We study a model with a down-type SU(2) singlet vector-like quark (VLQ) as a minimal extension of the standard model (SM). In this model, flavor changing neutral currents (FCNCs) arise at tree level and the unitarity of the $3times 3$ Cabibbo-Kobayashi-Maskawa (CKM) matrix does not hold. In this paper, we constrain the FCNC coupling from $brightarrow s$ transitions, especially $B_srightarrow mu^+mu^-$ and $bar{B}rightarrow X_sgamma$ processes. In order to analyze these processes, we derive an effective Lagrangian which is valid below the electroweak symmetry breaking scale. For this purpose, we first integrate out the VLQ field and derive an effective theory by matching Wilson coefficients up to one-loop level. Using the effective theory, we construct the effective Lagrangian for $brightarrow sgamma^{(*)}$. It includes the effects of the SM quarks and the violation of the CKM unitarity. We show the constraints on the magnitude of the FCNC coupling and its phase by taking account of the current experimental data on $Delta M_{B_s}$, $mathrm{Br}[B_srightarrowmu^+mu^-]$, $mathrm{Br}[bar{B}rightarrow X_sgamma]$ and CKM matrix elements as well as theoretical uncertainties. We find that the constraint from the $mathrm{Br}[B_srightarrowmu^+mu^-]$ is more stringent than that from the $mathrm{Br}[bar{B}rightarrow X_sgamma$]. We also obtain the bound for the mass of the VLQ and the strength of the Yukawa couplings related to the FCNC coupling of $brightarrow s$ transition. Using the CKM elements which satisfy above constraints, we show how the unitarity is violated on the complex plane.
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