The ratio of the CKM quark-mixing matrix elements \V-ub\/\V-cb\ has been me
asured using B hadron semileptonic decays. The analysis uses the reconstruc
ted mass M-X of the secondary hadronic system produced in association with
an identified lepton. Since B --> X(u)l (v) over bar transitions are charac
terised by hadronic masses below those of the D mesons produced in B --> X(
c)l (v) over bar transitions, events with a reconstructed value of M-X sign
ificantly below the D mass are selected. Further signal enrichments are obt
ained using the topology of reconstructed decays and hadron identification.
A fit to the numbers of decays in the b --> u enriched and depleted sample
s with M-X above and below 1.6 GeV/c(2) and to the shapes of the lepton ene
rgy distribution in the B rest frame gives \V-ub\/\V-cb\ = 0.103 (+0.011)(-
0.012) (stat.) +/- 0.016(syst.) +/- 0.010(model) and, correspondingly, a ch
armless semileptonic B decay branching fraction of BR(B --> X(u)l (v) over
bar)= (1.57 +/- 0.35 (stat.) +/- 0.48(syst.) +/- 0.27(model)) x 10(-3). (C)
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