Searches for the decays $B^0\to l^{\pm }{\tau }^{\mp }$ and $B^{+}\to l^{+}\nu$ ($l=e$, $\mu$) using hadronic tag reconstruction

We present searches for the leptonic decays $B^{+}\to {\ell }^{+}\nu$ and the lepton flavor violating decays $B^0\to {\ell }^{\pm }{\tau }^{\mp }$, where $\ell =e$, $\mu$, with data collected by the BABAR experiment at SLAC. This search demonstrates a novel technique in which we fully reconstruct the accompanying $\overline{B}$ in ${\rm Y}(4S)\to B\overline{B}$ events, and look for a monoenergetic lepton from the signal $B$ decay. The signal yield is extracted from a fit to the signal lepton candidate momentum distribution in the signal $B$ rest frame. Using a data sample of approximately $378\times {10}^6$ $B\overline{B}$ pairs ($342{\mathrm{fb}}^{-1}$), we find no evidence of signal in any of the decay modes. Branching fraction upper limits of $\mathcal{B}(B^{+}\to e^{+}\nu )<5.2\times {10}^{-6}$, $\mathcal{B}(B^{+}\to {\mu }^{+}\nu )<5.6\times {10}^{-6}$, $\mathcal{B}(B^0\to e^{+}{\tau }^{-})<2.8\times {10}^{-5}$ and $\mathcal{B}(B^0\to {\mu }^{+}{\tau }^{-})<2.2\times {10}^{-5}$, are obtained at 90% confidence level.

Figure 1

$E_{\mathrm{extra}}$ distributions for the background simulation and data (left) and the signal (right) after all other selection criteria have been applied. The upper plots are for $B^{+}\to {\ell }^{+}\nu$ modes and the lower plots are for $B^0\to {\ell }^{+}{\tau }^{-}$ modes. The background distributions show electron and muon modes together, as they are nearly identical. The background is almost completely dominated by $B\overline{B}$ events. The signal modes are shown with a branching fraction of ${10}^{-5}$.

Figure 2

The unbinned maximum likelihood fits on the lepton momentum. The dashed line, representing the signal PDF with an arbitrary scaling, indicates where the signal is expected.