Determination of ${\rm |V_{ub}|}$ from the measurement of the inclusive charmless semileptonic branching ratio of b hadrons

Abstract.

From a study of the kinematic properties of the final state produced in the semileptonic decays ${\rm b\to X\ell\nu_{\ell}}$, the inclusive charmless semileptonic branching ratio of b hadrons is measured. With a sample of 3.6 million hadronic Z decays recorded between 1992 and 1995 with the ALEPH detector at LEP, the value ${\rm Br(b\to X_u\ell\nu_{\ell})}$ is determined to be $(1.73\pm 0.55_{{\rm stat}}\pm 0.55_{{\rm syst}})\times 10^{-3}$, where ${\rm X_u}$ represents any charmless hadronic state and b is a mixture of b hadrons weighted by their production rates. This measurement yields the result \[ {\rm |V_{ub}|^2}= (18.68\pm 5.94_{{\rm stat}}\pm 5.94_{{\rm syst}}\pm 1.45_{HQE})\times 10^{-6}, \] where the last error comes from the conversion of the branching ratio to the CKM matrix element squared.