Search for Decays of $B^0\to e^{+}{e}^{-}$, $B^0\to {\mu }^{+}{\mu }^{-}$, $B^0\to e^{\pm }{\mu }^{\mp }$

We present a search for the decays $B^0\to e^{+}{e}^{-}$, $B^0\to {\mu }^{+}{\mu }^{-}$, and $B^0\to e^{\pm }{\mu }^{\mp }$ in data collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the SLAC $B$ Factory. Using a data set of $111{\mathrm{fb}}^{-1}$, we find no evidence for a signal in any of the three channels investigated and set the following branching fraction upper limits at the 90% confidence level: $\mathcal{B}(B^0\to e^{+}{e}^{-})<6.1\times {10}^{-8}$, $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<8.3\times {10}^{-8}$, and $\mathcal{B}(B^0\to e^{\pm }{\mu }^{\mp })<18\times {10}^{-8}$.

Figure 1

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

Figure 2

Distributions of signal MC (hatched) and sideband data (points) for the $e^{+}{e}^{-}$ channel after the initial selection cuts for (a) $|\cos {\theta }_T|$, (b) $m_{\mathrm{ROE}}$, (c) $R_2$, and (d) $N_{\mathrm{mult}}$. Arrows indicate final cut values. All distributions are normalized to unit area.

Figure 3

Distribution of events in ${\mathrm{m}}_{\mathrm{ES}}$ and $\Delta E$ for $B^0\to e^{+}{e}^{-}$ (top left), $B^0\to {\mu }^{+}{\mu }^{-}$ (top right), and $B^0\to e^{\pm }{\mu }^{\mp }$ (bottom).