Observation of the rare decay $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}$e${}^{+}$e${}^{-}$

Abstract

A first observation of the rare decay $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}$e${}^{+}$e${}^{-}$ is reported by the CMS Collaboration at the CERN LHC. The result is based on a proton-proton collision data sample at $\sqrt{s}$ = 13.6 TeV corresponding to an integrated luminosity of 38.0 fb${}^{-1}$, acquired in 2022 using a high-rate dimuon trigger. Using the $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}\gamma$ decay channel for normalization, the branching fraction is measured to be $\mathcal{B}$($\eta$ $\to$ ${\mu }^{+}{\mu }^{-}$e${}^{+}$e${}^{-}$) = (2.4 $\pm$ 0.8)$\times$ 10${}^{-6}$, with the uncertainty including statistical and systematic sources as well as the $\mathcal{B}$($\eta$ $\to$ ${\mu }^{+}{\mu }^{-}\gamma$) uncertainty. This result is close to two orders of magnitude smaller than the existing limit, and is consistent with recent theoretical predictions.

arXiv commentSubmitted to Physical Review Letters. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/BPH-24-001 (CMS Public Pages)