Observation of $\overline{B}\to D^{(\ast )}{\pi }^{+}{\pi }^{-}{\ell }^{-}\overline{\nu }$ decays in $e^{+}{e}^{-}$ collisions at the ${\rm Y}(4S)$ resonance

Abstract

We report on measurements of the decays of $\overline{B}$ mesons into the semileptonic final states $\overline{B}\to D^{(\ast )}{\pi }^{+}{\pi }^{-}{\ell }^{-}\overline{\nu }$, where $D^{(\ast )}$ represents a $D$ or $D^{\ast }$ meson and ${\ell }^{-}$ is an electron or a muon. These measurements are based on $471\times 10^6$ $B\overline{B}$ pairs recorded with the $BABAR$ detector at the SLAC asymmetric $B$ Factory PEP-II. We determine the branching fraction ratios $R_{{\pi }^{+}{\pi }^{-}}^{(\ast )}=\mathcal{B}(\overline{B}\to D^{(\ast )}{\pi }^{+}{\pi }^{-}{\ell }^{-}\overline{\nu })/\mathcal{B}(\overline{B}\to D^{(\ast )}{\ell }^{-}\overline{\nu })$ using events in which the second $B$ meson is fully reconstructed. We find $R_{{\pi }^{+}{\pi }^{-}}=0.067\pm 0.010\pm 0.008$ and $R_{{\pi }^{+}{\pi }^{-}}^{\ast }=0.019\pm 0.005\pm 0.004$, where the first uncertainty is statistical and the second is systematic. Based on these results, we estimate that $\overline{B}\to D^{(\ast )}\pi \pi {\ell }^{-}\overline{\nu }$ decays, where $\pi$ denotes both a ${\pi }^{\pm }$ and ${\pi }^0$ meson, account for up to half the difference between the measured inclusive semileptonic branching fraction to charm hadrons and the corresponding sum of previously measured exclusive branching fractions.

8 pages, 2 figures, submitted to PRL