Probing the B meson DA with B to l+l-l’nu

Studying radiative leptonic B decays provide insight into the internal structure of the B-meson. Among these processes, the radiative decay (B to l nu gamma) offers a particularly clean probe of the nonperturbative dynamics of the B meson. In this talk, I will discuss how this can be generalised to the decay (B to l+l-l' nu ), where the photon is off shell and converts into a lepton pair, which is more suitable for LHCb. I will show how the corresponding form factors can be calculated in QCD factorization, and present a framework to quantify soft QCD effects and improve theoretical predictions for these rare decays.