meson to meson form factors. The dependency on the CKM matrix elements exactly cancels in these ratios. Similarly, the uncertainties due to the form factors also largely cancel in these ratios and a clean prediction of $R_D$ and $R_{D^{\ast}}$ can be made within the standard model (SM). Hence, any deviation from the SM prediction would clearly indicate the presence of new physics (NP). At present, combining the results of $R_D$ and $R_{D^{\ast}}$ measured by various experiments such as BABAR [@Lees:2012xj; @Lees:2013uzd], BELLE [@Huschle:2015rga; @Sato:2016svk; @Hirose:2016wfn], and LHCb [@Aaij:2015yra; @Aaij:2017uff], i.e, $R_D = 0.407 \pm 0.039 \pm 0.024$ and $R_{D^{\ast}} = 0.304 \pm 0.013 \pm 0.007$ exceed the SM predictions by $2.3\sigma$ and $3.4\sigma$, respectively. Again, including the $R_D-R_{D^{\ast}}$ correlation, the discrepancy with SM prediction [@Lattice:2015rga; @Na:2015kha; @Aoki:2016frl; @Bigi:2016mdz; @Fajfer:2012vx] currently stands at about $4.1\sigma$ [@hflag]. Recently, LHCb [@LHCb; @LHCb2] has measured the value of the ratio of branching ratio $$\begin{aligned}
R_{J/\Psi}=\frac{\mathcal B(B_c \to J/\Psi\,\tau\nu)}{\mathcal B(B_c \to J/\Psi\,l\nu)} = 0.71\pm 0.17\pm 0.18\,,\end{aligned}$$ where $l$ is either an electron or muon, which is at more than $2\sigma$ away from the standard model (SM) prediction [@Wen-Fei:2013uea; @Dutta:2017xmj]. In Ref. [@Hirose:2016wfn], the $\tau$ polarization fraction in $B \to D^{\ast}\tau\nu$ decays has also been measured 