$\bar{B}\to D\tau {\bar{\nu }}_{\tau }$ vs. $\bar{B}\to D\mu {\bar{\nu }}_{\mu }$

Abstract

Recent experimental results for the ratio of the branching fractions of $\bar{B}\to D^{(\ast )}\tau {\bar{\nu }}_{\tau }$ and $\bar{B}\to D^{(\ast )}\mu {\bar{\nu }}_{\mu }$ decays came as a surprise and lead to a discussion of possibility to constraining New Physics through these modes. Here we focus on $\mathcal{B}(\bar{B}\to D\tau {\bar{\nu }}_{\tau })/\mathcal{B}(\bar{B}\to D\mu {\bar{\nu }}_{\mu })$ 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.

Comments: 10 pages (published version)