A search for weakly interacting massive darkmatter
particles produced in association with bottom or top
quarks is presented. Final states containing third-generation
quarks and missing transverse momentum are considered.
The analysis uses 36.1 fb−1 of proton–proton collision data
recorded by the ATLAS experiment at √s = 13 TeV in 2015
and 2016. No significant excess of events above the estimated
backgrounds is observed. The results are interpreted
in the framework of simplified models of spin-0 dark-matter
mediators. For colour-neutral spin-0 mediators produced in
association with top quarks and decaying into a pair of darkmatter
particles, mediator masses below 50 GeV are excluded
assuming a dark-matter candidate mass of 1 GeV and unitary
couplings. For scalar and pseudoscalar mediators produced in
association with bottom quarks, the search sets limits on the
production cross-section of 300 times the predicted rate for
mediators with masses between 10 and 50 GeV and assuming
a dark-matter mass of 1 GeV and unitary coupling. Constraints
on colour-charged scalar simplified models are also
presented. Assuming a dark-matter particle mass of 35 GeV,
mediator particles with mass below 1.1 TeV are excluded
for couplings yielding a dark-matter relic density consistent
with measurements.