Hint of a new scalar interaction in LHCb data?

Abstract

We explain recent LHCb measurements of the lepton universality ratios, $R_{D^{(*)}}^{τ/\ell}\equiv \frac{\mathcal{B}(\bar B \to D^{(*)+} τ^- \barν_τ)} {\mathcal{B}(\bar B \to D^{(*)+}\ell^- \barν_\ell)}$ and ${R(Λ_c^+)}^{τ/\ell} \equiv \frac{\mathcal{B}(Λ_b \to Λ_c^+ τ^- \barν_τ)}{\mathcal{B}(Λ_b \to Λ_c^+ \ell^- \barν_{\ell})}$ with $\ell=μ$, via new physics that affects $R_D^{τ/\ell}$ and $R(Λ_c^+)^{τ/\ell}$ but not $R_{D^*}^{τ/\ell}$. The scalar operator in the effective theory for new physics is indicated. We find that the forward-backward asymmetry and $τ$ polarization in $\bar{B} \to D^+ τ^{-} \barν_τ$ and $Λ_b \to Λ_c^+ τ^- \barν_τ$ decays are significantly affected by the scalar interaction. We construct a simple two Higgs doublet model as a realization of our scenario and consider lepton universality in semileptonic charm and top decays, radiative $B$ decay, $B$-mixing, and $Z \to b \bar b$.