Measurement of the branching fraction of $B^{+}\to {\tau }^{+}{\nu }_{\tau }$ decays with the semileptonic tagging method

We report a measurement of the branching fraction of $B^{+}\to {\tau }^{+}{\nu }_{\tau }$ decays using a data sample of $772\times {10}^{6}B\overline{B}$ pairs, collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We reconstruct the accompanying $B$ meson in a semileptonic decay and detect the recoiling $B$ candidate in the decay channel $B^{+}\to {\tau }^{+}{\nu }_{\tau }$. We obtain a branching fraction of $\mathcal{B}(B^{+}\to {\tau }^{+}{\nu }_{\tau })=[1.25\pm 0.28(\text{stat}.)\pm 0.27(\text{syst}.)]\times {10}^{-4}$. This result is in good agreement with previous measurements and the expectation from the calculation based on the Standard Model.

Figure 1

Comparison of the data and MC distribution in the double-tagged sample after the efficiency correction. The orange boxes show MC and the black markers the data. The upper and lower plots show the $E_{\mathrm{ECL}}$ distribution in the $B^{+}\to {\overline{D}}^0{\pi }^{+}$ sample and the $p_{\mathrm{sig}}^{*}$ distribution in the $B^{+}\to {\overline{D}}^{*0}(\to {\overline{D}}^0{\pi }^0){\ell }^{+}{\nu }_{\ell }$ sample, respectively. Both samples are selected corresponding to the ${\tau }^{+}\to {\mu }^{+}{\overline{\nu }}_{\tau }{\nu }_{\mu }$ decay channel.

Figure 2

Distributions for (a) ${\tau }^{+}\to {\mu }^{+}{\overline{\nu }}_{\tau }{\nu }_{\mu }$, (b) ${\tau }^{+}\to e^{+}{\overline{\nu }}_{\tau }{\nu }_e$, (c) ${\tau }^{+}\to {\pi }^{+}{\overline{\nu }}_{\tau }$, (d) ${\tau }^{+}\to {\rho }^{+}{\overline{\nu }}_{\tau }$, and (e) the sum of them. The left and right columns show the distributions of $E_{\mathrm{ECL}}$ and $p_{\mathrm{sig}}^{*}$ projected in the region $E_{\mathrm{ECL}}<0.2\mathrm{GeV}$, respectively. The markers show the data distribution, the solid line the total fitted distribution, and the dashed line the signal component. The orange (red) filled distribution represents the $B\overline{B}$ (continuum) background.