In 1984, Alvaro de Rujula pointed out that the proton-proton collisions at the LHC would produce large fluxes of very high energy neutrinos in the forward direction [1]. In 2023, almost 40 years later, the FASER and SND experiments ushered in the era of collider neutrinos at the LHC with their observation of TeV neutrinos [2]. These are currently situated in the LHC tunnel 480m from the ATLAS collision point. A new generation of detectors looking at the very forward collision products generated in ATLAS is envisioned for the Forward Physics Facility (FPF), a proposed cavern that would be 620m away from the ATLAS collision point. FPF emerged within the Physics Beyond Colliders Initiative (PBC) to exploit the full physics potential of the HL-LHC in the best possible way and would host experiments hunting for physics beyond the Standard Model (BSM) as well as studying neutrino physics and open questions in QCD. 

For example, the recent measurement of the collider neutrino-cross sections at previously unconstrained energies [3] paves the way for much more precise measurements at the FPF including the first measurements with tau and anti-tau neutrinos at the LHC. The high-energy neutrino beam allows precise probes of the proton structure via deep-inelastic scattering [4], where the improved PDFs will reduce theory uncertainties for measurements and searches at ATLAS/CMS, as well as lepton universality tests, and searches for non-standard neutrino interactions. The FPF experiments are expected to detect about 1000 neutrinos per day, enabling detailed studies of all three neutrino species. The expected energy spectra of reconstructed electron neutrinos at FLArE are sensitive to different QCD and BSM effects. FLArE will use a liquid argon time projection chamber similar to those used in protoDUNE to identify neutrinos and to search for dark-matter particles, benefitting a lot from synergies in the expertise from the CERN neutrino platform.

From SM to BSM

The FASER experiment has set world leading constraints on long-lived particles (LLP) [5,6] and its successor FASER2 would increase the sensitivity for detecting LLPs by a factor of 10000. These hypothesised particles may be created through a combination of small couplings, heavy mediator particles and small mass differences of the parent states. These characteristics lead to a wide mass range prediction and thus large spread in particle lifetimes that will need to be covered. This, however, makes it almost impossible to create a single detector that is able to detect every possible type of LLPs. If found, LLPs would manifest themselves as very displaced tracks in the detector.

At the FPF, the FORMOSA collaboration is planning searches for hypothetical millicharged particles carrying only a tiny fraction of the elementary charge and being potential candidates for dark matter [7]. The full version of this experiment offers a world-leading sensitivity of detecting such millicharged particles over a huge mass range. A version that is already taking data and looking for millicharged particles at large angles is the MilliQan experiment, which is installed close to the CMS experiment.

The FPF experiments would furthermore target light LLPs that might occur at small angles. They are complemented by experiments emerged within the PBC searching for similar types of BSM particles in a different energy range, which can be observed, if they exist, especially at large angles. Two pathfinder experiments searching for new physics at large angles are CODEX-beta and proANUBIS. With their cube-sized demonstrator consisting of Resistive Plate Chambers (RPC), which is about to be installed 25m behind the shielding wall of the LHCb experiment, the researchers at CODEX-beta aim at validating their setup to start hunting for many exotic particles. This initial test phase is foreseen for 2025. The final CODEX-b experiment would be 25 times larger in volume and will be able to search in particular for exotic very long-lived particles from B and Higgs decays to which the forward search experiments, such as FASER and SHiP, as well as the ATLAS and CMS experiments have only limited sensitivity [8].

The projects within PBC: Technical drawing of the proposed Forward Physics Facility (upper row, left), the expected neutrino-energy spectra (upper row, middle) as well as FORMOSA’s potential sensitivity (lower row, middle). CODEX-beta's assembly of the RPCs (lower row, left) and the installed proANUBIS demonstrator at the ATLAS experiment (right). Credits (in order of listing): L Krzempek; FLArE collaboration; FORMOSA collaboration; CODEX-b collaboration; Aashaq Shah.

At the ATLAS experiment, the ANUBIS sub-detector will consist of three RPC layers, covering the ceiling of the experimental cavern, separated by air from each other [9]. This year, the refined proof of principle experiment proANUBIS is taking data to validate the background model and nicely complements the search programmes of CODEX-b(eta) and the FPF at the LHC. The design of ANUBIS allows for a dramatic extension of the fiducial volume to observe LLPs at ATLAS, extending the range from about 7.5m to roughly 23m, increasing the potential sensitivity to some benchmark models by several orders of magnitude.

These PBC activities are covering energy and precision regions inaccessible by the four big four LHC experiments, and thus round off searches at ongoing and future fixed-target experiments. Another striking aspect is that all experiments offer a unique discovery potential while being cost-efficient and aiming at exploiting an otherwise uncovered region.

With this in mind, everybody is looking ahead of the exciting results yet to come from these projects during the upcoming HL-LHC era.
 

Further Reading

[1] https://inspirehep.net/literature/210810 & https://cds.cern.ch/record/1337790?ln=en
[2] https://home.cern/news/news/physics/new-lhc-experiments-enter-uncharted-territory
[3] https://arxiv.org/abs/2403.12520
[4] Eur. Phys. J. C 84 (2024) 369
[5] Phys. Lett. B 848 (2024) 138378
[6] CERN-FASER-CONF-2024-001
[7] https://home.cern/news/news/experiments/hunting-millicharged-particles-lhc 
[8] https://arxiv.org/abs/1911.00481
[9] https://arxiv.org/abs/1909.13022