Hayrapetyan, A.Tumasyan, A.Adam, W.Andrejkovic, J. W.Bergauer, T.Chatterjee, S.Damanakis, K.Gürpınar Güler, EmineGüler, Yalçın2024-04-202024-04-2020230031-90071079-7114https://doi.org/10.1103/PhysRevLett.131.091903-2https://hdl.handle.net/20.500.13091/5370A search for the rare ri -> 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 fb(-1). A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the eta -> mu+ mu(-) decay as normalization, the branching fraction B(eta -> mu(+) mu(-) mu(+) mu(-) = [5.0 +/- 0.8 (stat) +/- 0.7 (syst) +/- 0.7(B-2 mu)] x 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.eninfo:eu-repo/semantics/openAccessDegenerate Electronic StatesPolyatomic-MoleculesQuantum SimulationsUltracold AtomsInformationStabilityGatesArticle10.1103/PhysRevLett.131.091903-2