| Abstract |
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We report a measurement of the branching fraction e^+e^- --> B0B0bar
with a data sample 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}
we obtain a preliminary result of f00 = 0.486 +- 0.010(stat.) +- 0.009(sys.).
Our result does not depend on branching fractions of the B0bar
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 our understanding of isospin violation
in the Upsilon(4S) system.
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