  This thesis presents results and method developments in both experimental and
theoretical particle physics. The main part shows measurements of
$\ell^+\ell^-\ell'^+\ell'^-$ production (where $\ell$, $\ell'$ is either an
electron or a muon) in proton-proton collisions at 13 TeV centre-of-mass
energy. The collisions were produced by the Large Hadron Collider in 2015 and
2016 and observed with the ATLAS detector. In a phase space sensitive to Z
boson pair production, the integrated cross section as well as differential
cross sections with respect to twenty-one observables are measured. Ten of
these directly measure associated jet activity. The measurements provide an
important test of the Standard Model of particle physics. A direct search for
effects beyond the Standard Model affecting ZZ production is performed in a
generic effective field theory approach. No significant deviations from the
Standard Model predictions are observed. Exclusion limits are set on the
parameters describing new physics in the effective field theory. In theoretical
developments, the automated description of loop-induced processes with the
Herwig 7 event generator is presented. These are processes that can only occur
via a quantum loop of virtual particles. Preliminary results in leading-order
quantum chromodynamics are shown for the production of a Higgs boson, of a pair
of Higgs bosons, and of four leptons. The Higgs boson results show that the
full loop-induced description can deviate significantly from the common
approximation where the mass of the top quark is treated as infinitely large.
Thus, including loop effects is crucial to obtaining precise predictions to
compare to measurements at the Large Hadron Collider. Developments towards a
next-to-leading-order description of arbitrary loop-induced processes are
shown.
