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Register a new userRare decays of the top quark mediated by Z gauge bosons and flavor violation
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2020
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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 $tto cV, uV$ induced at the one loop level, where $V =gamma, g$, 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 $m_{Z^prime}geq$ 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.
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In this work we study the flavor changing neutral current(FCNC) decays of the top quark, $tto cgamma$ and $tto c g$. The Standard Model, predictions for the branching ratios of these decays are about $sim 5times 10^{-14}$, and $sim 1times 10^{-12}$, respectively. The recent study presented by the ATLAS Collaboration gives a sensitivity on these branching ratios about $sim 10^{-5}$ at $%95$ C.L. The parameter space of $lambda$, $Lambda$, and $d$ where the branching ratios of $tto cgamma$ and $tto c g$ decays exceed these predictions is obtained.
We propose a novel strategy to test lepton flavor universality (LFU) in top decays, applicable to top pair production at colliders. Our proposal exploits information in kinematic distributions and mostly hinges on data-driven techniques, thus having very little dependence on our theoretical understanding of top pair production. Based on simplified models accommodating recent hints of LFU violation in charged current B meson decays, we show that existing LHC measurements already provide non-trivial information on the flavor structure and the mass scale of such new physics (NP). We also project that the measurements of LFU in top decays at the high-luminosity LHC could reach a precision at the percent level or below, improving the sensitivity to LFU violating NP in the top sector by more than an order of magnitude compared to existing approaches.
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 $Z^prime$ gauge boson considered, the best prediction for the magnetic dipole moment of the tau lepton, $|a_tau|$, is of the order of $10^{-8}$, while the highest value for the electric one, $|d_tau|$, corresponds to $10^{-24}$ $e,$cm; our main result for the chromomagnetic dipole moment of the top quark, $|hat{mu}_t|$, is $10^{-6}$, and the value for the chromoelectric one, $|d_t|$, 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 possibility of detecting double flavor violating top quark transitions $t to u_itau mu$ ($u_i=u,c$) is explored in a model--independent manner, using the effective Lagrangian approach. Low--energy data, on high precision measurements, and current experimental limits are used to constraint the $tu_iH$ and $Htau mu$ vertices and then to calculate the branching ratio BR$(t to u_itau mu)$. If in the Standard Model BR$(t to u_itau mu)$ is of the order of $10^{-13}$$-10^{-14}$, higgs--mediated double flavor violating top quark decays can occur with branching ratios ranging from $10^{-3}$ to $10^{-4}$ for 114.4 GeV$/c^2$ $< m_H<$ $2m_W$, that is at the reach of the CERN Large Hadron Collider.
Minimal Flavor Violation in the up-type quark sector leads to particularly interesting phenomenology due to the interplay of flavor physics in the charm sector and collider physics from flavor changing processes in the top sector. We study the most general operators that can affect top quark properties and $D$ meson decays in this scenario, concentrating on two CP violating operators for detailed studies. The consequences of these effective operators on charm and top flavor changing processes are generically small, but can be enhanced if there exists a light flavor mediator that is a Standard Model gauge singlet scalar and transforms under the flavor symmetry group. This flavor mediator can satisfy the current experimental bounds with a mass as low as tens of GeV and explain observed $D$-meson direct CP violation. Additionally, the model predicts a non-trivial branching fraction for a top quark decay that would mimic a dijet resonance.
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