Charm-loop effect in $B\to K^{(\ast )}{\ell }^{+}{\ell }^{-}$ and $B\to K^{\ast }\gamma$

Abstract

We calculate the long-distance effect generated by the four-quark operators with $c$-quarks in the $B\to K^{(\ast )}{\ell }^{+}{\ell }^{-}$ decays. At the lepton-pair invariant masses far below the $\bar{c}c$-threshold, $q^2\ll 4m_c^2$, we use OPE near the light-cone. The nonfactorizable soft-gluon emission from $c$-quarks is cast in the form of a nonlocal effective operator. The $B\to K^{(\ast )}$ matrix elements of this operator are calculated from the QCD light-cone sum rules with the $B$-meson distribution amplitudes. As a byproduct, we also predict the charm-loop contribution to $B\to K^{\ast }\gamma$ beyond the local-operator approximation. To describe the charm-loop effect at large $q^2$, we employ the hadronic dispersion relation with $\psi =J/\psi ,\psi (2S),...$ contributions, where the measured $B\to K^{(\ast )}\psi$ amplitudes are used as inputs. Matching this relation to the result of QCD calculation reveals a destructive interference between the $J/\psi$ and $\psi (2S)$ contributions. The resulting charm-loop effect is represented as a $q^2$-dependent correction $\Delta C_9(q^2)$ to the Wilson coefficient $C_9$. Within uncertainties of our calculation, at $q^2$ below the charmonium region the predicted ratio $\Delta C_9(q^2)/C_9$ is $\le 5$ for $B\to K{\ell }^{+}{\ell }^{-}$, but can reach as much as 20% for $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$, the difference being mainly caused by the soft-gluon contribution.

Comments: A few comments added, version to appear in JHEP