Hadronic invariant mass spectrum in B→X_ulv decay with a lepton energy cut

We discuss the implications of a charged lepton energy cut to the hadronic invariant mass spectrum in charmless semileptonic B decays. A charged-lepton energy cut is inevitable in order to remove secondary leptonic events such as b→c, τ→l, and to identify the charged-leptons at detectors experimentally. We consider three possible lepton energy cuts, E^cuts_l=0.6, 1.5, and 2.3 GeV, and find that with the most probable cuts E^cut_l=1.5 GeV and M^max_X=1.5 (1.86) GeV, 45-60% (58-67%) of decay events survive. Therefore, B→X_ulν decay events can be efficiently distinguished from B→X_clν decay events. We also discuss the possible model dependence of the results.