Search for $B^{+}\to {\tau }^{+}\nu$ decays with hadronic $B$ tags

We present a search for the decay $B^{+}\to {\tau }^{+}\nu$ using $383\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the SLAC PEP-II $B$ Factory. We select a sample of events with one completely reconstructed tag $B$ in a hadronic decay mode ($B^{-}\to D^{(*)0}{X}^{-}$), and examine the rest of the event to search for a $B^{+}\to {\tau }^{+}\nu$ decay. We identify the $\tau$ lepton in the following modes: ${\tau }^{+}\to e^{+}\nu \overline{\nu }$, ${\tau }^{+}\to {\mu }^{+}\nu \overline{\nu }$, ${\tau }^{+}\to {\pi }^{+}\overline{\nu }$, and ${\tau }^{+}\to {\pi }^{+}{\pi }^0\overline{\nu }$. We find a $2.2\sigma$ excess in data and measure a branching fraction of $\mathcal{B}(B^{+}\to {\tau }^{+}\nu )=({1.8}_{-0.8}^{+0.9}(\mathrm{stat}.)\pm 0.4(\mathrm{bkg}.\mathrm{syst}.)\pm 0.2(\mathrm{other}\mathrm{syst}.))\times {10}^{-4}$. We calculate the product of the $B$ meson decay constant $f_B$ and $|V_{ub}|$ to be $f_B·|V_{ub}|=({10.1}_{-2.5}^{+2.3}(\mathrm{stat}.{)}_{-1.5}^{+1.2}(\mathrm{syst}.))\times {10}^{-4}\mathrm{GeV}$.

Figure 1

Distribution of the energy-substituted mass, $m_{\mathrm{ES}}$, of the tag $B$ candidates in data. The combinatorial background is overlaid.

Figure 2

$E_{\mathrm{extra}}$ distribution after all selection criteria have been applied. The upper plot shows the distribution of all the modes combined while lower plots show the (a) ${\tau }^{+}\to e^{+}\nu \overline{\nu }$, (b) ${\tau }^{+}\to {\mu }^{+}\nu \overline{\nu }$, (c) ${\tau }^{+}\to {\pi }^{+}\overline{\nu }$, and (d) ${\tau }^{+}\to {\pi }^{+}{\pi }^0\overline{\nu }$ modes separately. The on-resonance data (black dots) distribution is compared with the total background prediction (continuous histogram). The hatched histrogram represents the combinatorial background component. $B^{+}\to {\tau }^{+}\nu$ signal MC (dashed histogram), normalized to a branching fraction of $3\times {10}^{-3}$ for illustrative purposes, is shown for comparison.