Measuring [Vub] with B→Ds+Xu transitions -: art. no. 093017

We propose the determination of the CKM matrix element \V-ub\ by the measurement of the spectrum of B --> Ds+Xu, dominated by the spectator quark model mechanism (b) over bar-->D-s((*)+)(u) over bar. The interest in considering B-->Ds+Xu versus the semileptonic decay is that more than 50% of the spectrum for B-->D-s(+) X-u occurs above the kinematical limit for B-->Ds+Xc, while most of the spectrum B-->lvX(u) occurs below the B-->lvX(c) one. Furthermore, the measure of the hadronic mass M-X is easier in the presence of an identified D-s than when a v has been produced. As a consistency check, we point out that the rate (b) over bar-->D-s((*)+)(c) over bar (including QCD corrections) is consistent with the measured BR(B-->Ds+/-X) by CLEO. Although the hadronic complications may be more severe in the decay mode that we propose than in the semileptonic inclusive decay, the end of the spectrum in B-->lvX(u), is not well understood on theoretical grounds. We argue that, in our case, the excited mesons D-s**. decaying into DK, do not contribute and, if there is tagging of the B meson, the other mechanisms to produce a D-s of the right sign are presumably small, of O(10(-2)) relative to the spectator amplitude, or can be controlled by kinematical cuts. We discuss in detail the hadronic uncertainties of the method and present an error calculation on \V-qb\ (q = u,c) and on the ratio \V-ub\/\V-cb\, for which most of the systematic errors cancel. In the absence of tagging, other hadronic backgrounds deserve careful study. We present a feasibility study with the BaBar detector.