Soft photon problem in leptonic B-decays

We point out at the peculiarity of B→μνμ decay, namely the enhancement of the soft photon events which originate from the structure dependent part of the B→μνμγ amplitude. This may be a dominant source of systematic uncertainty and compromise the projected experimental uncertainty on Γ(B→μνμ). We show that the effect of these soft photons can be controlled if the experimental cut on identification of soft photons is lowered and especially if the better resolution in identifying the momentum of muon emerging from B→μνμ is made. A lattice QCD computation of the relevant form factors would be highly helpful for a better numerical control over the structure dependent soft photon emission.