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

Abstract

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 momentum spectrum, above the threshold for $B\to X_c\ell {\nu }_{\ell }$ decays. The analysis is based on a sample of 383 million ${\rm Y}(4S)$ 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 ${\bar{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays then select $B^0\to X_u^{-}{\ell }^{+}{\nu }_{\ell }$ 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 $\Delta \mathcal{B}(B^0\to X_u\ell \nu )=(1.30\pm 0.21_{stat}\pm 0.07_{syst})\times 10^{-4}$ 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 $R^{+/0}=\Delta \mathcal{B}(B^0\to X_u\ell \nu )/\Delta \mathcal{B}(B^{+}\to X_u\ell \nu )=1.18\pm 0.35_{stat}\pm 0.17_{syst}$. 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 2007