The Hadronic Recoil Mass Spectrum in Semileptonic B Decays and Extracting |V_(ub)| in a Model-Insensitive Way

We present an extended discussion of the previously noted possibility to extract |V_{ub}| from an analysis of the hadronic recoil mass spectrum in B->X_u l nu decays. Invariant mass spectra containing perturbative as well as nonperturbative corrections are given; their shape is manifestly sensitive to the three basic quantities mu_pi^2, m_b, and alpha_s$, whereas the total integrated rate is much less so. Only a small fraction of b->u transitions generates a recoil mass M_X of at least M_D. We find that the fraction of events with M_X < 1.5 GeV (to reject leakage from b->c) exhibits fairly little dependence on mu_pi^2, m_b and alpha_s; |V_{ub}| can then be extracted in a largely model-insensitive way. This conclusion is based on the applicability of the OPE to actual semileptonic B decays. A direct cross-check of this assumption and a determination of the required basic parameters of the heavy quark expansion will be possible in the future with more experimental data.