2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/135999We present the first measurement of the $B^0\to X_{u}^- \ell^+ {ν_\ell} $ partial branching fraction in the end-point region of the lepton momentum spectrum, above the threshold for $B\to X_c\ell {ν_\ell}$ decays. The analysis is based on a sample of 383 million $Υ(4S)$ decays into $B{\bar B}$ pairs collected with the {\sc BaBar} detector at the PEP-II $e^+e^-$ storage rings. We select $B^0{\bar B}^0$ events by partially reconstructing one $\B$ meson via the ${\bar B}^0\to D^{*+} \ell^- \barν_\ell$ decays then select $B^0\to X_{u}^- \ell^+ {ν_\ell} $ decays identifying a second high momentum lepton. In the momentum interval ranging from 2.3 to 2.6 GeV/c we measure the partial branching fraction $Δ{\cal B}(B^0\to X_u\ellν)=(1.30\pm0.21_{stat}\pm0.07_{syst})\times 10^{-4}$ where the first error is statistical and the second is systematic. By comparing this measurement with the one obtained from untagged B decays we obtain $R^{+/0}=Δ{\cal B}(B^0\to X_u\ellν)/Δ{\cal B}(B^+\to X_u\ellν)= 1.18 \pm 0.35_{stat} \pm 0.17_{syst}$. Using this measurement we extract a limit on the contributions from processes breaking isospin symmetry in charmless semileptonic B decays.17 pages, 8 postscript figures, submitted to Lepton Photon 2007High Energy Physics - Experimenttext