HECATE

Abstract The next generation of circular high energy collider is expected to be a lepton collider, FCC-ee at CERN or CEPC in China. However, the civil engineering concepts foresee equip these colliders with bigger detector caverns than one would need for so that they can used hadron may installed same tunnel without further engineering. This opens up possibility install extra instrumentation cavern walls search new long-lived particles collider. We use example heavy neutral leptons show such an installation could improve sensitivity squared mixing parameter by almost half order magnitude.