Motivated by the first search for the rare charged-current $B$ decay to four leptons, $\ell \bar{ \nu}_\ell \ell^{ (\prime)} \bar{ \ell}^{ (\prime)}$, we calculate the decay amplitude with factorization methods. We obtain the $B\to \gamma^*$ form factors, which depend on the invariant masses of the two lepton pairs, at leading power in an expansion in $\Lambda_{ \rm QCD}/m_b$ to next-to-leading order in $\alpha_s$, and at $\mathcal{ O}(\alpha_s^0)$ at next-to-leading power. Our calculations predict branching fractions of a few times $10^{ -8}$ in the $\ell^{ (\prime)} \bar{ \ell}^{ (\prime)}$ mass-squared bin up to $q^2=1~\text{ GeV}^2$ with $n_+q>3~$GeV. The branching fraction rapidly drops with increasing $q^2$. An important further motivation for this investigation has been to explore the sensitivity of the decay rate to the inverse moment $\lambda_B$ of the leading-twist $B$ meson light-cone distribution amplitude. We find that in the small-$q^2$ bin, the sensitivity to $\lambda_B$ is almost comparable to $B^- \rightarrow \ell^- \bar{ \nu}_\ell\gamma$ when $\lambda_B$ is small, but with an added uncertainty from the light-meson intermediate resonance contribution. The sensitivity degrades with larger $q^2$.
Categories: hep-ph