B → D* τ(ν)over-barτ sensitivity to new physics

B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B -> tau(nu) over bar (tau) decay, where the large tau mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B -> D-(*())tau(nu) over bar (tau) rates. Because of the rich spin structure of the final state, the decay mode B -> D*tau(nu) over bar (tau) offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b -> c (semi) leptonic transitions. We explore such contributions to the B -> D*tau(nu) over bar (tau) decay amplitudes by determining the differential decay rate, longitudinal D* polarization fraction, D* - tau opening angle asymmetry and the tau helicity asymmetry. We identify the size of possible new physics contributions to these observables constrained by the present B -> D-(*())tau(nu) over bar (tau) rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the tau helicity asymmetry can still deviate by as much as 80%.