First observation of B → D ¯ 1 (→ D ¯ π+π-) ℓ+ν ℓ and measurement of the B → D ¯ (∗)π ℓ+ν ℓ and B → D ¯ (∗)π+π- ℓ+ν ℓ branching fractions with hadronic tagging at Belle

We report measurements of the ratios of branching fractions for B→D¯(∗)πℓ+νℓ and B→D¯(∗)π+π-ℓ+νℓ relative to B→D¯∗ℓ+νℓ decays with ℓ=e, μ. These results are obtained from a data sample that contains 772×106BB¯ pairs collected near the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider. Fully reconstructing both B mesons in the event, we obtain B(B0→D¯0π-ℓ+νℓ)B(B0→D∗-ℓ+νℓ)=(7.23±0.36±0.14)%, B(B+→D-π+ℓ+νℓ)B(B+→D¯∗0ℓ+νℓ)=(6.78±0.24±0.18)%, B(B0→D¯∗0π-ℓ+νℓ)B(B0→D∗-ℓ+νℓ)=(11.10±0.48±0.23)%, B(B+→D∗-π+ℓ+νℓ)B(B+→D¯∗0ℓ+νℓ)=(9.50±0.33±0.34)%, B(B0→D-π+π-ℓ+νℓ)B(B0→D∗-ℓ+νℓ)=(2.91±0.37±0.26)%, B(B+→D¯0π+π-ℓ+νℓ)B(B+→D¯∗0ℓ+νℓ)=(3.10±0.26±0.22)%, B(B0→D∗-π+π-ℓ+νℓ)B(B0→D∗-ℓ+νℓ)=(0.99±0.43±0.20)%, B(B+→D¯∗0π+π-ℓ+νℓ)B(B+→D¯∗0ℓ+νℓ)=(1.25±0.27±0.15)%, where the uncertainties are statistical and systematic, respectively. These are the most precise measurements of these branching fraction ratios to date. The invariant mass spectra of the Dπ, D∗π, and Dππ systems are studied, and the branching fraction products B(B0→D2∗-ℓ+νℓ)×B(D2∗-→D¯0π-)=(0.157±0.015±0.005)%, B(B+→D¯0∗0ℓ+νℓ)×B(D¯0∗0→D-π+)=(0.054±0.022±0.005)%, B(B+→D¯2∗0ℓ+νℓ)×B(D¯2∗0→D-π+)=(0.163±0.011±0.008)%, B(B0→D1-ℓ+νℓ)×B(D1-→D¯∗0π-)=(0.306±0.050±0.029)%, B(B0→D1′-ℓ+νℓ)×B(D1′-→D¯∗0π-)=(0.206±0.068±0.025)%, B(B0→D2∗-ℓ+νℓ)×B(D2∗-→D¯∗0π-)=(0.051±0.040±0.010)%, B(B+→D¯10ℓ+νℓ)×B(D¯10→D∗-π+)=(0.249±0.023±0.015)%, B(B+→D¯1′0ℓ+νℓ)×B(D¯1′0→D∗-π+)=(0.138±0.036±0.009)%, B(B+→D¯2∗0ℓ+νℓ)×B(D¯2∗0→D∗-π+)=(0.137±0.026±0.009)%, B(B0→D1-ℓ+νℓ)×B(D1-→D-π+π-)=(0.102±0.013±0.009)%, B(B+→D¯10ℓ+νℓ)×B(D¯10→D¯0π+π-)=(0.105±0.011±0.009)%, are extracted. This is the first observation of the decays B→D¯1ℓ+νℓ with D1→Dπ+π-.