Study of $CP$ violation in mixing with partially reconstructed $D^{*-} \ell^+ \nu$ events

We present a simultaneous measurement of the neutral $B$-meson lifetime difference $\Delta \Gamma$ and of the parameter $|q/p|$, relative to the $CP$ violation in $B$ mixing, using $D^{*-} \ell^+ \nu$ events. The semileptonic final state is partially reconstructed, i.e. only the slow pion from $D^{*-}$ decay and the lepton are used. The flavour of the other $B$ is determined by means of lepton tagging. The measurement is performed using a fit to the proper time difference distribution of the two mesons on a sample of 240 million $\Upsilon(4S) \rightarrow B\bar{B}$ events, collected by the BaBar detector at the PEP-II asymmetric-energy $B$ factory.