Charged-Higgs on R^D(∗) , τ polarization, and FBA

We study the influence of a charged-Higgs on the excess of branching fraction ratio, R_M= BR(B¯ → M _τ) / BR(B¯ → M _ℓ) (M= D, D^∗) , in a generic two-Higgs-doublet model. In order to investigate the lepton polarization, the detailed decay amplitudes with lepton helicity are given. When the charged-Higgs is used to resolve excesses, it is found that two independent Yukawa couplings are needed to explain the R_D and RD∗ anomalies. We show that when the upper limit of BR(B_c→ _τ) < 30 % is included, R_D can be significantly enhanced while RD∗<0.27. With the BR(B_c→ _τ) constraint, we find that the τ-lepton polarizations can be still affected by the charged-Higgs effects, where the standard model (SM) predictions are obtained: PDτ≈0.324 and PD-0.500, and they can be enhanced to be PDτ≈0.5 and PD-0.41 by the charged-Higgs. The integrated lepton forward–backward asymmetry (FBA) is also studied, where the SM result is A¯FBD(∗),τ≈-0.359(0.064), and they can be enhanced (decreased) to be A¯FBD(∗),τ≈-0.33(0.02).