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

Abstract

We model-independently study the impacts of anomalous $tqZ$ couplings ($q=u,c$), which lead to the $t\to qZ$ 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 $BR(t\to uZ)<1.7\times 10^{-4}$, 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 $Re({ϵ}^{\prime }/ϵ)\lesssim 0.8\times 10^{-3}$; (b) $BR(K^{+}\to {\pi }^{+}\nu \bar{\nu })\lesssim 12\times 10^{-11}$ and $BR(K_L\to {\pi }^0\nu \bar{\nu })\lesssim 7.9\times 10^{-11}$; (c) the BR for $K_S\to {\mu }^{+}{\mu }^{-}$ including the long-distance effect can be enhanced by $11\%$ with respect to the standard model result, and (d) $BR(B_d\to {\mu }^{+}{\mu }^{-})\lesssim 1.97\times 10^{-10}$. In addition, although $Re({ϵ}^{\prime }/ϵ)$ cannot be synchronously enhanced with $BR(K_L\to {\pi }^0\nu \bar{\nu })$ and $BR(K_S\to {\mu }^{+}{\mu }^{-})$ in the same region of the CP-violating phase, the values of $Re({ϵ}^{\prime }/ϵ)$, $BR(K^{+}\to {\pi }^{+}\nu \bar{\nu })$, and $BR(B_d\to {\mu }^{+}{\mu }^{-})$ can be simultaneously increased.

Comments: 19 pages, 3 figures, version to be published in the European Physical Journal C