Test of lepton flavour universality using $B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau }$ decays with hadronic $\tau$ channels

Abstract

The branching fraction $\mathcal{B}(B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau })$ is measured relative to that of the normalization mode $B^0\to D^{\ast -}{\pi }^{+}{\pi }^{-}{\pi }^{+}$ using hadronic ${\tau }^{+}\to {\pi }^{+}{\pi }^{-}{\pi }^{+}({\pi }^0){\overline{\nu }}_{\tau }$ decays in proton-proton collision data at a center-of-mass energy of 13 TeV collected by the LHCb experiment, corresponding to an integrated luminosity of 2 fb${}^{-1}$. The measured ratio is $\mathcal{B}(B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B^0\to D^{\ast -}{\pi }^{+}{\pi }^{-}{\pi }^{+})=1.79\pm 0.11\pm 0.11$, where the first uncertainty is statistical and the second is related to systematic effects. Using established branching fractions for the $B^0\to D^{\ast -}{\pi }^{+}{\pi }^{-}{\pi }^{+}$ and $B^0\to D^{\ast -}{\mu }^{+}{\nu }_{\mu }$ modes, the lepton universality test, $\mathcal{R}(D^{\ast -})\equiv \mathcal{B}(B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B^0\to D^{\ast -}{\mu }^{+}{\nu }_{\mu })$ is calculated, $$\mathcal{R}(D^{\ast -})=0.260\pm 0.015\pm 0.016\pm 0.012,$$ where the third uncertainty is due to the uncertainties on the external branching fractions. This result is consistent with the Standard Model prediction and with previous measurements.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-052.html (LHCb public pages), updated to correct the central value of $R(D^{\ast })$ following an erratum

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