Compact Muon Solenoid
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| CMS-PAS-BPH-24-001 | ||
| Observation of the rare decay $ \eta \to \mu^+\mu^-\mathrm{e^+e^-} $ | ||
| CMS Collaboration | ||
| 18 December 2025 | ||
| Abstract: A first observation of the rare decay $ \eta \to \mu^+\mu^-\mathrm{e^+e^-} $ with significance well in excess of 5 standard deviations is reported by the CMS Collaboration at the CERN LHC. The result is based on a data sample corresponding to an integrated luminosity of 38.0 $ \mathrm{fb^{-1}} $, acquired in 2022 using a special high-rate dimuon trigger. A measurement of the branching fraction of this decay mode, using the decay mode $ \eta \to \mu^+\mu^-\gamma $ as normalization, yields a value $ \mathcal{B}(\eta\to\mu^+\mu^-\mathrm{e^+e^-}) = ( $ 2.1 $ \pm $ 0.7 $) \times 10^{-6} $, including statistical and systematic sources of uncertainty, along with the uncertainty in the branching fraction of the reference mode. This observed branching fraction is close to two orders of magnitude smaller than the existing limit, and is consistent with the latest standard model prediction. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
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Compact Muon Solenoid LHC, CERN |
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