Determination of |Vub|/|Vcb| with DELPHI at LEP

The ratio of the CKM quark-mixing matrix elements \V-ub\/\V-cb\ has been measured using B hadron semileptonic decays. The analysis uses the reconstructed 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 characterised 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 significantly below the D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the b --> u enriched and depleted samples with M-X above and below 1.6 GeV/c(2) and to the shapes of the lepton energy 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 charmless 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) 2000 Published by Elsevier Science B.V. All rights reserved.