A measurable angular distribution for B ¯ → D ∗ τ − v ¯ τ $$ \overline{B}\to {D}^{\ast }{\tau}^{-}{\overline{v}}_{\tau } $$ decays

Abstract

Abstract At present, the measurements of R D ∗ $$ {R}_{D^{\left(\ast \right)}} $$ and R J/ψ hint at new physics (NP) in b → c τ − v ¯ $$ b\to c{\tau}^{-}\overline{v} $$ decays. The angular distribution of B ¯ → D ∗ → Dπ τ − v ¯ τ $$ \overline{B}\to {D}^{\ast}\left(\to D\pi \right){\tau}^{-}{\overline{v}}_{\tau } $$ would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum p → τ $$ {\overrightarrow{p}}_{\tau } $$ cannot be determined precisely since the decay products of the τ − include an undetected ν τ . In this paper, we construct a measurable angular distribution by considering the additional decay τ − → π − ν τ . The full process is B ¯ → D ∗ → D π ′ τ − → π − v τ v ¯ τ $$ \overline{B}\to {D}^{\ast}\left(\to D{\pi}^{\prime}\right){\tau}^{-}\left(\to {\pi}^{-}{v}_{\tau}\right){\overline{v}}_{\tau } $$ , which includes three final-state particles whose three-momenta can be measured: D, π ′, π −. 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 q 2 distribution and the D ∗ polarization, and (ii) new observables associated with the π − emitted in the τ decay: the forward-backward asymmetry of the π − and the CP-violating triple-product asymmetry.

Keywords

• Beyond Standard Model
• CP violation