Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic $B\to X_u \ell\bar\nu$ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of $\Lambda_{QCD}/m_b$ but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width $\Delta E_\ell \sim \Lambda_{QCD}$ near the endpoint the contributions proportional to $\delta(E_\ell - m_b/2)$ 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.