Search for the lepton flavor violating decay tau to 3mu in proton-proton collisions at sqrt(s) = 13 TeV

A search for lepton flavor violating $\tau\to3\mu$ decays is performed using 97.7\fbinv of proton-proton collisions at a center-of-mass energy of 13\TeV collected by the CMS experiment at the LHC in 2017 and 2018. Tau leptons produced in heavy-flavor hadron decays and produced in \PW boson decays are exploited in the analysis. The results of this search are combined with the previous result based on data collected in 2016, to obtain a total integrated luminosity of 131\fbinv. The observed (expected) upper limit at 90\% confidence level on the branching fraction $\mathcal{B}(\tau\to3\mu)$ is $2.9 \times 10^{-8}$ ($2.4 \times 10^{-8}$). The observed (expected) upper limit at 95\% confidence level on the branching fraction $\mathcal{B}(\tau\to3\mu)$ is $3.6 \times 10^{-8}$ ($3.0 \times 10^{-8}$).