A Closer Look at R_D and R_D∗

The measurement of R_D (R_D∗), the ratio of the branching fraction of B → Dτν¯_τ(B → D^∗τν¯_τ) to that of B → Dlν¯_l(B → D^∗lν¯_l), shows 1.9σ (3.3σ) deviation from its Standard Model (SM) prediction. The combined deviation is at the level of 4σ according to the Heavy Flavour Averaging Group (HFAG). In the paper [1], we perform an effective field theory analysis (at the dimension 6 level) of these potential New Physics (NP) signals assuming SU(3)_C ×SU(2)_L ×U(1)_Y gauge invariance. We first show that, in general, R_D and R_D∗ are theoretically independent observables and hence, their theoretical predictions are not correlated. We identify the operators that can explain the experimental measurements of R_D and R_D∗ individually and also together. Motivated by the recent measurement of the τ polarisation in B → D^∗τν¯_τ decay, P_τ ^D∗ by the BELLE collaboration, we study the impact of a more precise measurement of P_τ ^D∗ (and a measurement of P_τ ^D) on the various possible NP explanations. Furthermore, we show that the measurement of R_D∗ in bins of q², the square of the invariant mass of the lepton neutrino system, along with the information on τ polarisation, can completely distinguish the various operator structures.