Rare top-quark decays $t \to cg(g)$ in the aligned two-Higgs-doublet model

TL;DR

This paper updates the predictions for rare top-quark decays in the Standard Model and two-Higgs-doublet models, finding that the aligned 2HDM can produce branching ratios near current experimental sensitivities, unlike other models.

Contribution

It provides the first comprehensive analysis of $t o cg(g)$ decays in the aligned two-Higgs-doublet model, including updated constraints and maximum predicted branching ratios.

Findings

Branching ratios in the aligned 2HDM can reach up to 3.36×10⁻⁹ for $t o cg$.

Branching ratios in the aligned 2HDM can reach up to 2.95×10⁻⁹ for $t o cgg$.

Predicted rates are below the sensitivities of future colliders.

Abstract

We update the Standard Model (SM) predictions for the branching ratios of the rare top-quark decays $t \to cg(g)$, 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 $t \to cg$ and $t \to cgg$ decays can reach up to $3.36\times 10^{-9}$ and $2.95\times 10^{-9}$ 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 $t \to cgg$ is about two orders of magnitude larger than that of the two-body decay $t \to cg$. 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 $t \to cg$ and $t \to cgg$ 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.