$\eta$ and ${\eta }^{\prime }$ decays into lepton pairs

Abstract

In this work, we calculate the branching ratios for the $\eta ({\eta }^{\prime })\to \bar{\ell }\ell$ decays, where $\ell =e,\mu$. These processes have tiny rates in the standard model due to spin flip, loop, and electromagnetic suppression, for what they could be sensitive to New Physics effects. In order to provide a reliable input for the Standard Model, we exploit the general analytical properties of the amplitude. For that purpose, we invoke the machinery of Canterbury approximants, which provides a systematic description of the underlying hadronic physics in a data-driven fashion. Given the current experimental discrepancies, we discuss in detail the role of the resonant region and comment on the reliability of $\chi$PT calculations. Finally, we discuss the kind of new physics which we think would be relevant to account for them.

Comments: 35 pages, 18 figures. V2 matches the published version