To date, LHC searches have provided no definitive evidence for physics beyond the Standard Model (BSM). What if we’re looking in the wrong place? Long-lived particles (LLPs) arise from a variety of well-motivated BSM theories. However, they remain weakly constrained at the LHC: most reconstruction algorithms target prompt decays of particles rather than the displaced decays of LLPs. LLP searches are generally signature-driven, both because several different models can often predict the same signature and because the unconventional nature of LLP signatures requires extensive knowledge of the detector and dedicated reconstruction algorithms. In this seminar, I will discuss the first ever ATLAS search for a displaced and delayed diphoton resonance and the intricate detector-level expertise necessary to isolate this challenging and striking signature. I will focus on the development and performance of our specialized analysis tools, including a novel method of photon vertexing, which exploits detailed low-level calorimeter information. Our unique approach to event reconstruction has greatly enhanced the sensitivity of the analysis and contributed to some of the first exclusions for long-lived higgsinos.