Recent experimental results for the ratio of the branching fractions of $\bar B\to D^{ (*)}\tau\bar \nu_\tau$ and $\bar B\to D^{ (*)}\mu\bar \nu_\mu$ decays came as a surprise and lead to a discussion of possibility to constraining New Physics through these modes. Here we focus on ${ \cal B}(\bar B\to D\tau\bar \nu_\tau)/{ \cal B}(\bar B\to D\mu\bar \nu_\mu)$ and argue that the result is consistent with the Standard Model within $2\sigma$, and that the test of compatibility of this ratio with the Standard Model can be done experimentally with a minimal theory input. We also show that these two decay channels can provide us with quite good constraints of the New Physics couplings.
Categories: hep-ph hep-ex hep-lat