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

Abstract

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^{-} \barν_{\ell}$,wherethepresenceofasignaldecayisdeterminedusingonlytheleptonandthesoftpionfromthe{D}^{\ast }decay.Byreconstructingeventswithoneortwosignaldecaysweobtainapreliminaryresultof{e}^{+}{e}^{-}-->B0B0bar=0.486\pm 0.010(stat.)\pm 0.009(sys.)$. 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.

Latex, 5 pages, 2 figures, proceedings to the DPF 2004, 26 August-31 August 2004, Riverside, CA, USA