Measurement of the Branching Fraction of Upsilon(4S) --> B0B0bar

We report the first measurement of the branching fraction f00 for Upsilon(4S) --> B0B0bar. The data sample consists of 81.7 fb^-1 collected at the Upsilon(4S) resonance with the BABAR detector at the PEP-II asymmetric-energy e^+e^- storage ring. Using partial reconstruction of the decay B0bar --> D^{*+} \ell^{-} \bar{\nu}_{\ell} in which only the charged lepton and the soft pion from the decay D^{*+} --> D0 pi^+ are reconstructed, we obtain f00 = 0.487 \pm 0.010(stat) \pm 0.008(sys). Our result does not depend on the branching fractions of B0bar --> D^{*+} \ell^{-} \bar{\nu}_{\ell} and D^{*+} --> D0 pi^+ decays, on the ratio of the charged and neutral B meson lifetimes, nor on the assumption of isospin symmetry.