Testing the Standard Model in $B \to D \tau \bar \nu$ decay with minimal theory input

Abstract

Recent experimental results for the ratio of the branching fractions of $\bar B \to D^{(*)}\tau \nu_\tau$ and $B\to D^{(*)} \mu \nu_\mu$ decays came as a surprise and lead to a discussion of possibility to constraining new physics through these modes. Here we focus on $Br(B \to D \tau \bar\nu_\tau)/Br(B \to D \mu \bar\nu_\mu)$ 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.