a subset of these $55$ NP structures. We do not propose any NP model that can generate such NP structures in particular. Rather we study it from a purely phenomenological point of view. It was shown in Ref. [@Alonso:2016oyd] that lifetime of $B_c$ meson put a severe constraint on scalar type NP interactions. Based on various SM calculations [@Bigi:1995fs; @Beneke:1996xe; @Chang:2000ac], it was inferred that $\mathcal B(B_c \to \tau\nu) \le 5\%$ is necessary to comply with the current world average of the $B_c$ lifetime. However, the constraint can be relaxed up to $30\%$ depending on the value of the total decay width of $B_c$ meson that is used as input for the SM calculation of various partonic transitions. Very recently, in Ref. [@Akeroyd:2017mhr], a more significant bound of $\mathcal B(B_c \to \tau\nu) \le 10\%$ was obtained by taking the LEP data at the $Z$ peak. Our analysis also takes into account the indirect constraint coming from $\mathcal B(B_c \to \tau\nu)$ to rule out various NP scenarios. We, however, have not included the measured value of $P_{\tau}^{D^{\ast}}$ reported by Belle in our fitting method since the uncertainties associated with it is rather large.

The paper is organized as follows. In section. 