# $B_c^- \to J/\psi (\to \mu^+ \mu^-)\tau^- (\to \pi^- \nu_\tau, \rho^- \nu_\tau, \ell^-\bar{\nu}_\ell\nu_\tau)\bar{\nu}_\tau$ decays with visible final-state kinematics

## TL;DR

This paper analyzes the semitauonic $B_c$ decay with polarized particles to identify potential new physics effects, deriving detailed kinematic distributions and observables sensitive to beyond Standard Model interactions.

## Contribution

It introduces a comprehensive framework for analyzing $B_c^- o J/ au^- ar{
u}_ au$ decays with polarized states, deriving decay rates and constructing numerous observables including CP-violating asymmetries.

## Key findings

- Derived five-fold differential decay rate in terms of visible kinematics.
- Constructed 34 normalized observables, including CP-violating asymmetries.
- Estimated sensitivities of observables at LHCb, highlighting $ au^- o
ho^-
u_ au$ channel.

## Abstract

The semitauonic $B_c^- \to J/\psi \tau^-\bar{\nu}_\tau$ decay is optimal to scrutinize possible new physics effects in $b \to c \tau^- \bar{\nu}_\tau$ transitions as indicated by the current data on $R(D^{(*)})$ anomalies. In this work, we study the $B_c^- \to J/\psi \tau^-\bar{\nu}_\tau$ decay with both polarized $\tau$ and $J/\psi$. Their subsequent decays, with $J/\psi\to \mu^+ \mu^-$ as well as $\tau^- \to \pi^- \nu_\tau$, $\tau^- \to \rho^- \nu_\tau$ and $\tau^- \to \ell^-\bar{\nu}_\ell\nu_\tau$, are exploited to extract the energy and angular distributions of the charged final-state particles in the processes. Starting with the most general effective Hamiltonian relevant for the $b \to c \tau^- \bar{\nu}_\tau$ transitions, including all possible Lorentz structures of the dimension-six operators with both left- and right-handed neutrinos, we first derive the five-fold differential decay rate in terms of the visible final-state kinematics. From this distribution, we then construct in total 34 normalized observables, among which nine refer to the CP-violating triple product asymmetries that vanish within the Standard Model. We also construct five new observables based on the combinations of these normalized observables that can only be attributed to the right-handed neutrinos. On the other hand, considering the low statistics of the fully differential distribution, we introduce some integrated observables with only one kinematic variable left, which are more promising to be measured due to the largely increased statistics. The sensitivities of all these observables to the different new physics scenarios are investigated in detail. Finally, assuming an ideal circumstance, we give an estimate of the statistical uncertainties of the nine CP-conserving observables at LHCb and found that $\tau^-\to \pi^-\nu_\tau$ has the highest analyzing power among the three $\tau$ decay channels.