Search for charged lepton flavor violating Z and Z' boson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper reports a search for charged lepton flavor violating decays of Z and Z' bosons in proton-proton collisions at 13 TeV, setting the most stringent limits to date on certain decay channels and exploring potential Z' decays.

Contribution

It provides the first stringent direct limits on Z boson flavor violating decays and searches for Z' decays in the eμ final state using CMS data at 13 TeV.

Findings

No evidence for flavor violating Z or Z' decays was observed.

The most stringent upper limit on Z → eμ branching fraction is 1.9×10⁻⁷.

Limits on Z' → eμ decays range from 11.4×10⁻⁶ to 13.8×10⁻⁶.

Abstract

A search for flavor violating decays of the Z boson to charged leptons is performed using data from proton-proton collisions at $\sqrt{s}$ = 13 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Each of the decays Z $\to$ e$\mu$, Z $\to$ e$\tau$, and Z $\to$ $\mu\tau$ is considered. The data are consistent with the backgrounds expected from standard model processes. For the Z $\to$ e$\mu$ channel the observed (expected) 95% confidence level upper limit on the branching fraction is 1.9 (2.0) $\times$ 10$^{-7}$, which is the most stringent direct limit to date on this process; the corresponding limits for the Z $\to$ e$\tau$ and Z $\to$ $\mu\tau$ channels are 13.8 (11.4) $\times$ 10$^{-6}$ and 12.0 (5.3) $\times$ 10$^{-6}$, respectively. Additionally, the e$\mu$ 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.