  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 to tau nu 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 to D(*) tau nu
rates. Due to the rich spin structure of the final state, the decay mode B to
D* tau nu 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 to c (semi)leptonic
transitions. We explore such contributions to the B to D* tau nu 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
constrained by the present B to D(*) tau nu 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%.
