This thesis presents an analysis of events with large transverse missing
momentum in association with jets using 139 fb−1 of proton-proton
collisions at a centre of mass energy of 13 TeV , that was delivered at
the Large Hadron Collider and recorded using the ATLAS detector
from 2015 − 2018. The dominant process which contributes to these
events is the Z boson decaying to two neutrinos followed closely
by the contribution from W bosons decaying leptonically, in which
the charged lepton is outside the detector acceptance. The similarity
of these processes to Z and W bosons decaying leptonically can be
exploited by measuring the one lepton and two lepton regions, and
treating the leptons as invisible in order to constrain modelling along
with the experimental and theoretical uncertainties. These lepton
regions are known as the auxiliary regions.
This analysis was performed using three different phase-spaces
that are sensitive to different Dark Matter production channels. These
three phase-spaces require the ≥ 1 jet , ≥ 2 jet and VBF topologies.
The total yield is measured at the detector level and the differential
cross section is measured at the particle level as a function of the
missing transverse momentum, the dijet invariant mass and the dijet
azimuthal angle. Ratios of these cross sections are also presented
at the particle level to facilitate comparisons between regions and
minimise the systematic uncertainties.
These results are interpreted using a likelihood fit and the agree-
ment between the modelling and the data is quantified using a χ2
test and p-value. Constraints are made on the axial-vector and pseu-
doscalar simplified Dark Matter models and are shown to be competitive with results from the recent dedicated monojet-like search in ATLAS.