At present, the measurements of $R_{D^{(*)}}$ and $R_{J/psi}$ hint at new physics (NP) in $b to c tau^- {bar u}$ decays. The angular distribution of ${bar B} to D^* (to D pi) , tau^{-} {bar u}_tau$ 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 $ u_tau$. In this paper, we construct a measurable angular distribution by considering the additional decay $tau^- to pi^- u_tau$. The full process is ${bar B} to D^* (to D pi) , tau^{-} (to pi^- u_tau) {bar u}_tau$, which includes three final-state particles whose three-momenta can be measured: $D$, $pi$, $pi^-$. The magnitudes and relative phases of all the NP parameters can be extracted from a fit to this angular distribution. One can measure CP-violating angular asymmmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the $q^2$ distribution and the $D^*$ polarization, and (ii) new observables associated with the $pi^-$ emitted in the $tau$ decay: the forward-backward asymmetry of the $pi^-$ and the CP-violating triple-product asymmetry.
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