Measurement of $\bar{B^0}\to\pi^+\ell^-\bar{\nu}$ on the recoil of partially reconstructed $B^0$ decays

We present a measurement of the branching fraction $\bar{B^0}\rightarrow \pi^+ \ell^- \bar{\nu_\ell}$ and of $|V_{ub}|$ from a sample of $\Upsilon(4S) \to B\bar{B}$ decays collected by the BaBar detector at the asymmetric $e^+e^-$ collider PEP-II. We search for events compatible with $\bar{B^0}\rightarrow \pi^+ \ell^- \bar{\nu_\ell}$ decay inside an enriched $\bar{B^0}B^0$ sample selected using partial reconstruction of the decay $B^0 \rightarrow D^{*-} \ell^+ \nu_\ell$. With this technique, we can perform softer cuts on pion and lepton momenta, as compared to untagged analyses, and therefore obtain a smaller systematic error due to the cut on the phase-space.