FACET: Forward Aperture CMS ExTension for LLP Searches

The FACET project to search for Beyond the Standard Model long-lived particles at the LHC is being developed as a new very forward subsystem of CMS. Located 100 - 120 m from the CMS collision point it will be sensitive to particles that can penetrate up to 50 m of iron absorber and then decay in a vacuum tank approximately 1 m diameter and 12 m long to standard model particles. These are measured in silicon tracking, calorimeters and a toroidal muon spectrometer using CMS Phase-2 upgrade detector technology. FACET has sensitivity to dark photons, heavy neutral leptons, axion-like particles and dark Higgs bosons in a unique region of mass and coupling strength.