FASER's Physics Reach for Long-Lived Particles

TL;DR

FASER targets a largely unexplored LLP landscape by placing a compact detector 480 m downstream in the LHC’s far forward region, leveraging extremely collimated, high-energy LLP decays in a low-background environment. The paper unifies detector assumptions and analyzes a comprehensive set of models—dark photons, B−L and L_i−L_j gauge bosons, dark Higgs, HNLs, ALPs, and dark pseudoscalars—across production channels and decay signatures, presenting sensitivities for both Run 3 (FASER) and HL-LHC (FASER 2). It provides detailed reach plots, background assessments, and systematic studies (beam offset, MC generators, energy thresholds, and efficiencies), demonstrating substantial discovery potential across a wide mass and coupling range and highlighting the detectors’ role in complementing other experiments. The results underscore FASER’s potential to illuminate dark sectors and their cosmological implications, with FASER 2 extending reach to larger masses and weaker couplings.

Abstract

FASER,the ForwArd Search ExpeRiment,is a proposed experiment dedicated to searching for light, extremely weakly-interacting particles at the LHC. Such particles may be produced in the LHC's high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons (HNLs); light B-L and $L_i - L_j$ gauge bosons; axion-like particles (ALPs) that are coupled dominantly to photons, fermions, and gluons through non-renormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

Figures (17)

• Figure 1: Left panel: The arrow points to FASER's location in service tunnel TI12, roughly 480 m east of the ATLAS IP. Credit: CERN Geographical Information System. Right panel: View of FASER in tunnel TI12. The trench lowers the floor by 45 cm at the front of FASER to allow FASER to be centered on the beam collision axis. Credit: CERN Site Management and Buildings Department.
• Figure 2: 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 and decay within FASER in TI12.
• Figure 3: Layout of the FASER detector. LLPs enter from the left and the entire length of the detector is roughly 5 m. The detector components include scintillators (gray), dipole magnets (red), tracking stations (blue), a calorimeter (dark purple), and support structures (green).
• Figure 4: Representative Feynman diagrams for the LLP production processes outlined in this section: dark photon production from pion decay (left), dark photon production via dark bremsstrahlung (center left), dark photon production in hard scattering (center right), and ALP production via the Primakoff process from photons scattering in the TAN (right).
• Figure 6: Benchmark Model V1. The dark photon decay length (top left panel), its branching fractions into hadronic and leptonic final states (bottom left panel) and FASER's reach (right panel). In the right panel, the gray-shaded regions are excluded by current bounds, and the projected future sensitivities of other experiments are shown as colored contours. See the text for details.