The ratios of branching fractions $\mathcal{R}(D^{*})\equiv \mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\mu }^{-}{\overline{\nu }}_{\mu })$ and $\mathcal{R}(D^0)\equiv \mathcal{B}(B^{-}\to D^0{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(B^{-}\to D^0{\mu }^{-}{\overline{\nu }}_{\mu })$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to $3.0{\mathrm{fb}}^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode ${\tau }^{-}\to {\mu }^{-}{\nu }_{\tau }{\overline{\nu }}_{\mu }$. The measured values are $\mathcal{R}(D^{*})=0.281\pm 0.018\pm 0.024$ and $\mathcal{R}(D^0)=0.441\pm 0.060\pm 0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho =-0.43$. The results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the standard model.

• Received 9 February 2023
• Revised 18 April 2023
• Accepted 29 June 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.111802

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