Subleading shape function contributions to the hadronic invariant mass spectrum in B -> X_u l \nu decay

We study the O(Lambda/mb) corrections to the singly and doubly differential hadronic invariant mass spectra d\Gamma/dsH and d\Gamma/dsH dq^2 in b -> u decays, and discuss the implications for the extraction of the CKM matrix element |Vub|. Using simple models for the subleading shape functions, the effects of subleading operators are estimated to be at thefew percent level for experimentally relevant cuts. The subleading corrections proportional to the leading shape function are larger, but largely cancel in the relation between the hadronic invariant mass spectrum and the photon spectrum in B -> X_s \gamma. We also discuss the applicability of the usual prescription of convoluting the partonic level rate with the leading light-cone wavefunction of the b quark to subleading order.