First Measurement of the Branching Fraction of e^+e^- --> B0B0bar

We report the first measurement of the absolute branching fraction e^+e^- --> B0B0bar at the Upsilon(4S) resonance using data collected with the BABAR detector at the PEP-II asymmetric-energy e^+e^- storage ring. The analysis is performed with partial reconstruction of the decay B0bar --> D^{*+}\ell^{-} \bar{\nu}_{\ell}$, where the presence of a signal decay is determined using only the lepton and the soft pion from the D^{*} decay. By reconstructing events with one or two signal decays we obtain a preliminary result of e^+e^- --> B0B0bar = 0.486 \pm 0.010(stat.) \pm 0.009(sys.)$. Our result does not depend on branching fractions of the B0bar and the D^{*+} decay chains, on the individual simulated reconstruction efficiencies, on the ratio of the charged and neutral B meson lifetimes, or on the assumption of isospin symmetry.