Observation of the Rare Decay of the n Meson to Four Muons

Abstract

A search for the rare η → μ+μ−μ+μ− 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 η → μ+μ− decay as normalization, the branching fraction B(η → μ+μ−μ+μ−) = [5.0 ± 0.8(stat) ± 0.7(syst) ± 0.7(β₂μ)] × 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.