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

TL;DR

This paper measures the branching ratios of specific semileptonic B meson decays involving two pions, providing insights into the composition of inclusive semileptonic decay rates and their relation to exclusive channels.

Contribution

It presents the first measurements of the ratios of branching fractions for B meson decays into D(*) mesons with two pions and a lepton, using a large data sample from the BABAR experiment.

Findings

Measured the ratios R_{π+π-} and R^*_{π+π-} with statistical and systematic uncertainties.

Estimated that these decays account for up to half of the difference between inclusive and exclusive semileptonic branching fractions.

Provided new data to help understand the composition of semileptonic B decays.

Abstract

We report on measurements of the decays of $\overline{B}$ mesons into the semileptonic final states $\overline{B} \to D^{(*)} \pi^+\pi^- \ell^-\overline{\nu}$, where $D^{(*)}$ represents a $D$ or $D^*$ 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 $R^{(*)}_{\pi^+\pi^-} = \mathcal{B}(\overline{B} \to D^{(*)} \pi^+\pi^- \ell^- \overline{\nu})/\mathcal{B}(\overline{B} \to D^{(*)} \ell^-\overline{\nu})$ 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 $R^*_{\pi^+\pi^-} = 0.019 \pm 0.005 \pm 0.004$, where the first uncertainty is statistical and the second is systematic. Based on these results, we estimate that $\overline{B} \to D^{(*)}\pi\pi\ell^-\overline{\nu}$ 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.