Revisit to the b → cτν transition: In and beyond the SM

We perform an analysis of the b→cτν data, including R(D(⁎)), R(J/ψ), Pτ(D⁎) and FLD⁎, within and beyond the Standard Model (SM). We fit the B→D(⁎) hadronic form factors in the HQET parametrization to the lattice and the light-cone sum rule (LCSR) results, applying the general strong unitarity bounds corresponding to JP=1−, 1+, 0− and 0+. Using the obtained HQET relations between helicity amplitudes, we give the strong unitarity bounds on individual helicity amplitudes, which can be used in the BGL fits. Using the fitted form factors and taking into account the most recent Belle measurement of R(D(⁎)) we investigate the model-independent and the leptoquark model explanations of the b→cτν anomalies. Specifically, we consider the one-operator, the two-operator new physics (NP) scenarios and the NP models with a single R2, S1 or U1 leptoquark which is supposed to be able to address the b→cτν anomalies, and our results show that the R2 leptoquark model is in tension with the limit B(Bc→τν)<10%. Furthermore, we give predictions for the various observables in the SM and the NP scenarios/leptoquark models based on the present form factor study and the analysis of NP.