Search for the decays $B^0\to e^{+}{e}^{-}\gamma$ and $B^0\to {\mu }^{+}{\mu }^{-}\gamma$

We present results of a search for the decays $B^0\to {\ell }^{+}{\ell }^{-}\gamma$ ($\ell =e$ or $\mu$). The search is performed using $320\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the PEP-II $B$ Factory at SLAC. We find no significant signal and set the following branching fraction upper limits at the 90% confidence level: $\mathcal{B}(B^0\to e^{+}{e}^{-}\gamma )<1.2\times {10}^{-7}$ and $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-}\gamma )<1.6\times {10}^{-7}$.

Figure 1

The penguin (top) and box (bottom) Feynman diagrams for $B^0\to {\ell }^{+}{\ell }^{-}\gamma$ ($\ell =e,\mu$) decays. The photon can be emitted from any of the quarks or leptons, but the amplitudes are largest if the photon is emitted from one of the initial quarks.

Figure 2

Definitions of the signal box, blinded area (equal to the sum of the signal box and of the shaded area), and sideband areas in the $\Delta E$ vs $m_{\mathrm{ES}}$ plot: Upper1 (U1), Upper2 (U2), Lower1 (L1), Lower2 (L2), and Middle1 (M1). The signal box has the same $\Delta E$ range as the M1 box (different for each mode), and the same $m_{\mathrm{ES}}$ limits as the U2 and L2 boxes. The figure is not drawn to scale.

Figure 3

Distribution of events in $m_{\mathrm{ES}}$ and $\Delta E$. The left plot is for the $e^{+}{e}^{-}\gamma$ mode and the right plot is for the ${\mu }^{+}{\mu }^{-}\gamma$ mode. The dots are the events outside the signal box (rectangular region), and the triangles are the events inside the signal box.