Measurement of the relative $B^{-}\text{\hspace{0.17em}⁣}\to D^0\mathrm{/}{D}^{\ast 0}\mathrm{/}{D}^{\ast \ast 0}{\mu }^{-}{\bar{\nu }}_{\mu }$ branching fractions using $B^{-}$ mesons from $\bar{B}{}_{s2}^{\ast 0}$ decays

The decay of the narrow resonance $\overline{B}{}{s2}^{*0}!\rightarrow B^- K^+$canbeusedtodeterminethe$B^-$ momentum in partially reconstructed decays without any assumptions on the decay products of the $B^{-}$ meson. This technique is employed for the first time to distinguish contributions from $D^0$, $D^{\ast 0}$, and higher-mass charmed states ($D^{\ast \ast 0}$) in semileptonic $B^{-}$ decays by using the missing-mass distribution. The measurement is performed using a data sample corresponding to an integrated luminosity of 3.0 fb${}^{-1}$ collected with the LHCb detector in $pp$ collisions at center-of-mass energies of 7 and 8 TeV. The resulting branching fractions relative to the inclusive $B^{-}\text{\hspace{0.17em}⁣}\to D^{0}X{\mu }^{-}{\bar{\nu }}_{\mu }$ are $f{D^0} = $\mathcal{B}$( B^- \rightarrow D^0\mu^-\overline{\nu}\mu )/$\mathcal{B}$( B^- \rightarrow D^0 X \mu^- \overline{\nu}\mu ) = 0.25 \pm 0.06$,$f_{D^{**0}} = $\mathcal{B}$( B^- \rightarrow ( D^{**0} \rightarrow D^0 X)\mu^-\overline{\nu}\mu )/$\mathcal{B}$( B^- \rightarrow D^0 X \mu^- \overline{\nu}\mu ) = 0.21 \pm 0.07$,with$f_{D^{*0}} = 1 - f_{D^0} - f_{D^{**0}}$ making up the remainder.