Top-quark flavor-changing tqZ couplings and rare processes

Chuan-Hung Chen; Takaaki Nomura

doi:10.1140/epjc/s10052-019-7133-5

EPJ

Top-quark flavor-changing tqZ couplings and rare $\Delta F=1$ processes

Chuan-Hung Chen¹ and Takaaki Nomura²^*

¹ Department of Physics, National Cheng-Kung University, Tainan, 70101, Taiwan
² School of Physics, KIAS, Seoul, 02455, Republic of Korea

Received: 10 March 2019
Accepted: 12 July 2019
Published online: 7 August 2019

Abstract

We study the impacts of anomalous tqZ couplings ( $$q=u,c$$

), which lead to the $t\rightarrow q Z$ decays, on low energy flavor physics. It is found that the tuZ-coupling effect can significantly affect the rare K and B decays, whereas the tcZ-coupling effect is small. Using the ATLAS’s branching ratio (BR) upper bound of $BR(t\rightarrow uZ) < 1.7\times 10^{-4}$ , the influence of the anomalous tuZ-coupling on the rare decays can be found as follows: (a) The contribution to the Kaon direct CP violation can be up to $Re(\epsilon '/\epsilon ) \lesssim 0.8 \times 10^{-3}$ ; (b) $BR(K^+\rightarrow \pi ^+ \nu {\bar{\nu }}) \lesssim 12 \times 10^{-11}$ and $BR(K_L \rightarrow \pi ^0 \nu {\bar{\nu }})\lesssim 7.9 \times 10^{-11}$ ; (c) the BR for $K_S \rightarrow \mu ^+ \mu ^-$ including the long-distance effect can be enhanced by $11\%$ with respect to the standard model result, and (d) $BR(B_d \rightarrow \mu ^+ \mu ^-) \lesssim 1.97 \times 10^{-10}$ . In addition, although $Re(\epsilon '/\epsilon )$ cannot be synchronously enhanced with $BR(K_L\rightarrow \pi ^0 \nu {\bar{\nu }})$ and $BR(K_S\rightarrow \mu ^+ \mu ^-)$ in the same region of the CP-violating phase, the values of $Re(\epsilon '/\epsilon )$ , $BR(K^+ \rightarrow \pi ^+ \nu {\bar{\nu }})$ , and $BR(B_d \rightarrow \mu ^+ \mu ^-)$ can be simultaneously increased.

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EPJ

Top-quark flavor-changing tqZ couplings and rare $\Delta F=1$ processes

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