Letter of Intent for FASER: ForwArd Search ExpeRiment at the LHC

TL;DR

FASER is a compact experiment positioned along the LHC beam axis to search for light, weakly-interacting particles, expanding the collider's discovery reach for new physics phenomena like dark photons and axion-like particles.

Contribution

This paper proposes the design, placement, and potential of FASER, a new experiment at the LHC, including its detection capabilities and background suppression strategies.

Findings

FASER can detect long-lived particles decaying within its volume.

Simulations show backgrounds are highly suppressed at FASER's location.

FASER's design is cost-effective and ready for installation during LHC's Long Shutdown 2.

Abstract

FASER is a proposed small and inexpensive experiment designed to search for light, weakly-interacting particles at the LHC. Such particles are dominantly produced along the beam collision axis and may be long-lived, traveling hundreds of meters before decaying. To exploit both of these properties, FASER is to be located along the beam collision axis, 480 m downstream from the ATLAS interaction point, in the unused service tunnel TI18. We propose that FASER be installed in TI18 in Long Shutdown 2 in time to collect data from 2021-23 during Run 3 of the 14 TeV LHC. FASER will detect new particles that decay within a cylindrical volume with radius R= 10 cm and length L = 1.5 m. With these small dimensions, FASER will complement the LHC's existing physics program, extending its discovery potential to a host of new particles, including dark photons, axion-like particles, and other CP-odd scalars. A FLUKA simulation and analytical estimates have confirmed that numerous potential backgrounds are highly suppressed at the FASER location, and the first in situ measurements are currently underway. We describe FASER's location and discovery potential, its target signals and backgrounds, the detector's layout and components, and the experiment's preliminary cost estimate, funding, and timeline.