A search for flavor violating decays of the Z boson to charged leptons is performed using data from proton-proton collisions at s = 13 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb − 1 . Each of the decays Z → e μ , Z → e τ , and Z → μ τ is considered. The data are consistent with the backgrounds expected from standard model processes. For the Z → e μ channel the observed (expected) 95% confidence level upper limit on the branching fraction is 1.9 ( 2.0 ) × 10 − 7 , which is the most stringent direct limit to date on this process; the corresponding limits for the Z → e τ and Z → μ τ channels are 13.8 ( 11.4 ) × 10 − 6 and 12.0 ( 5.3 ) × 10 − 6 , respectively. Additionally, the e μ final state is used to search for lepton flavor violating decays of Z ′ resonances in the mass range from 110 to 500 GeV . No significant excess is observed above the predicted background levels.
Hayrapetyan et al. (Mon,) studied this question.