Search for decays of $B^0$ mesons into $e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$ and $e^{\pm }{\mu }^{\mp }$ final states

We present a search for the decays $B^0\to e^{+}{e}^{-}$, $B^0\to {\mu }^{+}{\mu }^{-}$, and $B^0\to e^{\pm }{\mu }^{\mp }$ using data collected with the BABAR detector at the PEP-II $e^{+}{e}^{-}$ collider at SLAC. Using a data set corresponding to $384\times {10}^6$ $B\overline{B}$ pairs, we do not find evidence of any of the three decay modes. We obtain upper limit on the branching fractions, at 90% confidence level, of $\mathcal{B}(B^0\to e^{+}{e}^{-})<11.3\times {10}^{-8}$, $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<5.2\times {10}^{-8}$, and $\mathcal{B}(B^0\to e^{\pm }{\mu }^{\mp })<9.2\times {10}^{-8}$.

Figure 1

Representative Feynman diagrams for $B^0\to l^{+}{l}^{-}$ in the standard model.

Figure 2

Distribution of events in $m_{\mathrm{ES}}$ (a, b, c), $\Delta E$ (d, e, f), and $\mathcal{F}$ (g, h, i) for $B^0\to e^{+}{e}^{-}$ (left), $B^0\to {\mu }^{+}{\mu }^{-}$ (middle), and $B^0\to e^{\pm }{\mu }^{\mp }$ (right). The overlaid solid curve in each plot is the background $s\mathrm{Plot}$ [22] distribution obtained by maximizing the likelihood not using the information from the corresponding component. The dotted line is the PDF obtained from signal Monte Carlo with arbitrary normalization.

Figure 3

Distribution of the likelihood as function of the $BF$ for $e^{+}{e}^{-}$ (a), ${\mu }^{+}{\mu }^{-}$ (b), and $e^{\pm }{\mu }^{\mp }$ (c) decays.