  The first collider search for dark matter arising from a strongly coupled
hidden sector is presented and uses a data sample corresponding to 138
fb$^{-1}$, collected with the CMS detector at the CERN LHC, at $\sqrt{s} =$ 13
TeV. The hidden sector is hypothesized to couple to the standard model (SM) via
a heavy leptophobic Z' mediator produced as a resonance in proton-proton
collisions. The mediator decay results in two "semivisible" jets, containing
both visible matter and invisible dark matter. The final state therefore
includes moderate missing energy aligned with one of the jets, a signature
ignored by most dark matter searches. No structure in the dijet transverse mass
spectra compatible with the signal is observed. Assuming the Z' has a universal
coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model
that exhibits this kinematic behavior, excludes mediator masses of 1.5-4.0 TeV
at 95% confidence level, depending on the other signal model parameters. To
enhance the sensitivity of the search for this particular class of hidden
sector models, a boosted decision tree (BDT) is trained using jet substructure
variables to distinguish between semivisible jets and SM jets from background
processes. When the BDT is employed to identify each jet in the dijet system as
semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges
of the other signal model parameters. These limits exclude a wide range of
strongly coupled hidden sector models for the first time.
