Determination of the form factors for the decay $B^{0}\rightarrow D\ast^-l^{+}\nu_{l}$ and of the CKM matrix element |V$_{cb}$|
B. Aubert \fIet al.\fP

We present a combined measurement of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$ and of the parameters $\rho^2$, $R_1(1)$, and $R_2(1)$, which fully characterize the form factors for the $B^0 \rightarrow D^{*-}\ell^{+}\nu_\ell$ decay in the framework of HQET. The results, based on a selected sample of about 52,800 $B^0 \rightarrow D^{*-}\ell^{+}\nu_\ell$ decays, recorded by the \mbox{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}} detector, are $\rho^2 = 1.157 \pm 0.094 \pm 0.027$, $R_1(1) = 1.327 \pm 0.131 \pm 0.043$, $R_2(1) = 0.859 \pm 0.077 \pm 0.021$, and $\mathcal{F}(1)|V_{cb}| = (34.7 \pm 0.4 \pm 1.0) \times 10^{-3}$. The first error is the statistical and the second is the systematic uncertainty. Combining these measurements with the previous \mbox{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}} measurement of the form factors, which employs a different fit technique on a partial sample of the data, we improve the statistical precision of the result, $\rho^2 = 1.191 \pm 0.048 \pm 0.028, R_1(1) = 1.429 \pm 0.061 \pm 0.044, R_2(1) = 0.827 \pm 0.038 \pm 0.022, $ and $ \mathcal{F}(1)|V_{cb}| = (34.4 \pm 0.3 \pm 1.1) \times 10^{-3}.$ Using lattice calculations for the axial form factor $\mathcal{F}(1)$, we extract $|V_{cb}| =(37.4 \pm 0.3 \pm 1.2 \pm ^{1.2}_{1.4} ) \times 10^{-3}$, where the third error is due to the uncertainty in $\mathcal{F}(1)$. We also present a measurement of the exclusive branching fraction, ${\cal B} = (4.69 \pm 0.04 \pm 0.34)\%$.

© 2008 The American Physical Society.