Model-Independent Prediction of $R({\eta }_c)$

Abstract

We present a model-independent prediction for $R({\eta }_c)\equiv \mathcal{B}\mathcal{R}(B_c\to {\eta }_c{\tau }^{+}{\nu }_{\tau })/\mathcal{B}\mathcal{R}(B_c\to {\eta }_c{\mu }^{+}{\nu }_{\mu })$. This prediction is obtained from the form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing determinations from lattice QCD. The resulting prediction, $R({\eta }_c)=0.29(5)$, agrees with the weighted average of previous model predictions, but with reduced uncertainties.

Comments: 8 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:1807.02730