Hint of a new scalar interaction in LHCb data?

Abstract

We explain recent LHCb measurements of the lepton universality ratios, $R_{D^{(\ast )}}^{\tau /\ell }\equiv \frac{\mathcal{B}(\bar{B}\to D^{(\ast )+}{\tau }^{-}{\bar{\nu }}_{\tau })}{\mathcal{B}(\bar{B}\to D^{(\ast )+}{\ell }^{-}{\bar{\nu }}_{\ell })}$ and ${R({\Lambda }_c^{+})}^{\tau /\ell }\equiv \frac{\mathcal{B}({\Lambda }_b\to {\Lambda }_c^{+}{\tau }^{-}{\bar{\nu }}_{\tau })}{\mathcal{B}({\Lambda }_b\to {\Lambda }_c^{+}{\ell }^{-}{\bar{\nu }}_{\ell })}$ with $\ell =\mu$, via new physics that affects $R_D^{\tau /\ell }$ and $R({\Lambda }_c^{+}{)}^{\tau /\ell }$ but not $R_{D^{\ast }}^{\tau /\ell }$. The scalar operator in the effective theory for new physics is indicated. We find that the forward-backward asymmetry and $\tau$ polarization in $\bar{B}\to D^{+}{\tau }^{-}{\bar{\nu }}_{\tau }$ and ${\Lambda }_b\to {\Lambda }_c^{+}{\tau }^{-}{\bar{\nu }}_{\tau }$ 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 $Z\to b\bar{b}$.

Comments: 7 pages, 2 figures, 3 tables