Anomalous $tqγ$ coupling effects in exclusive radiative B-meson decays

Abstract

The top-quark FCNC processes will be searched for at the CERN LHC, which are correlated with the B-meson decays. In this paper, we study the effects of top-quark anomalous interactions $tqγ$ in the exclusive radiative $B\to K^*γ$ and $B\toργ$ decays. With the current experimental data of the branching ratios, the direct CP and the isospin asymmetries, bounds on the coupling $κ_{tcR}^γ$ from $B\to K^*γ$ and $κ_{tuR}^γ$ from $B\to ργ$ decays are derived, respectively. The bound on $|κ_{tcR}^γ|$ from ${\mathcal B}(B\to K^{*}γ)$ is generally compatible with that from ${\mathcal B}(B\to X_{s}γ)$. However, the isospin asymmetry $Δ(K^{*}γ)$ further restrict the phase of $κ_{tcR}^γ$, and the combined bound results in the upper limit, $\mathcal B(t\to cγ)<0.21%$, which is lower than the CDF result. For real $κ_{tcR}^γ$, the upper bound on $\mathcal B(t\to cγ)$ is about of the same order as the $5σ$ discovery potential of ATLAS with an integrated luminosity of $10 {\rm fb}^{-1}$. For $B\toργ$ decays, the NP contribution is enhanced by a large CKM factor $|V_{ud}/V_{td}|$, and the constraint on $tuγ$ coupling is rather restrictive, $\mathcal B(t\to uγ)<1.44\times 10^{-5}$. With refined measurements to be available at the LHCb and the future super-B factories, we can get close correlations between $B\to V γ$ and the rare $t\to qγ$ decays, which will be studied directly at the LHC ATLAS and CMS.