The ratios of branching fractions
$\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to
D^{*}τ^{-}\barν_τ)/\mathcal{B}(\bar{B}\to
D^{*}μ^{-}\barν_μ)$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to
D^{0}τ^{-}\barν_τ)/\mathcal{B}(B^{-}\to
D^{0}μ^{-}\barν_μ)$ are measured, assuming isospin symmetry, using a
sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of
integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The
tau lepton is identified in the decay mode
$τ^{-}\toμ^{-}ν_τ\barν_μ$. The measured values are
$\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and
$\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is
statistical and the second is systematic. The correlation between these
measurements is $ρ=-0.43$. 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.
Stephen Sekula
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