  The first search for the Z boson decay to $\tau\tau\mu\mu$ at the CERN LHC is
presented, based on data collected by the CMS experiment at the LHC in
proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding
to an integrated luminosity of 138 fb$^{-1}$. The data are compatible with the
predicted background. For the first time, an upper limit at the 95% confidence
level of 6.9 times the standard model expectation is placed on the ratio of the
Z $\to$ $\tau\tau\mu\mu$ to Z $\to$ 4$\mu$ branching fractions. Limits are also
placed on the six flavor-conserving four-lepton effective-field-theory
operators involving two muons and two tau leptons, for the first time testing
all such operators.
