| Abstract |
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$CP$ violation in $B^0 \kern 0.18em\overline{\kern -0.18em B}{}^0$ mixing
is characterized by the value of the parameter
$|q/p|$ being different from 1, and the Standard Model predicts this
difference to be smaller than $10^{-3}$. We present a measurement of this
parameter using a partial reconstruction of one of the $B$ mesons in the
semileptonic channel $D^{*-} \ell^+ \nu_{\ell}$, where only the hard lepton
and the soft pion from the $D^{*-} \rightarrow {\bar D^0} \pi^-$ decay are
reconstructed.
The flavor of the other $B$ is determined by means of lepton tagging.
The determination of $|q/p|$ is then performed with a fit to the proper time
difference of the two $B$ decays.
We use a luminosity of 200.8 $\mathrm{fb}^{-1}$, collected
at the $\Upsilon(4S)$ resonance by the BaBar detector at the PEP-II
asymmetrical-energy $e^+e^-$ collider, in the period 1999-2004. We obtain
the preliminary result:
\begin{equation}
\nonumber
|q/p| - 1 = (6.5 \pm 3.4(\mathrm{stat.}) \pm 2.0(\mathrm{syst.})) \cdot 10^{-3}
\end{equation}
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