Measurement of an excess of B ¯ → D ( * ) τ − ν ¯ τ decays and implications for charged Higgs bosons

Abstract

Based on the full BABAR data sample, we report improved measurements of the ratios $\mathcal{R}(D)=\mathcal{B}(\overline{B}\ensuremath{\rightarrow}D{\ensuremath{\tau}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}})/\mathcal{B}(\overline{B}\ensuremath{\rightarrow}D{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}})$ and $\mathcal{R}({D}^{*})=\mathcal{B}(\overline{B}\ensuremath{\rightarrow}{D}^{*}{\ensuremath{\tau}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}})/\mathcal{B}(\overline{B}\ensuremath{\rightarrow}{D}^{*}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}})$, where $\ensuremath{\ell}$ refers to either an electron or muon. These ratios are sensitive to new physics contributions in the form of a charged Higgs boson. We measure $\mathcal{R}(D)=0.440\ifmmode\pm\else\textpm\fi{}0.058\ifmmode\pm\else\textpm\fi{}0.042$ and $\mathcal{R}({D}^{*})=0.332\ifmmode\pm\else\textpm\fi{}0.024\ifmmode\pm\else\textpm\fi{}0.018$, which exceed the standard model expectations by $2.0\ensuremath{\sigma}$ and $2.7\ensuremath{\sigma}$, respectively. Taken together, the results disagree with these expectations at the $3.4\ensuremath{\sigma}$ level. This excess cannot be explained by a charged Higgs boson in the type II two-Higgs-doublet model. Kinematic distributions presented here exclude large portions of the more general type III two-Higgs-doublet model, but there are solutions within this model compatible with the results.