Technical Proposal for FASER: ForwArd Search ExpeRiment at the LHC

TL;DR

FASER proposes a compact, low-cost forward detector positioned 480 m from the ATLAS IP to search for light, weakly interacting long-lived particles produced along the beam axis. The design integrates a 0.5–0.6 T Halbach permanent magnet decay volume, a Silicon SCT-based tracker, scintillator veto/timing layers, and an electromagnetic calorimeter, with a trigger/DAQ system to operate in Run 3 and collect substantial data. Backgrounds are constrained through FLUKA simulations and in situ measurements, while the detector’s modular approach leverages spare ATLAS/LHCb components and ACTS-based software to enable robust reconstruction and analysis. The document provides a detailed civil, mechanical, electrical, and computing plan, cost estimates, and a realistic LS2–Run 3 schedule to establish FASER as a feasible addition to the LHC program with potential broad implications for particle physics and cosmology.

Abstract

FASER is a proposed small and inexpensive experiment designed to search for light, weakly-interacting particles during Run 3 of the LHC from 2021-23. Such particles may be produced in large numbers along the beam collision axis, travel for hundreds of meters without interacting, and then decay to standard model particles. To search for such events, FASER will be located 480 m downstream of the ATLAS IP in the unused service tunnel TI12 and be sensitive to particles that decay in a cylindrical volume with radius R=10 cm and length L=1.5 m. FASER will complement the LHC's existing physics program, extending its discovery potential to a host of new, light particles, with potentially far-reaching implications for particle physics and cosmology. This document describes the technical details of the FASER detector components: the magnets, the tracker, the scintillator system, and the calorimeter, as well as the trigger and readout system. The preparatory work that is needed to install and operate the detector, including civil engineering, transport, and integration with various services is also presented. The information presented includes preliminary cost estimates for the detector components and the infrastructure work, as well as a timeline for the design, construction, and installation of the experiment.

Figures (62)

• Figure 1: Schematic view of the far-forward region downstream of ATLAS and various particle trajectories. Upper panel: FASER is located $480~\text{m}$ downstream of ATLAS along the beam collision axis (dotted line) after the main LHC tunnel curves away. Lower left panel: High-energy particles produced at the IP in the far-forward direction. Charged particles are deflected by LHC magnets, and neutral hadrons are absorbed by either the TAS or TAN, but LLPs pass through the LHC infrastructure without interacting. Note the extreme difference in horizontal and vertical scales. Lower right panel: LLPs may then travel $\sim 480~\text{m}$ further downstream, passing through 10 m of concrete and 90 m of rock, and decay within FASER in TI12.
• Figure 2: Layout of the proposed FASER detector. LLPs enter from the left. The detector components include scintillators (gray), dipole magnets (red), tracking stations (blue), and a calorimeter (dark purple).
• Figure 3: A model of the FASER detector situated at the proposed location (centered on the nominal LOS) in the TI12 tunnel.
• Figure 4: FLUKA simulation estimation of the particle flux as a function of energy at the FASER location: (top) for negative and positive muons; (bottom) for different neutrino species. These are normalized to a luminosity of $2 \times 10^{34}~\text{cm}^{-2}~\text{s}^{-1}$.
• Figure 5: FLUKA simulation estimates of negative muon fluxes (top left) and positive muon fluxes (top right) in the transverse plane at the FASER location. These results assume the TI18 location, 485 m from the IP. The diagram at the bottom shows the geometry used in the simulations. The FASER detector is visible as a small, partially-cut circle of radius 20 cm at the bottom right of the tunnel.