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 during 2017 and 2018 and corresponding to an integrated luminosity of $101{\mathrm{fb}}^{-1}$. 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 $\mathcal{B}(\eta \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})=[5.0\pm 0.8(\mathrm{stat})\pm 0.7(\mathrm{syst})\pm 0.7({\mathcal{B}}_{2\mu })]\times {10}^{-9}$ is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over 5 orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.

• Received 8 May 2023
• Accepted 14 June 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.091903

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