Measurement of the Branching Fraction of ${\rm Y}(4S)\to B^0{\overline{B}}^0$

We report the first measurement of the branching fraction $f_{00}$ for ${\rm Y}(4S)\to B^0{\overline{B}}^0$. The data sample consists of $81.7{\mathrm{fb}}^{-1}$ collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the SLAC PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring. Using partial reconstruction of the decay ${\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell }$ in which only the charged lepton and the soft pion from the decay $D^{*+}\to D^0{\pi }^{+}$ are reconstructed, we obtain $f_{00}=0.487\pm 0.010(\mathrm{stat})\pm 0.008(\mathrm{syst})$. Our result does not depend on the branching fractions of ${\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell }$ and $D^{*+}\to D^0{\pi }^{+}$ decays, on the ratio of the charged and neutral $B$ meson lifetimes, nor on the assumption of isospin symmetry.

Figure 1

The ${\mathcal{M}}_s^2$ (top) and ${\mathcal{M}}_2^2$ (bottom) distributions for the on-resonance sample. The continuum background has been subtracted from the distributions. In addition, the ${\mathcal{M}}_1^2$-combinatorial and the ${\mathcal{M}}_1^2$-peaking backgrounds have been subtracted from the ${\mathcal{M}}_2^2$ distribution. The levels of the simulated signal, peaking $B\overline{B}$, and combinatorial $B\overline{B}$ background contributions are obtained from the fit.

Figure 2

The ${\mathcal{M}}_s^2$ (top) and ${\mathcal{M}}_2^2$ (bottom) distributions for the same-charge on-resonance sample. The continuum background has been subtracted from the distributions. The ${\mathcal{M}}_1^2$-combinatorial and the ${\mathcal{M}}_1^2$-peaking backgrounds have been subtracted from the ${\mathcal{M}}_2^2$ distribution. The level of the simulated combinatorial $B\overline{B}$ background is obtained from the fit.