Wednesday 25 May
13:00 - 16:00
This thesis presents a search for dark matter with the ATLAS experiment using 139 fb−1 of pp collision data at
√s = 13 TeV delivered by the LHC from 2015 to 2018. The search targets a simplified model where a spin-0 scalar or
pseudoscalar mediator particle, decaying to a pair of dark matter particles, is produced in association with a pair of top
quarks. The final state of interest includes one charged lepton, multiple jets and missing transverse momentum. No
excess over the Standard Model background is observed and new exclusion limits at 95% confidence level are placed on
the mediator mass. Mediator masses up to 200 GeV are excluded for a dark matter mass of 1 GeV and a coupling
strength of one. The one-lepton analysis is statistically combined with those targeting final states with zero or two
leptons. The combined results extend the exclusion limits for mediator masses up to 370 GeV. These results are also
used to set an upper limit on the Higgs boson branching ratio into invisible particles of 0.40 at 95% confidence level.
Luminosity is a key measure of the LHC performance which relates the number of events of a physics process to its
cross-section. This thesis presents systematic studies of the track-counting algorithm which is used to measure the
luminosity in ATLAS.
Keywords: LHC, ATLAS, luminosity, track counting, dark matter, BSM.