Search for the Rare Leptonic Decay $B^{+}\to {\mu }^{+}{\nu }_{\mu }$

We have performed a search for the rare leptonic decay $B^{+}\to {\mu }^{+}{\nu }_{\mu }$ with data collected at the ${\rm Y}(4S)$ resonance by the BABAR experiment at the PEP-II storage ring. In a sample of $88.4\times {10}^6$ $B\overline{B}$ pairs, we find no significant evidence for a signal and set an upper limit on the branching fraction $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })<6.6\times {10}^{-6}$ at the $90\%$ confidence level.

Figure 1

Results of the ARGUS fit to the on-resonance data satisfying $-0.75<\Delta E<0.5\mathrm{GeV}$. The two dashed lines indicate the lower boundaries of the blinded region and signal box at 5.24 and $5.27\mathrm{GeV}/c^2$, respectively. The histogram shows the sum of all simulated background sources normalized to the on-resonance luminosity.

Figure 2

The distributions of $\Delta E$ and $m_{\mathrm{ES}}$ of the companion $B$ in the $B^{+}\to D^0{\pi }^{+}$ control sample after all previous cuts have been applied. The points are the on-resonance data while the histogram is the MC simulation normalized to the number of reconstructed $B^{+}\to D^0{\pi }^{+}$ decays. The observed discrepancies between data and simulation are accounted for by correcting the signal efficiency obtained from the simulation.

Figure 3

The distribution of $\Delta E$ vs $m_{\mathrm{ES}}$ in the on-resonance data after all selection criteria have been applied. The signal box is represented by the solid lines while the dashed lines indicate the region used to estimate the background.