  We study dark matter (DM) production in the mono-Z channel at the 13 TeV LHC
both in an effective field theory framework as well as in simplified models
with vector mediators, using a multivariate analysis. For DM-quark effective
operators with scalar, vector, and tensor couplings and DM mass of 100 GeV, the
5$\sigma$ reach in the DM interaction scale $\Lambda$ is around 2, 1, and 3
TeV, respectively, for 3 ab$^{-1}$ and assuming a 5\% systematic uncertainty on
the total background normalization. For simplified models with leptophobic
vector mediators, the 5$\sigma$ reach for the mass of the mediator is 1.7 TeV
also assuming a 5\% systematics and 3 ab$^{-1}$ of integrated luminosity. The
reach for the dark matter interaction scale obtained with the multivariate
analysis using a likelihood function discriminant is at least twice as high as
that obtained from a simple cut and count analysis, once systematics on the
background normalization larger than a few percent are taken into account.
Moreover, the reach is much more stable against degradation due these
systematic uncertainties.
