Measurement of the top quark coupling to the Z boson, and applications to searches for physics Beyond the Standard Model

This thesis presents results using data collected by the ATLAS experiment in the 2015–2016 and 2015–2018 periods, corresponding to integrated luminosities at a centre-of-mass energy √s = 13 TeV of 36.1fb−1 and 139fb−1 respectively. It explores a connection between the top quark sector of the Standard Model and searches for supersymmetry in the all-hadronic final state.
In Part II, a measurement of the inclusive cross section of the tt ̄Z process in multi-lepton
events results in σ(tt ̄Z) = 0.95±0.13pb with 36.1fb−1 of data, and σ(tt ̄Z) = 1.09±0.10pb with the
full LHC Run 2 dataset. Both results are compatible with the Standard Model prediction of
σ(theory) = 0.863+0.09-0.10(scale) ± 0.03(PDF+αs) pb. In the former analysis, exclusion limits are set on relevant dimension-6 effective field theory operators, while in the latter, the very first measurement of the differential tt ̄Z cross section at ATLAS is presented.
In Part III, a search for the supersymmetric partner to the top quark in the all-hadronic final state, characterised by six or more jets and large ETmiss, is described. No significant excess over the expected Standard Model background is observed, using 36.1fb of ATLAS data. Assuming a
100% branching ratio of t ̃1 → t + χ ̃01, stop masses are excluded up to 1TeV for neutralino masses smaller than 350GeV. The estimation of the irreducible tt ̄Z(→ νν ̄) through a boson replacement technique is described in detail. Following the results obtained in Part II, the use of a multi-lepton tt ̄Z control sample is instead proposed and shown to significantly improve modelling uncertainties in a subsequent analysis using the full LHC Run 2 dataset.