\times 10^{-12}\,{\rm s}$ and $\tau_{B_c}= 0.507 \times 10^{-12}\,{\rm s}$ [@pdg]. We use $f_{B_c}= 0.434(0.015)\,{\rm GeV}$ from Ref. [@Colquhoun:2015oha]. The value of CKM matrix element is taken to be $V_{cb}= 0.0409(0.0011)$ [@pdg]. The uncertainty associated with $f_{B_c}$ and $V_{cb}$ are indicated by the number in parentheses.

In order to compute the branching fractions and other observables, we need information on various hadronic form factors that parametrizes the hadronic matrix elements of vector, axial vector, scalar, pseudoscalar, and tensor currents between two mesons. For the $B_c \to J/\Psi$ hadronic form factors, we follow ref. [@Wen-Fei:2013uea]. The relevant formula for $V(q^2)$, $A_0(q^2)$, $A_1(q^2)$, and $A_2(q^2)$ pertinent for our discussion, taken from ref. [@Wen-Fei:2013uea] is $$\begin{aligned}
F(q^2) = F(0)\,\exp\Big[a\,q^2 + b\,(q^2)^2\Big]\,,\end{aligned}$$ where $F$ stands for the form factors $V$, $A_0$, $A_1$, and $A_2$ and $a$, $b$ are the fitted parameters. The numerical values of $B_c \to J/\Psi$ form factors at $q^2 = 0$ and their fitted parameters $a$ and $b$, calculated in perturbative QCD (PQCD) approach, are collected from ref. [@Wen-Fei:2013uea]. Similarly for the $B_c \to J/\Psi$ tensor form factors, we follow Ref. [@Watanabe:2017mip]. The relevant formulas pertinent for our numerical computation are $$\begin{aligned}
&&T_1(q^2) = \frac{m_b + m_c}{m_{B_c} + m_{J/\Psi}}\,V(q^2)\,,\qquad
T_2(q^2) = \frac{m_b 