Scrutinizing new physics in semi-leptonic B _c → J/ψτν decay

We perform a global analysis of the b → cτν data using the recent lattice results on the B c → J / ψ vector and axial-vector form factors. To explore the effects from the tensor operator of new physics beyond the Standard Model, we determine the tensor form factors by using the non-relativistic QCD (NRQCD) relations between tensor and (axial-)vector form factors. Based on the lattice + NRQCD form factors, we fit the Wilson coefficients and the new physics couplings in R 2 , S 1 and U 1 leptoquark models by including the recently measured R (Λ c ) and imposing the relaxed constraint B ( B c → J / ψ ) < 30 % in light of the recent studies on LEP1 data and B c lifetime. We give predictions for the experimental observables including R ( J / ψ ), P τ ( J / ψ ), F L ( J / ψ ) and A FB ( J / ψ ) as well as their q 2 distribution in new physics scenarios/models. Our results suggest that the longitudinal τ polarization fraction P τ ( J / ψ ) and the forward–backward asymmetry A FB ( J / ψ ) are useful for testing the R 2 leptoquark model.