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This thesis presents results and method developments in both experimental andtheoretical particle physics. The main part shows measurements of$\ell^+\ell^-\ell'^+\ell'^-$ production (where $\ell$, $\ell'$ is either anelectron or a muon) in proton-proton collisions at 13 TeV centre-of-massenergy. The collisions were produced by the Large Hadron Collider in 2015 and2016 and observed with the ATLAS detector. In a phase space sensitive to Zboson pair production, the integrated cross section as well as differentialcross sections with respect to twenty-one observables are measured. Ten ofthese directly measure associated jet activity. The measurements provide animportant test of the Standard Model of particle physics. A direct search foreffects beyond the Standard Model affecting ZZ production is performed in ageneric effective field theory approach. No significant deviations from theStandard Model predictions are observed. Exclusion limits are set on theparameters describing new physics in the effective field theory. In theoreticaldevelopments, the automated description of loop-induced processes with theHerwig 7 event generator is presented. These are processes that can only occurvia a quantum loop of virtual particles. Preliminary results in leading-orderquantum chromodynamics are shown for the production of a Higgs boson, of a pairof Higgs bosons, and of four leptons. The Higgs boson results show that thefull loop-induced description can deviate significantly from the commonapproximation where the mass of the top quark is treated as infinitely large.Thus, including loop effects is crucial to obtaining precise predictions tocompare to measurements at the Large Hadron Collider. Developments towards anext-to-leading-order description of arbitrary loop-induced processes areshown.

Measurements of ZZ production with the ATLAS detector and simulation of loop-induced processes with the HERWIG event generator