Letter of Intent: The Forward Physics Facility

TL;DR

The Forward Physics Facility Letter of Intent presents a comprehensive plan to exploit TeV-energy collider neutrinos and forward hadron production at the HL-LHC by constructing an underground cavern near IP1 and hosting four complementary detectors (FLArE, FASERν2, FASER2, FORMOSA). The physics program spans neutrino physics, QCD, astroparticle physics, dark matter, and new particles, with collider-forward measurements poised to sharpen PDFs, test lepton-flavor universality, search for LLPs, and probe exotic sectors. The document details the facility design, site rationale, radiation and safety considerations, detector layouts, R&D priorities, and a simulation framework (FPFSim) to optimize performance and guide integration. A structured collaboration model and phased funding timeline are outlined to mature the project from LOI to construction, emphasizing modest cost relative to the potential physics payoff and the development of a new generation of researchers. Overall, the FPF aims to open a new frontier in collider neutrino physics and forward-sector BSM searches, delivering high-impact measurements and broad, cross-disciplinary scientific benefits.

Abstract

The Forward Physics Facility (FPF) is a proposed extension of the HL-LHC program designed to exploit the unique scientific opportunities offered by the intense flux of high energy neutrinos, and possibly new particles, in the far-forward direction. Located in a well-shielded cavern 627 m downstream of one of the LHC interaction points, the facility will support a broad and ambitious physics program that significantly expands the discovery potential of the HL-LHC. Equipped with four complementary detectors -- FLArE, FASER$ν$2, FASER2, and FORMOSA -- the FPF will enable breakthrough measurements that will advance our understanding of neutrino physics, quantum chromodynamics, and astroparticle physics, and will search for dark matter and other new particles. With this Letter of Intent, we propose the construction of the FPF cavern and the construction, integration, and installation of its experiments. We summarize the physics case, the facility design, the layout and components of the detectors, as well as the envisioned collaboration structure, cost estimate, and implementation timeline.

Figures (70)

• Figure 1: Physics Overview. The FPF will probe topics that span multiple frontiers, including new particles, neutrinos, dark matter, QCD, and astroparticle physics.
• Figure 2: New Particle Searches and Neutrino Measurements at the FPF. Representative examples of DM and other new particles that can be discovered and studied at the FPF (top) and of some of the many topics that can be illuminated by TeV-energy neutrino measurements at the FPF (bottom).
• Figure 3: Neutrino Yields at the FPF. The panels show the expected event rates and energy spectra of neutrinos undergoing CC interactions at the existing FASER$\nu$ and SND@LHC detectors with about 1 ton target mass and 350 fb$^{-1}$ integrated luminosity (dashed); a detector at the FASER location with 1 ton target mass operating during the full HL-LHC era with 3 ab$^{-1}$ integrated luminosity (dash-dotted); and the FPF, with the 20 ton FASER$\nu$2 and 10 ton FLArE detectors, with 2 ab$^{-1}$ integrated luminosity (solid). The spectra shown are for electron (left), muon (middle), and tau (right) neutrinos. Spectra from previous accelerator experiments ParticleDataGroup:2020ssz and the planned SHiP experiment Ahdida:2023okr are also shown for comparison. Details of the simulation are discussed in the text.
• Figure 4: Neutrino Event Rate Timeline. Accumulated number of CC neutrino interaction events recorded by existing and proposed detectors at the end of each year in logarithmic (left) and linear (right) scales assuming $150~\text{fb}^{-1}$ of data collected in 2031 and $300~\text{fb}^{-1}$ in all later years. The projection includes the currently operating FASER and SND@LHC experiments, assuming --- for simplicity --- that similarly-sized detectors continue to operate at the same locations during the HL-LHC era. It also includes the proposed FASER$\nu$2 and FLArE detectors at the FPF, including the FLArE HCAL, which are assumed to begin full data-taking at he beginning of LHC Run 5.
• Figure 5: Neutrino Cross Sections at the FPF. The expected precision of FPF measurements of neutrino CC interaction cross sections (statistical errors only) as a function of energy for electron (left), muon (middle), and tau (right) neutrinos with an integrated luminosity of $2~\text{ab}^{-1}$. In the case of muon and tau neutrinos, separate measurements of the neutrino and antineutrino measurement can be performed at FASER$\nu$2 and FLArE using muons passing through the FASER2 spectrometer, where a 17% branching fraction of taus into muons was considered. For electron neutrinos, FASER$\nu$2 and FLArE measure a flux-averaged cross section, while a dedicated 0.6 ton plastic target placed in the veto system of the FASER2 spectrometer will make it possible to perform separate measurements Kling:2025lnt. Existing data from accelerator experiments ParticleDataGroup:2020ssz, IceCube IceCube:2017roe, and FASER FASER:2024hoeFASER:2024ref are also shown.