  Results of a search for new particles decaying into eight or more jets and
moderate missing transverse momentum are presented. The analysis uses 139
fb$^{-1}$ of proton$-$proton collision data at $\sqrt{s} = 13$ TeV collected by
the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The
selection rejects events containing isolated electrons or muons, and makes
requirements according to the number of $b$-tagged jets and the scalar sum of
masses of large-radius jets. The search extends previous analyses both in using
a larger dataset and by employing improved jet and missing transverse momentum
reconstruction methods which more cleanly separate signal from background
processes. No evidence for physics beyond the Standard Model is found. The
results are interpreted in the context of supersymmetry-inspired simplified
models, significantly extending the limits on the gluino mass in those models.
In particular, limits on the gluino mass are set at 2 TeV when the lightest
neutralino is nearly massless in a model assuming a two-step cascade decay via
the lightest chargino and second-lightest neutralino.
