Probing the structure of the $B$ meson with $B\to\ell\ell\ell'\nu$
We consider the decay $B\to\ell\ell\ell^{\prime}\nu$, taking into account the leading $1/m_b$ and $q^2$ corrections calculated in the QCD factorization framework as well as the soft corrections calculated employing dispersion relations and quark-hadron duality. We extend the existing results for the radiative decay $B\to\gamma\ell\nu$ to the case of non-zero (but small) $q^2$, the invariant mass squared of the dilepton pair $\ell^+\ell^-$. This restricts us to the case $\ell\neq\ell'$ as otherwise the same sign $\ell$ and $\ell'$ cannot be distinguished. We further study the sensitivity of the results to the leading moment of the $B$-meson distribution amplitude and discuss the potential to extract this quantity at LHCb and the Belle II experiment.