Eur. Phys. J. C (2022) 82: 881
https://doi.org/10.1140/epjc/s10052-022-10815-x

Regular Article - Theoretical Physics

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

¹ Institute of Particle Physics and Key Laboratory of Quark and Lepton Physics (MOE), Central China Normal University, 430079, Wuhan, Hubei, China
² School of Physics and Microelectronics, Zhengzhou University, 450001, Zhengzhou, Henan, China

^c xqli@mail.ccnu.edu.cn

Received: 22 March 2022
Accepted: 18 September 2022
Published online: 6 October 2022

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.