{"@context":"http://schema.org","@id":"https://doi.org/10.17182/hepdata.168684.v1","@reverse":{"isBasedOn":[{"@type":"ScholarlyArticle","identifier":{"@type":"PropertyValue","propertyID":"URL","value":"https://inspirehep.net/literature/3150815"}}]},"@type":"Dataset","additionalType":"Collection","author":{"@type":"Organization","name":"CMS Collaboration"},"creator":{"@type":"Organization","name":"CMS Collaboration"},"datePublished":"2026","description":"A first observation of the rare decay $\\eta \\to \\mu^+\\mu^-\\textrm{e}^+\\textrm{e}^-$ 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 $\\mathcal{B}(\\eta \\to \\mu^+ \\mu^- \\textrm{e}^+ \\textrm{e}^-) = (2.4 \\pm 0.8)\\times 10^{-6}$, 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.","hasPart":[{"@id":"https://doi.org/10.17182/hepdata.168684.v1/t1","@type":"Dataset","description":"Invariant mass histogram for signal channel, data with signal and resonant background MC distributions.","name":"Figure 2 (upper)"},{"@id":"https://doi.org/10.17182/hepdata.168684.v1/t2","@type":"Dataset","description":"Invariant mass histogram for reference channel, data with reference MC distribution.","name":"Figure 2 (lower)"},{"@id":"https://doi.org/10.17182/hepdata.168684.v1/t3","@type":"Dataset","description":"Measured ratio of $\\mathcal{B}(\\eta \\rightarrow 2 \\mu 2e) / \\mathcal{B}(\\eta \\rightarrow 2 \\mu \\gamma)$","name":"Ratio of branching fractions"},{"@id":"https://doi.org/10.17182/hepdata.168684.v1/t4","@type":"Dataset","description":"Measured branching fraction $\\mathcal{B}(\\eta \\rightarrow 2\\mu 2e)$","name":"Measured branching fraction"}],"identifier":[{"@type":"PropertyValue","propertyID":"HEPDataRecord","value":"https://www.hepdata.net/record/ins3150815?version=1"},{"@type":"PropertyValue","propertyID":"HEPDataRecordAlt","value":"https://www.hepdata.net/record/168684"}],"inLanguage":"en","name":"Observation of the rare decay $\\eta$ $\\to$ $\\mu^+\\mu^-$e$^+$e$^-$","provider":{"@type":"Organization","name":"HEPData"},"publisher":{"@type":"Organization","name":"HEPData"},"url":"https://www.hepdata.net/record/ins3150815?version=1","version":1}