Observation of the rare decay of the η 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 (B 2 μ)] × 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.