  We describe a proposal to add a set of very forward detectors to the CMS
experiment for the high-luminosity era of the Large Hadron Collider to search
for beyond the standard model long-lived particles, such as dark photons, heavy
neutral leptons, axion-like particles, and dark Higgs bosons. The proposed
subsystem is called FACET for Forward-Aperture CMS ExTension, and will be
sensitive to any particles that can penetrate at least 50 m of magnetized iron
and decay in an 18 m long, 1 m diameter vacuum pipe. The decay products will be
measured in detectors using identical technology to the planned CMS Phase-2
upgrade.
