Search for lepton-number-violating $B^{-}\to D^{(\ast )+}{\mu }^{-}{\mu }^{-}$ decays

A search is performed for lepton-number-violating $B^{-}\to D^{(\ast )+}{\mu }^{-}{\mu }^{-}$ decays, using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb${}^{-1}$. No significant signal is observed, and upper limits are set on the branching fractions, $\mathcal{B}(B^{-}\to D^{+}{\mu }^{-}{\mu }^{-})<4.6\times {10}^{-8}$ and $\mathcal{B}(B^{-}\to D^{\ast +}{\mu }^{-}{\mu }^{-})<5.9\times {10}^{-8}$, at the 95% confidence level.