Semileptonic B → D∗ decays from light to τ leptons: the extraction of the form factor F2 from data

We extend the Standard Model (SM) analysis of Martinelli et al. (Phys Rev D Ser 111:013005, 2025. https://doi.org/10.1103/PhysRevD.111.013005), which was limited to light leptons in the final state, to the semileptonic B→D∗τντ decay. By using quantities that can be analised without the knowledge of |Vcb|, we derive important information about the helicity amplitudes and the hadronic form factors that can be compared with the predictions of lattice QCD calculations. In particular, there is a difficulty in reproducing simultaneously the experimental values of R(D∗) and of other quantities relevant for the semitauonic decays within the SM. As a byproduct of our analysis, we also present a determination of |Vcb| from the total decay rate.