Measurements of the production cross section of a Z boson in association with high-transverse-momentum jets with the ATLAS detector at the Large Hadron Collider

In this thesis, the measurements of the production cross section of a $Z$ boson produced in association with high-transverse-momentum jets and decaying into a charged-lepton pair are presented. The data used for these measurements are analyzed from proton--proton collisions at the Large Hadron Collider. The dataset corresponds to an integrated luminosity of $139$ fb$^{-1}$ collected at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector between 2015 and 2018. The cross-section measurements are performed separately for the electron and muon decay channels of the $Z$ boson and then combined in order to increase the precision of the final $Z(\to \ell^+\ell^-)\text{+jets}$ measurements. Events with at least one jet with $p_{\mathrm{T}} \geq 500~GeV/c$ are selected and populate a high-$p_{\mathrm{T}}$ region. In this region, the production of an on-shell $Z$ boson radiated by a quark is enhanced and results in events with a small angle between the $Z$ boson and the associated quark jet such that the two objects are measured to be collinear. A fraction of the events in the high-$p_{\mathrm{T}}$ region are also described by $Z\text{+1 jet}$ events where the $Z$ boson and the jet recoil against each-other such that the two objects are measured to be back-to-back. The large dataset has made it possible for the first time to separately study these two event topologies where a $Z$ boson and a jet are either collinear or back-to-back. This is achieved by measuring the angular correlation between the $Z$ boson and the closest jet. The production of a $Z$ boson in association with jets, $Z\text{+jets}$, provides a near inexhaustible amount of information about the physics mechanism for the production of vector bosons at the Large Hadron Collider. This analysis allows to measure the cross section of $Z\text{+jets}$ as a function of characteristic observables. The resulting differential cross sections are compared with state-of-the-art theoretical predictions. The differential cross sections themselves provide a powerful way to test the Standard Model and in particular quantum chromodynamics. The data are found to agree with the latest next-to-next-to-leading-order and next-to-leading-order theoretical predictions for $Z\text{+jets}$ production.