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We present the first measurement of the $B^0\to X_{u}^- \ell^+ {\nu_\ell} $partial branching fraction in the end-point region of the lepton momentumspectrum, above the threshold for $B\to X_c\ell {\nu_\ell}$ decays. Theanalysis is based on a sample of 383 million $\Upsilon(4S)$ decays into $B{\barB}$ 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 ${\bar B}^0\to D^{*+} \ell^- \bar{\nu}_\ell$ decays thenselect $B^0\to X_{u}^- \ell^+ {\nu_\ell} $ decays identifying a second highmomentum lepton. In the momentum interval ranging from 2.3 to 2.6 GeV/c wemeasure the partial branching fraction $\Delta{\cal B}(B^0\toX_u\ell\nu)=(1.30\pm0.21_{stat}\pm0.07_{syst})\times 10^{-4}$ where the firsterror is statistical and the second is systematic. By comparing thismeasurement with the one obtained from untagged B decays we obtain$R^{+/0}=\Delta{\cal B}(B^0\to X_u\ell\nu)/\Delta{\cal B}(B^+\to X_u\ell\nu)=1.18 \pm 0.35_{stat} \pm 0.17_{syst}$. Using this measurement we extract alimit on the contributions from processes breaking isospin symmetry incharmless semileptonic B decays.

Measurement of the $B^0\to X_{u}^- \ell^+ {\nu_\ell} $ decays near the  kinematic endpoint of the lepton spectrum and search for violation of isospin  symmetry