The hadronic recoil mass spectrum in semileptonic decays and extracting in a model-insensitive way

.   We present an extended discussion of the previously noted possibility to extract from an analysis of the hadronic recoil mass spectrum in decays. Invariant mass spectra containing perturbative as well as nonperturbative corrections are given; their shape is manifestly sensitive to the three basic quantities , and , whereas the total integrated rate is much less so. Only a small fraction of transitions generates a recoil mass of at least . Moreover we find that the fraction of events with (to reject leakage from due to measurement errors) exhibits fairly little dependence on , and ; can then be extracted in a largely model-insensitive way. This conclusion is based on the applicability of the OPE to actual semileptonic 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.