  Long-lived particles have significant enough lifetimes as to, when produced
in collisions, leave a distinct signature in the detectors. Driven by
increasingly higher energies, trigger and reconstruction algorithms at particle
colliders are optimized for increasingly heavier particles, which in turn, tend
to be short-lived. This makes searches for long-lived particles difficult,
usually requiring dedicated methods and sometimes dedicated hardware top spot
them. However, taking upon the challenge brings enormous potential, since new,
long-lived particles feature in a variety of promising new physics models that
could answer most of the open questions of the standard model, such as:
neutrino masses, Dark Matter, or the matter-antimatter unbalance in the
Universe. Currently, the international high energy physics community is
planning future facilities post-LHC, and various particle colliders have been
proposed. Crucial physics cases connected to long-lived particles will be
accessible then, and in this presentation, three interesting examples are
highlighted: Heavy Neutral Leptons, Hidden Sectors connected to Dark Matter,
and exotic Higgs boson decays. This is followed by a small review of the
preliminary studies assuming different future colliders, exploiting the
complementary advantages that different colliding particles and accelerator
types provide.
