THE HADRONIC RECOIL MASS-SPECTRUM IN SEMILEPTONIC B-DECAYS AND EXTRACTING VERTICAL-BAR-V-UB-VERTICAL-BAR 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 spectru m in B --> Xu rho upsilon decays. Invariant mass spectra containing pe rturbative as well as nonperturbative corrections are given; their sha pe 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. On ly a small fraction of b --> u transitions generates a recoil mass M-X of at least M-D. Moreover we find that the fraction of events with M- X less than or equal to 1.5 GeV (to reject leakage from b --> c due to measurement errors) exhibits fairly little dependence on mu(pi)(2), m (b) and alpha(s); \V-ub\ can then be extracted in a largely model-inse nsitive way This conclusion is based on the applicability of the OPE t o actual semileptonic B decays. A direct cross-check of this assumptio n and a determination of the required basic parameters of the heavy qu ark expansion will be possible in the future with more experimental da ta.