Observation of B̅ → D* π⁺π⁻ℓ⁻ν̅ Decays in e⁺e⁻ Collisions at the ϒ(4s) Resonance

We report on measurements of the decays of B̅ mesons into the semileptonic final states B̅ → D^{()} π⁺π⁻ℓ⁻ν̅, where D^{(())} represents a D or D* meson and ℓ⁻ is an electron or a muon. These measurements are based on 471 x 10⁶ BB̅ pairs recorded with the BABAR detector at the SLAC asymmetric B factory PEP-II. We determine the branching fraction ratios R(π⁺π⁻)* = β(B̅ → D* π⁺π⁻ℓ⁻ν̅ /B (B̅ → Dℓ⁻ν̅) using events in which the second B meson is fully reconstructed. We find Rπ⁺π⁻ = 0.067 ± 0.010 ± 0.008 and R(π⁺π⁻) = 0.019 ± 0.005 ± 0.004, where the first uncertainty is statistical and the second is systematic. Based on these results and assuming isospin invariance, we estimate that B̅ → D^{(*)}ππ ℓ⁻ν̅ decays, where π denotes either a π^± and π⁰ 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.