First Measurement of the Branching Fraction of e+e- to B0B0bar

We report the first measurement of the absolute branching fraction e+e- → B0(bar B)0 at the Υ(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 (bar B)0 → D*+(ell)-(bar ν)paralell, 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- → (bar B)0(bar B)0 = 0.486 ± 0.010(stat.) ± 0.009(sys.). Our result does not depend on branching fractions of the (bar B)0 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