Evidence for $B^{-}\to D^{\ast \ast 0}{\tau }^{-}\overline{{\nu }_{\tau }}$ decays

The first evidence for the decay $B^{-}\to D^{\ast \ast 0}{\tau }^{-}\overline{{\nu }_{\tau }}$ is obtained using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb${}^{-1}$ , at centre-of-mass energies of 7, 8 and 13 Tev. Here, the $D^{\ast \ast 0}$ meson represents any of the three excited charm mesons $D_1(2420{)}^0$, $D_2^{\ast }(2460{)}^0$, and $D_1^{{}^{\mathrm{\prime }}}(2400{)}^0$. The $B^{-}\to D^{\ast \ast 0}{\tau }^{-}\overline{{\nu }_{\tau }}$ signal is measured with a significance of 3.5 $\sigma$, including systematic uncertainties. The combined branching fraction $BR(B^{-}\to D_{1,2}^{\ast \ast 0}{\tau }^{-}\overline{{\nu }_{\tau }})\times BR(D_{1,2}^{\ast \ast 0}\to D^{\ast +}{\pi }^{-})$, where $D_{1,2}^{\ast \ast 0}$ denotes both $D_1(2420{)}^0$ and $D_2^{\ast }(2460{)}^0$ contributions, is measured to be $(0.051\pm 0.013(stat)\pm 0.006(syst)\pm 0.009(\mathrm{e}\mathrm{x}\mathrm{t}))\mathrm{\%}$, where the last uncertainty reflects that of the branching fraction of the normalisation channel $B^{-}\to D_{1,2}^{\ast \ast 0}{D}_s^{(\ast )-}$. The ratio between the tauonic and muonic semileptonic $B$ decays, with the latter taken from world average values, is also determined and found to be $\mathcal{R}(D_{1,2}^{\ast \ast 0})=0.13\pm 0.03(stat)\pm 0.01(syst)\pm 0.02(\mathrm{e}\mathrm{x}\mathrm{t})$.