Search for the lepton flavor violating decay $τ$ $\to$ 3$μ$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Abstract: Results are reported from a search for the lepton flavor violating decay $\tau$ $\to$ 3$\mu$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data sample corresponds to an integrated luminosity of 33.2 fb$^{-1}$ recorded by the CMS experiment at the LHC in 2016. The search exploits $\tau$ leptons produced in both W boson and heavy-flavor hadron decays. No significant excess above the expected background is observed. An upper limit on the branching fraction $\mathcal{B}(\tau$ $\to$ 3$\mu)$ of 8.0 $\times$ 10$^{-8}$ at 90% confidence level is obtained, with an expected upper limit of 6.9 $\times$ 10$^{-8}$.