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

Abstract

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 $\sqrt{s}=13$ TeV of $36.1\,\mathrm{fb}^{-1}$ and $139\,\mathrm{fb}^{-1}$ 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 $t\bar{t}Z$ process in multi-lepton events results in $$\sigma_{t\bar{t}Z}= 0.95\pm 0.13\,\text{pb}$$ with $36.1\,\mathrm{fb}^{-1}$ of data, and $$\sigma_{t\bar{t}Z}= 1.09\pm 0.10\,\text{pb}$$ with the full LHC Run 2 dataset. Both results are compatible with the Standard Model prediction of $$\sigma_{t\bar{t}Z}^\text{theory}= 0.863^{+0.09}_{-0.10}\text{(scale)}\pm 0.03(\mathrm{PDF}+\alpha_s)\,\text{pb}.$$ 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 $t\bar{t}Z$ 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 $E_\mathrm{T}^\text{miss}$, is described. No significant excess over the expected Standard Model background is observed, using $36.1\,\mathrm{fb}^{-1}$ of ATLAS data. Assuming a 100% branching ratio of $\tilde{t}_1\to t+\tilde{\chi}_1^0$, stop masses are excluded up to 1 TeV for neutralino masses smaller than 350 GeV. The estimation of the irreducible $t\bar{t}Z(\to\nu\bar{\nu})$ through a boson replacement technique is described in detail. Following the results obtained in Part II, the use of a multi-lepton $t\bar{t}Z$ control sample is instead proposed and shown to significantly improve modelling uncertainties in a subsequent analysis using the full LHC Run 2 dataset.

Baptiste Ravina
Baptiste Ravina
Saltire Fellow (Royal Society of Edinburgh)

I’m an experimental particle physicist looking for new physics at the ATLAS experiment with machine learning.