  We discuss the potential of using long-lived particle (LLP) searches for
right-handed neutrinos (RHNs) to test resonant leptogenesis and the seesaw
mechanism. This is challenging if only RHNs are added to the Standard Model
(SM), as naturally the active-sterile mixing strengths $|V_{\ell N}|^2$ are
small, for 1 GeV $\lesssim M_N \lesssim 1000$ GeV. Instead, we consider the
minimal $B-L$ gauge model, where a $Z^\prime$ gauge boson couples to fermions
including the RHNs. During leptogenesis, this gauge coupling introduces
scattering processes that washout the $B-L$ asymmetry. At colliders, it can
lead to abundant production of RHNs which allows probing the associated seesaw
mechanism using LLP searches. We find that LLP searches at the FCC-hh can test
leptogenesis and the seesaw mechanism simultaneously, and probe the
active-sterile mixing at or below the seesaw floor.
