Evidence of the Purely Leptonic Decay $B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau }$

We present the first evidence of the decay $B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau }$, using $414{\mathrm{fb}}^{-1}$ of data collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. Events are tagged by fully reconstructing one of the $B$ mesons in hadronic modes. We detect the signal with a significance of 3.5 standard deviations including systematics and measure the branching fraction to be $\mathcal{B}(B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau })=({1.79}_{-0.49}^{+0.56}(\mathrm{stat}{)}_{-0.51}^{+0.46}(\mathrm{syst}))\times {10}^{-4}$. This implies that $f_B={0.229}_{-0.031}^{+0.036}(\mathrm{stat}{)}_{-0.037}^{+0.034}(\mathrm{syst})\mathrm{GeV}$ and is the first direct measurement of this quantity.

Figure 1

Purely leptonic $B$ decay proceeds via quark annihilation into a $W$ boson.

Figure 2

$E_{\mathrm{ECL}}$ distribution for double tagged events, where one $B$ is fully reconstructed in the hadronic mode and the other $B$ is reconstructed as $B^{-}\to D^{*0}{\ell }^{-}\overline{\nu }$. The dots indicate the data. The solid histogram is the background from $B\overline{B}$ MC events ($B^{+}{B}^{-}+B^0{\overline{B}}^0$), while the dashed one shows the contribution from $B^0{\overline{B}}^0$ events.

Figure 3

$E_{\mathrm{ECL}}$ distributions in the data after all selection criteria except the one on $E_{\mathrm{ECL}}$. The data and background MC samples are represented by the points and the solid histogram, respectively. The solid curve shows the result of the fit with the sum of the signal (dashed curve) and background (dotted curve) contributions.