Compressed dark sectors, with invisible decay products, are notoriously hard to probe at hadron colliders, particularly in the region of masses that are most relevant for thermal relics. Current search strategies rely mainly on hard radiation off the initial state. If the compressed sector contains an electrically-charged state, it is also possible to search for charged tracks that 'disappear' within the collider volume. A well-motivated example of a framework that contains such particles is the Minimal Supersymmetric Standard Model in the pure higgsino or wino limits. The current hadron-collider search strategy has no sensitivity to the upper range of pure higgsino masses that are consistent with the thermal relic density, even at a future circular collider (FCC) at 100~TeV centre-of-mass - the proper lifetime of its charged component, at around 10 picoseconds, is too short to be detected.
In this seminar I will discuss what changes to analysis strategy and detector setup will be necessary to definitively discover or exclude the experimentally-elusive pure-higgsino thermal relic at the FCC, thus closing the window on neutralino dark matter.