  Is Dark Matter part of a Dark Sector? The possibility of a dark sector
neutral under Standard Model (SM) forces furnishes an attractive explanation
for the existence of Dark Matter (DM), and is a compelling new-physics
direction to explore in its own right, with potential relevance to fundamental
questions as varied as neutrino masses, the hierarchy problem, and the
Universe's matter-antimatter asymmetry. Because dark sectors are generically
weakly coupled to ordinary matter, and because they can naturally have
MeV-to-GeV masses and respect the symmetries of the SM, they are only mildly
constrained by high-energy collider data and precision atomic measurements. Yet
upcoming and proposed intensity-frontier experiments will offer an
unprecedented window into the physics of dark sectors, highlighted as a
Priority Research Direction in the 2018 Dark Matter New Initiatives (DMNI) BRN
report. Support for this program -- in the form of dark-sector analyses at
multi-purpose experiments, realization of the intensity-frontier experiments
receiving DMNI funds, an expansion of DMNI support to explore the full breadth
of DM and visible final-state signatures (especially long-lived particles)
called for in the BRN report, and support for a robust dark-sector theory
effort -- will enable comprehensive exploration of low-mass thermal DM
milestones, and greatly enhance the potential of intensity-frontier experiments
to discover dark-sector particles decaying back to SM particles.
