Observation of B¯ → D* π+π-l− ¯ν Decays in e+e− Collisions at the Y(4S) Resonance

Abstract

We report on measurements of the decays of B¯ mesons into the semileptonic final states B¯ → DðÞπþπ−l−ν¯, where DðÞ represents a D or D meson and l− 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 RðÞ πþπ− ¼ BðB¯ → DðÞπþπ−l−ν¯Þ=BðB¯ → DðÞl−ν¯Þ using events in which the second B meson is fully reconstructed. We find Rπþπ− ¼ 0.067 0.010 0.008 and R πþπ− ¼ 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¯ → DðÞππl−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.