Measurements for the Production Cross-section of a Z Boson in Association with High-transverse-momentum Jets with the ATLAS Detector at the Large Hadron Collider

Abstract

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(→ l+l−)+jets measurements. Events with at least one jet with pT ≥ 500 GeV/c are selected and populate a high-pT 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-pT region are also described by Z+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+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+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 chromo-dynamics. The data are found to agree with the latest next-to-next-to-leading-order and next-to-leading-order theoretical predictions for Z+jets production.