CP asymmetries of and decays in the aligned two-Higgs-doublet model

Fang-Min Cai; Rui-Lin Fan; Xin-Qiang Li; Ya-Dong Yang

doi:10.1140/epjc/s10052-025-13965-w

2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2025) 85: 285
https://doi.org/10.1140/epjc/s10052-025-13965-w

Regular Article - Theoretical Physics

CP asymmetries of $t \to c γ$ $t \rightarrow c \gamma$ and $t \to c g$ $t \rightarrow cg$ decays in the aligned two-Higgs-doublet model

Fang-Min Cai¹, Rui-Lin Fan², Xin-Qiang Li²^,3^a and Ya-Dong Yang¹^,2

¹ Institute of Particle and Nuclear Physics, Henan Normal University, 453007, Xinxiang, China
² Institute of Particle Physics and Key Laboratory of Quark and Lepton Physics (MOE), Central China Normal University, 430079, Wuhan, Hubei, China
³ Center for High Energy Physics, Peking University, 100871, Beijing, China

^a xqli@mail.ccnu.edu.cn

Received: 29 October 2024
Accepted: 19 February 2025
Published online: 13 March 2025

Abstract

We study the CP asymmetries of the rare top-quark decays $t \to c γ$ $t \rightarrow c \gamma$ and $t \to c g$ $t \rightarrow cg$ in the aligned two-Higgs-doublet model (A2HDM), which is generically characterized by new sources of CP violation beyond the Standard Model (SM). Specifically, the branching ratios and CP asymmetries of these rare top-quark decays are explicitly formulated, with an emphasis on the origins of weak and strong phases in the A2HDM. Taking into account the most relevant constraints on this model, we evaluate the variations of these observables with respect to the model parameters. It is found that the branching ratios of $t \to c γ$ $t \rightarrow c \gamma$ and $t \to c g$ $t \rightarrow cg$ decays can maximally reach up to $1.47 \times 10^{- 10}$ $1.47\times 10^{-10}$ and $4.86 \times 10^{- 9}$ $4.86\times 10^{-9}$ respectively, which are about four and three orders of magnitude higher than the corresponding SM predictions. While the branching ratios are almost independent of the relative phase $φ$ $\varphi$ between the two alignment parameters $ς_{u}$ $\varsigma _u$ and $ς_{d}$ $\varsigma _d$ within the allowed parameter space, the CP asymmetries are found to be very sensitive to $φ .$ $\varphi .$ When the two alignment parameters are complex with a non-zero $φ$ $\varphi$ varied within the range $[50^{\circ}, 150^{\circ}],$ $[50^\circ ,150^\circ ],$ the magnitudes of the CP asymmetries can be significantly enhanced relative to both the SM and the real case. In particular, the maximum absolute values of the CP asymmetries can even reach up to $O (1)$ ${\mathcal {O}}(1)$ for these two decay modes, in the range $φ \in [70^{\circ}, 100^{\circ}] .$ $\varphi \in [70^\circ ,100^\circ ].$ These interesting observations could be utilized to discriminate the SM and the different scenarios of the A2HDM.

© The Author(s) 2025

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