A measurement of top quark pair production in association with a Z boson decaying to neutrinos

This thesis presents results for two ATLAS analyses measuring the cross section of ttZ,
analyzing events where the Z-boson decays into a neutrinos pair (Z −→ νν), and one
piece of technical research into improving the pile-up corrections for ATLAS’s forward
calorimeter using topological towers.
Both ttZ analyses use the ATLAS full Run 2 dataset with centre-of-mass energy √s =
13 TeV and an integrated luminosity of 140 fb−1. These analyses are presented in part
II, the first analysis, a standalone zero-lepton channel analysis, while the second analyses
is a combined zero, one and two-lepton analyses. The standalone analysis obtained a
cross-section of σ(ttZ)−→νν = 0.11+0.08
−0.07 pb in good agreement with standard model predictions
and a signal significance of Z = 2.47 σ, this region is then used in the combined analysis
with the one and two lepton channels, showing an expected signal significance of Z =
4.9 σ. This result will allow dark matter and SUSY searches to further constrain their main
background, improving their sensitivity.
The technical research is presented in part III and uses ATLAS Run 2 simulations to
exploit topological towers in the forward region of the ATLAS calorimeter to model the
impact of pile-up on jet reconstruction. This reconstruction method gives better modeling
of forward jet pile-up corrections, improving over previous attempts using calorimeter
topological clusters. We used this improved modeling of the pile-up to create an equation
that would give a value for the pile-up correction needed based on the in and out-of-time
pile-up conditions at ATLAS, which showed improved performance over previous methods.
The implementation of this algorithm for Run 3 data will result in better precision of
forward jets in measurements and will be especially important to rare standard model
searches.