Observation of B ̄ →d (∗) π+π- ν ̄ Decays in e+e- Collisions at the Υ (4S) Resonance

We report on measurements of the decays of B ̄ mesons into the semileptonic final states B ̄→D(∗)π+π-ν ̄, where D(∗) represents a D or D∗ meson and - is an electron or a muon. These measurements are based on 471×106 BB ̄ pairs recorded with the BABAR detector at the SLAC asymmetric B factory PEP-II. We determine the branching fraction ratios Rπ+π-(∗)=B(B ̄→D(∗)π+π-ν ̄)/B(B ̄→D(∗)-ν ̄) using events in which the second B meson is fully reconstructed. We find Rπ+π-=0.067±0.010±0.008 and Rπ+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 B ̄→D(∗)ππ-ν ̄ 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.