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

• Main Authors: Chuan-Hung Chen, Takaaki Nomura
• Format: Article
• Language: English
• Published: SpringerOpen 2019-08-01
• Series: European Physical Journal C: Particles and Fields
• Online Access: http://link.springer.com/article/10.1140/epjc/s10052-019-7133-5
• ISSN: 1434-6044; 1434-6052

Abstract We study the impacts of anomalous tqZ couplings ($$q=u,c$$ q=u,c ), which lead to the $$t\rightarrow q Z$$ t→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 $$BR(t\rightarrow uZ) < 1.7\times 10^{-4}$$ BR(t→uZ)<1.7×10-4 , 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 $$Re(\epsilon '/\epsilon ) \lesssim 0.8 \times 10^{-3}$$ Re(ϵ′/ϵ)≲0.8×10-3 ; (b) $$BR(K^+\rightarrow \pi ^+ \nu {\bar{\nu }}) \lesssim 12 \times 10^{-11}$$ BR(K+→π+νν¯)≲12×10-11 and $$BR(K_L \rightarrow \pi ^0 \nu {\bar{\nu }})\lesssim 7.9 \times 10^{-11}$$ BR(KL→π0νν¯)≲7.9×10-11 ; (c) the BR for $$K_S \rightarrow \mu ^+ \mu ^-$$ KS→μ+μ- including the long-distance effect can be enhanced by $$11\%$$ 11% with respect to the standard model result, and (d) $$BR(B_d \rightarrow \mu ^+ \mu ^-) \lesssim 1.97 \times 10^{-10}$$ BR(Bd→μ+μ-)≲1.97×10-10 . In addition, although $$Re(\epsilon '/\epsilon )$$ Re(ϵ′/ϵ) cannot be synchronously enhanced with $$BR(K_L\rightarrow \pi ^0 \nu {\bar{\nu }})$$ BR(KL→π0νν¯) and $$BR(K_S\rightarrow \mu ^+ \mu ^-)$$ BR(KS→μ+μ-) in the same region of the CP-violating phase, the values of $$Re(\epsilon '/\epsilon )$$ Re(ϵ′/ϵ) , $$BR(K^+ \rightarrow \pi ^+ \nu {\bar{\nu }})$$ BR(K+→π+νν¯) , and $$BR(B_d \rightarrow \mu ^+ \mu ^-)$$ BR(Bd→μ+μ-) can be simultaneously increased.