Measurement of $|{\rm V}_{\rm ub}|$ using $b$ hadron semileptonic decay

Abstract.

The magnitude of the CKM matrix element $|{\rm V}_{\rm ub}|$ is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on $b \to{\rm X}_{\rm u} \ell \nu$ event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br($b \to{\rm X}_{\rm u} \ell \nu$) to be ($1.63 \pm 0.53^{+0.55}_{-0.62}) \times 10^{-3}$. The first uncertainty is the statistical error and the second is the systematic error. From this analysis, $|{\rm V}_{\rm ub}|$ is determined to be:

\[|{\rm V}_{\rm ub}| = (4.00 \pm 0.65 ({\rm stat}) ^{+0.67}_{-0.76} ({\rm sys}) \pm 0.19 ({\rm HQE})) \times 10^{-3}. \]

The last error represents the theoretical uncertainties related to the extraction of $|{\rm V}_{\rm ub}|$ from Br($b \to{\rm X}_{\rm u} \ell \nu$) using the Heavy Quark Expansion.