Measurement of the Branching Fraction of Υ(4S)→B0¯¯¯B0

Abstract

We report the first measurement of the branching fraction f 00 for Υ ( 4 S ) → B 0 ¯¯¯ B 0 . The data sample consists of 81.7 fb − 1 collected at the Υ ( 4 S ) resonance with the BABAR detector at the SLAC PEP-II asymmetric-energy e + e − storage ring. Using partial reconstruction of the decay ¯¯¯ B 0 → D * + ℓ − ¯ ν ℓ in which only the charged lepton and the soft pion from the decay D * + → D 0 π + are reconstructed, we obtain f 00 = 0.487 ± 0.010 ( stat ) ± 0.008 ( syst ) . Our result does not depend on the branching fractions of ¯¯¯ B 0 → D * + ℓ − ¯ ν ℓ and D * + → D 0 π + decays, on the ratio of the charged and neutral B meson lifetimes, nor on the assumption of isospin symmetry.