FASER2 is a large-volume detector comprised of a spectrometer, electromagentic and hadronic calorimeters, veto detectors, and a muon detector. FASER2 is designed for sensitivity to a wide variety of models of BSM physics and for precise muon reconstruction. It builds on positive experience gained from the successful operation of the existing FASER detector, a much smaller detector that is constrained to lie within a LHC transfer tunnel. The FASER2 detector, which is designed for the FPF facility, is much larger (by a factor of ∼ 600 in decay volume size) and includes new detector elements. It has an increase in reach for various BSM signals of several orders of magnitude compared to FASER and allows sensitivity to particles that were previously out of reach, such as dark Higgs bosons, heavy neutral leptons, and some axion-like particles. In addition to the BSM case for FASER2, the SM neutrino program at the FPF will rely on the identification of muons from neutrino decays and precise measurement of their momentum and charge. The FASER2 spectrometer will be integral for these measurements for both FASERν2 and FLArE.

To enable realistic detector design studies, a Geant4 geometry of the proposed detector has been created:

The overall FASER2 design is largely driven by the spectrometer with further consideration having been given to deliverable and affordable magnet technology leading to a split spectrometer with a large-volume dipole magnet. The magnet has a rectangular aperture of 1 m in height and 3 m in width. This also defines the transverse size of decay volume, which is the 10 m un-instrumented region upstream of the first tracking station (a 3 × 1 × 10 m3 cuboid) and downstream of the first veto station. The transverse size is driven by the need to have sufficient sensitivity to BSM particles originating from heavy flavor decays.

The baseline magnetic field is one with 4 Tm of bending power. This field strength is required to achieve sufficient particle separation, momentum resolution, and charge ID performance for the BSM and neutrino program. Superconducting magnet technology is required to maintain such a field strength across a large aperture. Technology based on the magnet of the SAMURAI experiment is currently being pursued. The tracking detectors are foreseen to use a SiPM and scintillating fiber tracker technology, based on LHCb’s SciFi detector. This technology gives sufficient spatial resolution (∼ 100 μm) at a significantly reduced cost compared to silicon detectors. However, the use of silicon-based tracking detectors will be explored for the interface between FASER2 and FASERν2, and for the first tracking station downstream of the decay volume.

The calorimeter is foreseen to be based on dual-readout calorimetry [52, 53] technology, building from experience of existing prototypes for future collider R&D, but modified for the specific physics needs of FASER2: spatial resolution to be able to identify particles at ∼ 1−10 mm separation; good energy resolution; improved longitudinal segmentation with respect to FASER; and the capability to perform particle identification, separating, for example, electrons and pions. The ability to identify separate electrons and muons will be very important for signal characterization, background suppression, and for the interface with FASERν2. To achieve this, a mass of iron will be placed after the calorimeter, with sufficient depth to absorb pions and other hadrons, followed by a detector for muon identification, for which additional SciFi planes could be used.

Finally, the veto system will be required to reject muon rates of approximately 20 kHz. Scintillator-based approaches have proven to be sufficient for this in FASER, and a similar, but re-optimised, design is foreseen for FASER2. The event rate and size are much lower than most LHC experiments, so the trigger needs are not expected to be a limiting issue. For instance, it is expected that it will be possible to significantly simplify the readout of the tracker, with respect to what is used in the LHCb SciFi detector.