Measurement of gauge boson joint-polarisation states in WZ pair production at the LHC with the ATLAS detector

Within the Standard Model of particle physics, the electroweak spontaneous symmetry breaking mechanism is responsible for the appearance of a longitudinal polarisation state in the massive $W$± and $Z$ vector bosons. The study of joint-polarisation states in diboson processes, corresponding to the pair production of two such vector bosons, constitutes a detailed test of the Standard Model as it allows to probe indirectly this symmetry breaking mechanism. This thesis studies the $W$±$Z$ diboson process using the data recorded by the ATLAS detector during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb$^{−1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV. The ATLAS detector relies on a trigger system to manage the large amount of data produced at the LHC and be able to keep rare events such as those from diboson processes. The upgrade of this trigger system in preparation for the Run 3 of the LHC is also presented in this thesis, focusing on the new Digital Trigger of the Liquid Argon calorimeters. In the Run 2 dataset, events of $W$±$Z$ inclusive production with leptonic decay of the two gauge bosons into electrons or muons are selected. In such events, the four joint-polarisation fractions, considering longitudinal or transverse polarisation states for each boson, are extracted through a binned maximum-likelihood template fits. Polarisation fractions not being Lorentz invariant quantities, they are obtained in the $W$±$Z$ centre-of-mass frame. Special care is taken in the choice of a discriminating variable for the template fit and the development of accurate polarisation templates, relying in particular on advanced Deep Neural Networks techniques. The simultaneous pair-production of longitudinally polarised vector bosons is measured for the first time with a significance of 7.1 standard deviations. The measured joint-polarisation fractions integrated over the fiducial region are $f$$_{00}$ = 0.067 ± 0.010, $f$$_{0T}$ = 0.110 ± 0.029, $f$$_{T0}$ = 0.179 ± 0.023 and $f$$_{TT}$ = 0.644 ± 0.032, in agreement with the next-to-leading-order Standard Model predictions. Single boson polarisation fractions of the $W$ and $Z$ bosons are also measured and found to be consistent with joint-polarisation fractions within the expected amount of correlations. Both the joint and single boson polarisation fractions are also measured separately in $W$+$Z$ and $W$−$Z$ events.