A Measurable Angular Distribution for ${\bar B} \to D^{*} \tau^- {\bar\nu}_\tau$ Decays

Understand

At present, the measurements of $R_{D^{(*)}}$ and $R_{J/\psi}$ hint at new physics (NP) in $b \to c \tau^- {\bar\nu}$ decays.

• The angular distribution of ${\bar B} \to D^* (\to D \pi) \, \tau^{-} {\bar\nu}_\tau$ would be useful for getting information about the NP, but it cannot be measured.
• The reason is that the three-momentum ${\vec p}_\tau$ cannot be determined precisely since the decay products of the $\tau^-$ include an undetected $\nu_\tau$.
• In this paper, we construct a measurable angular distribution by considering the additional decay $\tau^- \to \pi^- \nu_\tau$.