A measurable angular distribution for B<overbar></mml:mover>→<mml:msup>D</mml:msup><mml:msup>τ-</mml:msup><mml:mover accent="true">v<mml:mo stretchy="true"><overbar></mml:mover>τ decays

At present, the measurements of RD<mml:mfenced close=")" open="("></mml:mfenced> and R<INF>J/psi</INF> hint at new physics (NP) in b -> c tau -v<overbar></mml:mover> decays. The angular distribution of B<overbar></mml:mover>-> D<mml:mfenced close=")" open="(">-> D pi</mml:mfenced>tau -v<overbar></mml:mover>tau would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum p -></mml:mover>tau cannot be determined precisely since the decay products of the tau - include an undetected nu <INF>tau</INF>. In this paper, we construct a measurable angular distribution by considering the additional decay tau --> pi -nu <INF>tau</INF>. The full process is B<overbar></mml:mover>-> D<mml:mfenced close=")" open="(">-> D pi <mml:mo>'</mml:mfenced>tau <mml:mo>-<mml:mfenced close=")" open="("><mml:mo>-> pi <mml:mo>-v tau</mml:mfenced><mml:mover accent="true">v<mml:mo stretchy="true"><overbar></mml:mover>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 asymmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the q2 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.