  Confining gauge dynamics in dark sector is promising to provide dark matter
with a mass in the range of sub-GeV to GeV. These dark sectors consist of
composite particles, such as dark baryons and dark mesons, that are neutral
under the standard-model charge. Dark photon is introduced as a portal matter
between the dark sector and the standard-model sector in order to alleviate
cosmological problems, and dark hadrons are produced through the same dark
photon at accelerator-based experiments. As dark vector mesons and dark pions
have the similar masses, dark vector mesons can be long-lived particles, which
will be explored by far-detector experiments. In this study, we study the
future prospect of the LHC forward-detector experiments, FASER2 and FACET, for
exploring the dark vector mesons. When dark photon is heavier than dark pions,
the LHC forward-detector searches can be comparable to DarkQuest, and the
invisible decay searches of dark photons can also explore the same parameter
space. Meanwhile, when dark photon is lightest in the dark sector, their future
prospect will be comparable to the visible decay searches for dark photons at
LHCb, Belle-II, and HPS.
