This talk describes contributions to a search for Heavy Neutral Leptons (HNLs), hypothetical particles beyond the Standard Model, using data from the ATLAS detector collected in 13 TeV proton-proton collisions. The search targets a signature involving a prompt lepton and a displaced dilepton vertex, corresponding to HNL production and decay from W boson to a prompt lepton and an HNL, with the HNL traveling some distance in the inner-detector and decaying into two leptons and a neutrino. A significant challenge is accurately estimating background processes, primarily from irreducible random track crossings, which occur when unrelated tracks coincidentally form a displaced vertex. My main contribution was the development and implementation of a data-driven method to estimate this background. This involved combining tracks from different events to generate a large, clean sample of random track crossings, significantly increasing the statistics available for studying this background. Along the way we had to tackle…