Abstract
We update the Standard Model (SM) predictions for the branching ratios of the rare top-quark decays t rightarrow cg(g), and evaluate the maximum rates that can be reached in the aligned as well as in the four conventional two-Higgs-doublet models (2HDMs) with mathcal {Z}_2 symmetries. Taking into account the relevant constraints on the model parameters resulting from a global fit obtained at the 95.5% confidence level, we find that the branching ratios of t rightarrow cg and t rightarrow cgg decays can reach up to 3.36times 10^{-9} and 2.95times 10^{-9} respectively, being therefore of the same order, in the aligned 2HDM (A2HDM). This is obviously different from the SM case, where the predicted branching ratio of the three-body decay t rightarrow cgg is about two orders of magnitude larger than that of the two-body decay t rightarrow cg. On the other hand, compared with the SM predictions, no significant enhancements are observed in the four conventional 2HDMs with mathcal {Z}_2 symmetries for the branching ratios of these two decays. Nevertheless, the predicted branching ratios of t rightarrow cg and t rightarrow cgg decays in the A2HDM are still out of the expected sensitivities of the future high-luminosity Large Hadron Collider and the Future Circular Collider in hadron-hadron mode.
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