Observation of (Formula presented) Production at the CMS Experiment

The first observation of single top quark production in association with a W and a Z boson in protonproton collisions is reported. The analysis uses data at center-of-mass energies of 13 and 13. 6 TeV recorded with the CMS detector at the CERN LHC, corresponding to a total integrated luminosity of 200 fb^−1. Events with three or four charged leptons, which can be electrons or muons, are selected. Advanced machine-learning algorithms and improved reconstruction methods, compared to an earlier analysis, result in an unprecedented sensitivity to tWZ production. The measured cross sections for tWZ production are 248 52 fb and 242 77 fb for s p ¼ 13 and 13. 6 TeV, respectively. The signal is established with a statistical significance of 5. 8 standard deviations, with 3. 5 expected, compared to the background-only hypothesis.