Observation of the rare decay of the $\eta$ meson to four muons

Abstract

A search for the rare $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers in 2017-2018 and corresponding to an integrated luminosity of 101 fb${}^{-1}$. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay as normalization, the branching fraction $\mathcal{B}(\eta$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})$ = [5.0 $\pm$ 0.8 (stat) $\pm$ 0.7 (syst) $\pm$ 0.7 (${\mathcal{B}}_{2\mu }$)] $\times$ 10${}^{-9}$ is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over five orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/BPH-22-003 (CMS Public Pages)

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Cited by in corpus (2)

• The anomalous magnetic moment of the muon in the Standard Model: an update
• Observation of the J/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay in proton-proton collisions at $\sqrt{s}$ = 13 TeV