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

Abstract We perform a global analysis of the b→cτν data using the recent lattice results on the Bc→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 R2, S1 and U1 leptoquark models by including the recently measured R(Λc) and imposing the relaxed constraint B(Bc→τν ) < 30% in light of the recent studies on LEP1 data and Bc lifetime. We give predictions for the experimental observables including R(J/ψ ), Pτ(J/ψ ), FL(J/ψ ) and AFB(J/ψ ) as well as their q2 distribution in new physics scenarios/models. Our results suggest that the longitudinal τ polarization fraction Pτ(J/ψ) and the forward-backward asymmetry AFB(J/ψ ) are useful for testing the R2 leptoquark model.