Abstract
The charged lepton spectrum in semileptonic B→X uℓ ν ̄ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of Λ QCD/ m b but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width ΔE ℓ∼ Λ QCD near the endpoint the contributions proportional to δ( E ℓ− m b /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 | V ub | are discussed.
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