The ratio of branching fractions ${ \cal{ R}}(D^{ *-})\equiv { \cal{ B}}(B^0 \to D^{ *-} \tau^+ \nu_{ \tau})/{ \cal{ B}}(B^0 \to D^{ *-} \mu^+\nu_{ \mu})$ is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 Tev, corresponding to an integrated luminosity of 3$~$fb$^{ -1}$. For the first time ${ \cal{ R}}(D^{ *-})$ is determined using the $\tau$ lepton decays with three charged pions in the final state. The $B^0 \to D^{ *-} \tau^+\nu_{ \tau}$ yield is normalized to that of the $B^0\to D^{ *-} \pi^+\pi^-\pi^+$ mode, providing a measurement of ${ \cal{ B}}(B^0\to D^{ *-}\tau^+\nu_{ \tau})/{ \cal{ B}}(B^0\to D^{ *-}\pi^+\pi^-\pi^+) = 1.97 \pm 0.13 \pm 0.18$, where the first uncertainty is statistical and the second systematic. The value of ${ \cal{ B}}(B^0 \to D^{ *-} \tau^+ \nu_{ \tau}) = (1.42 \pm 0.094 \pm 0.129 \pm 0.054)\% $ is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the $B^0 \to D^{ *-} \mu^+\nu_{ \mu}$ decay, a value of ${ \cal{ R}}(D^{ *-}) = 0.291 \pm 0.019 \pm 0.026 \pm 0.013$ is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and $B^0\to D^{ *-}\mu^+\nu_{ \mu}$ modes. This measurement is in agreement with the Standard Model prediction and with previous results.
Categories: hep-ex