The measurable angular distribution of $\Lambda_b^0 \to \Lambda_c^+ (\to \Lambda^0 \pi^+)\tau^- (\to \pi^- \nu_\tau)\bar{\nu}_\tau$ decay

In $\Lambda_b^0 \to \Lambda_c^+ (\to \Lambda^0 \pi^+) \tau^- \bar{\nu}_\tau$ decay, the solid angle of the final-state particle $\tau^-$ cannot be determined precisely since the decay products of the $\tau^-$ include an undetected $\nu_\tau$. Therefore, the angular distribution of this decay cannot be measured. In this work, we construct a {\it measurable} angular distribution by considering the subsequent decay $\tau^- \to \pi^- \nu_\tau$. The full cascade decay is $\Lambda_b^0 \to \Lambda_c^+ (\to \Lambda^0 \pi^+)\tau^- (\to \pi^- \nu_\tau)\bar{\nu}_\tau$. The three-momenta of the final-state particles $\Lambda^0$, $\pi^+$, and $\pi^-$ can be measured. Considering all Lorentz structures of the new physics (NP) effective operators and an unpolarized initial $\Lambda_b$ state, the five-fold differential angular distribution can be expressed in terms of ten angular observables ${\cal K}_i (q^2, E_\pi)$. By integrating over some of the five kinematic parameters, we define a number of observables, such as the $\Lambda_c$ spin polarization $P_{\Lambda_c}(q^2)$ and the forward-backward asymmetry of $\pi^-$ meson $A_{FB}(q^2)$, both of which can be represented by the angular observables $\widehat{{\cal K}}_i (q^2)$. We provide numerical results for the entire set of the angular observables $\widehat{{\cal K}}_i (q^2)$ and $\widehat{{\cal K}}_i$ both within the Standard Model and in some NP scenarios, which are a variety of best-fit solutions in seven different NP hypotheses. We find that the NP which can resolve the anomalies in $\bar{B} \to D^{(*)} \tau^- \bar{\nu}_\tau$ decays has obvious effects on the angular observables $\widehat{{\cal K}}_i (q^2)$, except $\widehat{{\cal K}}_{1ss} (q^2)$ and $\widehat{{\cal K}}_{1cc} (q^2)$.