence of a new neutral massive gauge boson (Z ) is predicted in the context of numerous extensions of the Standard Model (SM). The SU C (3) × SU L (2) × U Y (1) × U (1) extended electroweak gauge group is the simplest model that predicts the existence of the Z boson [1,2]. Although absent in the SM, the phenomenon of flavor violation can arise in many of its wellmotived extensions. One interesting feature of most models beyond the SM is the presence of generalized Yukawa sectors or generalized current sectors, which favor nondiagonal transitions mediated by neutral massive particles of spin-0 or spin-1 [1, 2]. Currently, experimental data have proven the existence of neutrino oscillations, which in simple words, tell us that lepton flavor conservation is violated in nature. However, the only signal of lepton flavor violation comes from transitions between neutral leptons. Thus, flavor violating transitions between charged fermions constitute an interesting subject of study, since if they occur in nature are additional evidence of flavor violation.The purpose of this work is to study the effect of quark flavor violation mediated by the Z boson on the b → sγ process, exploiting the rich phenomenology that offers the meson weak decays. Theoretically, it has been shown that this decay is highly sensitive to possible new physics effects in different contexts [3]. Thus, the main goal in this work is to estimate numerically the strength of the Z bs coupling from the b → sγ decay. However, it is well known that this process can not be tackled in isolation, but must be considered as part of the meson decay B → X s γ.