Observation of B[over ¯]→D^{(*)} π^{+}π^{-}ℓ^{-}ν[over ¯] Decays in e^{+}e^{-} Collisions at the Υ(4S) Resonance.

Lees JP , Poireau V , Tisserand V , Grauges E , Palano A , Eigen G , Stugu B , Brown DN , Kerth LT , Kolomensky YG , Lee MJ , Lynch G , Koch H , Schroeder T , Hearty C , Mattison TS , McKenna JA , So RY , Khan A , Blinov VE , Buzykaev AR , Druzhinin VP ,
Physical review letters
We report on measurements of the decays of B[over ¯] mesons into the semileptonic final states B[over ¯]→D^{(*)}π^{+}π^{-}ℓ^{-}ν[over ¯], where D^{(*)} represents a D or D^{*} meson and ℓ^{-} is an electron or a muon. These measurements are based on 471×10^{6} BB[over ¯] pairs recorded with the BABAR detector at the SLAC asymmetric B factory PEP-II. We determine the branching fraction ratios R_{π^{+}π^{-}}^{(*)}=B(B[over ¯]→D^{(*)}π^{+}π^{-}ℓ^{-}ν[over ¯])/B(B[over ¯]→D^{(*)}ℓ^{-}ν[over ¯]) using events in which the second B meson is fully reconstructed. We find R_{π^{+}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 B[over ¯]→D^{(*)}ππℓ^{-}ν[over ¯] decays, where π denotes either a π^{±} and π^{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.

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