  We show that in a confining hidden valley model where the lightest hidden
particles are dark hadrons that have mass splittings larger than
$\mathcal{O}(0.1)$ GeV, if the lightest dark hadron is either stable or decays
into Standard Model (SM) hadrons/charged leptons during the big-bang
nucleosynthesis (BBN), at least one of the heavier dark hadrons needs to decay
into SM particles within $\mathcal{O}(10)$ nanosec. Once being produced at
collider experiments, this heavier dark hadron is likely to decay within
$\mathcal{O}(1)$ meter distance, which strengthens the motivation of searching
for long-lived particles with sub-meter scale decay lengths at colliders. To
illustrate the idea, we study the lifetime constraint in scenarios where the
lightest dark particle is a pseudo-scalar meson, and dark hadrons couple to SM
particles either through kinetic mixing between the SM and dark photons or by
mixing between the SM and dark Higgs. We study the annihilation and decay of
dark hadrons in a thermal bath and calculate upper bounds on the lightest
vector meson (scalar hadron) lifetime in the kinetic mixing (Higgs portal)
scenario. We discuss the application of these lifetime constraints in
long-lived particle searches that use the LHCb VELO or the ATLAS/CMS inner
detectors.
