Measurement of the Branching Fraction of e+e- --> B0(bar B)0 at the Υ (4S) Resonance

The authors report a measurement of the branching fraction e+e- --> B0(bar B)0 with a data sample of 81.7 fb-1 collected at the Υ(4S) resonance with the BABAR detector at the PEP-II asymmetric-energy e+e- storage ring. Using partial reconstruction of the decay (bar B)0 --> D*+(ell)-(bar ν)ell they obtain a preliminary result of f00 = 0.486 ± 0.010(stat.) ± 0.009(sys.). The result does not depend on branching fractions of the (bar B)0 and the D*+ decay chains, on the simulated reconstruction efficiency, on the ratio of the charged and neutral B meson lifetimes, nor on assumption of isospin symmetry. This measurement is important for normalizing many B decay branching fractions, and contributes to an understanding of isospin violation in the Υ(4S) system