Measurement of the Branching Fraction of Υ(4S)→B0B¯0

We report the first measurement of the branching fraction f00 for Υ(4S)→B0B̅ 0. The data sample consists of 81.7  fb-1 collected at the Υ(4S) resonance with the BABAR detector at the SLAC PEP-II asymmetric-energy e+e- storage ring. Using partial reconstruction of the decay B̅ 0→D*+ℓ-ν̅ ℓ in which only the charged lepton and the soft pion from the decay D*+→D0π+ are reconstructed, we obtain f00=0.487±0.010(stat)±0.008(syst). Our result does not depend on the branching fractions of B̅ 0→D*+ℓ-ν̅ ℓ and D*+→D0π+ decays, on the ratio of the charged and neutral B meson lifetimes, nor on the assumption of isospin symmetry.