This thesis describes the analysis design and results of the ATLAS four-lepton measurement, using 139 fb−1 of data collected in 13 TeV proton-proton collisions at the Large Hadron Collider. The measurement is designed for maximal model-independence and inclusivity. Defined solely in terms of the final state particles, any process leading to the creation of four or more leptons is considered to be a part of the signal. The results are presented in the form of inclusive and fiducial differential cross-sections, and are corrected for detector effects via an iterative Bayesian technique. The measurement is compared to state- of-the-art Standard Model predictions, and the two are found to be consistent. Secondly, two re-interpretation studies are presented where existing precision fiducial measurements, including the aforementioned four-lepton measurement, are used to set constraints on two beyond the Standard Model theories. The first is a generic model of vector-like quarks, and the second is a model with a gauged and spontaneously broken B − L symmetry. These studies are conducted using the CONTUR re-interpretation toolkit. The derived limits are competitive with existing ATLAS limits, and exclude previously unexplored regions of parameter space
