The FACET Project: Forward Aperture CMS ExTension to search for new Long-Lived Particles

Abstract

FACET is a proposed new subsysem for CMS to search for portals such as dark photons, dark higgs, heavy neutral leptons and axion-like particles in the very forward direction at the High Luminosity LHC. Such particles can penetrate up to 50 m of iron and then decay inside a 14 m 3 vacuum pipe made by enlarging an 18 m long section of the LHC pipe to a radius of 50 cm. IntroductionFACET, short for Forward Aperture CMS ExTension, is a project under development to add a subsystem to CMS to search for beyond the standard model (BSM) long-lived particles (LLPs) in the high luminosity era of the LHC, in Run 4 (2028) and beyond. The project was initiated with a two-day meeting in April 2020 [1, 2], with one day discussing a forward hadron spectrometer for strong interaction physics, and one day on searching for long-lived particles. A description and more details of the physics potential are given in Ref. [3].We can compare FACET to the pioneering FASER experiment [4] which is approved to search for LLPs in the very forward direction in Run 3, and an upgrade FASER-2 [5] which is being developed for Run 4. Major differences with FACET are (a) FASER-2 is 480 m from IR5 (with ATLAS) while FACET is 100 -127 m from IR1 (with CMS). (b) FACET has 4× the solid angle: 54.5 µsr cf. 13.6 µsr. (c) FASER-2 has a 5 m-long decay volume; FACET has 18 m which is evacuated to eliminate background from particle interactions. (d) FASER-2 is centered at polar angle θ = 0 • while FACET covers 1 mrad < θ < 4 mrad. (e) FASER-2 is behind ∼ 100 m of rock absorber while FACET has ∼ 50 m of iron. However FACET is located inside the main LHC tunnel where radiation levels are much higher while FASER is located in a side tunnel.An important difference is that FASER is an independent experiment while FACET is not; it is proposed to be a new subsystem of CMS, fully integrated and using the same advanced technology for its detectors. This has the added benefit of allowing the study of correlations with the central event, and enables a standard model physics program especially in low pileup pp, pA and AA collisions.