Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic B→Xuℓν̄ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of ΛQCD/mb but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width ΔEℓ∼ΛQCD near the endpoint the contributions proportional to δ(Eℓ−mb/2) times (i) the matrix element of the chromomagnetic operator, and (ii) four-quark operators. These contribute to the total rate at the few percent level, but affect the endpoint region much more significantly. Implications for the determination of |Vub| are discussed.