Since its inception, the Large Hadron Collider (LHC) has been pivotal in validating and stress-testing the Standard Model. However, the scarcity of clear signals of new physics at the LHC motivates us to explore alternative experimental frontiers. In this talk, we investigate two complementary approaches that probe distinct particle lifetime regimes: cosmic-ray beam dumps which are highly sensitive to long-lived particles such as Heavy Neutral Leptons (HNLs), and future colliders like FCC-ee which excel at exploring the physics of prompt particles.
For the cosmic-ray beam dump scenario, we demonstrate that, under optimal conditions, such experiments can test the Type-I seesaw limit for HNLs. For the collider case, our projections for a leptophilic Z′ show that FCC-ee offers superior sensitivity compared to other proposed lepton colliders within the kinematically accessible mass range. Together, these complementary lifetime frontiers provide a broader and more versatile framework for uncovering physics beyond the Standard Model.
Ankita Budhraja, Phillipp Klose, Marvin Schnubel