Magnetic-dipole corrections to $R_{K}$ and $R_{K^*}$ in the Standard Model and Dark Photon scenarios

Abstract: In this work we evaluate the long-distance QED contributions, induced by the magnetic-dipole corrections to the final charged leptons, on the $B$ meson decay widths $B\to (K,K^*) \ell^+\ell^-$ and ratios $R_{K,K^*}=\Gamma(B\to (K,K^*) \mu^+\mu^-)/\Gamma(B\to (K,K^*) e^+e^-)$, as well as on $R^{\tau}_{K,K^*}$ (with $\mu$ replaced by the $\tau$ lepton). QED long-distance contributions induced by the Coulomb potential corrections (Fermi-Sommerfeld factors) were also included. Corresponding corrections to the inclusive decay widths of $B\to X_s \ell^+\ell^-$, with $\ell=e,\mu,\tau$, are also analyzed for completeness. The magnetic-dipole corrections, which are manifestly Lepton Flavor Universality violating and gauge-invariant, are expected to be particularly enhanced in $R_{K^*}$ for the dilepton mass region close to the threshold. However, we find that the largest contribution of all these corrections to the $R_{K,K^*}$ observables do not exceed a few per mille effect, thus reinforcing the validity of previous estimates about the leading QED corrections to $R_{K,K^*}$. Finally, viable new physics contributions to $R_{K,K^{*}}$ induced by the exchange of a massless dark-photon via magnetic-dipole interactions, which provide the leading contribution to the corresponding $B\to (K,K^*) \ell^+\ell^-$ amplitudes in this scenario, are analyzed in light of the present $R_{K,K^{*}}$ anomalies.