In December 2019 a proposed upgrade of the FASER detector designed to study neutrinos at the LHC was approved by CERN. The new detector called FASERν will be placed in front of the main FASER detector, 480-m in front of the ATLAS collision point, and exactly aligned with the collision axis.
Although FASERν will be the first experiment to detect neutrinos produced by a collider, this is not a new idea. CERN theorist Alvaro De Rujula proposed to study such neutrinos in the 1980’s and there were even discussions to have a dedicated neutrino experiment at the LHC. This did not happen, thus leaving the way for FASERnν to be the first such experiment.
FASER is designed to search for neutral, weakly interacting, light new-particles, produced in the decay of mesons that are produced in the LHC collisions. Neutrinos are very similar to the hypothetical signal that FASER is searching for, with one main difference being that we know that neutrinos exist. With this in mind, the FASER collaboration studied the possibility of detecting and studying neutrinos with FASER. Since neutrinos do not decay (unlike the new physics particles that FASER will search for) and are weakly-interacting, a heavy target is needed for the neutrino to interact with and therefore a new detector and target (FASERν) had to be added to FASER. Figure 1 shows a schematic of the LHC neutrino “beam-line” that FASERν will use.
Figure 1: High-energy mesons are produced at the interaction point of the ATLAS experiment in the far-forward direction, of which a fraction decay into neutrinos. The remaining charged particles are deflected by the LHC magnets. The neutrinos then travel to the FASERν detector 480m downstream of ATLAS, passing through 100 m of rock, which stops any other neutral particles produced in the collisions from reaching FASER.
The FASERν detector  is made up of interleaved films of emulsion and 1mm-thick tungsten plates. The very dense tungsten acts as the target where the neutrino may interact and the emulsion films can be used to track charged particles produced in the neutrino interaction. The space in front of FASER is limited and so the FASERν detector is only 1.3 m-long and 25 cm x 25 cm in the transverse direction, but despite this, due to the density of tungsten the detector weighs 1.3 tonnes. This is massive enough for more than 20000 neutrino interactions to occur in the detector during Run 3 of the LHC. Figure 2 shows the proposed location of FASERν directly in front of the FASER spectrometer in the TI12.
Figure 2 – Integration model of the FASER spectrometer and the FASERν detector installed in the TI12 tunnel in the LHC complex.
Emulsion detectors are based on silver bromide crystals (with diameter 0.2 μm) dispersed in a gelatine substrate. When a charged particle passes through the emulsion, the ionization is recorded quasi-permanently, and it can then be amplified and fixed by chemical development. Such detectors can make extremely precise position measurements of the trajectories of charged particles, which makes them ideal for observing short-lived particles like tau-leptons that could be produced in a neutrino interaction. However, they do not have any time resolution, and all charged particle tracks are recorded while the detector is in place. With a too high density of particle tracks the analysis becomes impossible, so the detector will have to be replaced during every Technical Stop of the LHC (about every 2-3 months), which should limit the track density to less than 106 tracks/cm2. This poses a logistical challenge as the detector needs to transported 500 m along the LHC and then lifted over the accelerator to be installed. A crane and protective cover have already been put in place to allow the installation of the main FASER detector components, while they can also be used to carry in and out the FASERν detector.
In order to understand the backgrounds for FASER a small emulsion detector was installed in the TI12 tunnel (the FASER location) during part of the 2018 LHC run. This detector was exposed to 12.5/fb of 13 TeV collision data. The results were used to measure the charged particle background rate in FASER, and to validate the background simulations. The results have also been used to validate the FASERν concept, and a number of neutral multi-track vertices have been seen in the analyzed data (as shown in Figure 3), which could be from neutrino or neutral hadron interactions. Analysis is ongoing to select a high purity set of neutrino interactions.
Figure 3: Three selected reconstructed neutral vertices from the test emulsion detector installed in TI12 in 2018 and exposed to 12.5/fb of 13 TeV pp collision data.
Due to the large luminosity of the LHC a huge number of neutrinos are produced and most in the very forward direction covered by FASER(ν). For example in Run 3 of the LHC (2021-2024 running, and assumed to be 150/fb of 14 TeV collisions) we expect 1012, 1011, and 109 muon neutrinos, electron neutrinos and tau neutrinos, respectively, to traverse FASER(ν). Due to the very low interaction cross-section only a tiny fraction of these will interact in the detector, but we still expect to have ~20000, 1300, and 20 interactions of the three types of neutrino in FASERν. Interestingly, the neutrinos interacting in FASERν will be in an energy regime never probed before, thanks to the high-energy of the LHC collisions. Figure 4 shows the projected cross-section measurements of the three neutrino flavours that FASERν could make during Run-3, as a function of the neutrino energy. It can be seen that for electron and tau neutrinos these will be the highest energy neutrino measurements ever made, whereas for muon neutrinos the FASERν results will fill the gap between lower energy fixed-target measurements, and those from the IceCube experiment. As well as these cross-section measurements FASERν results will provide insight on forward physics production in proton collisions, and may also help to constrain potential new-physics effects in the neutrino sector. The physics prospects of FASERν are discussed in detail in Ref. .
Figure 4: Projected FASERν sensitivity for neutrino cross-section measurements for electron (left), muon (middle) and tau (right) neutrinos as a function of the neutrino energy. The projections correspond 150/fb at 14 TeV (the expectation for LHC Run 3). Existing measurements are shown in grey.
Initially FASERν data will not be combined with information from the main FASER spectrometer. However, a possible future upgrade will include the installation of an interface tracking-detector, that would allow matching the tracks from vertices reconstructed in FASERν with events triggered in the FASER spectrometer. This would allow the charge of an outgoing muon to be measured by its bending in the spectrometer, so as to distinguish neutrino and anti-neutrino interactions. In addition the spectrometer information would improve the energy estimate of the reconstructed neutrino, and aid the background rejection.
Work is ongoing to finalize the design of the FASERν detector structure and to purchase and test the components along with progress on the civil engineering discussed by Jonathan Gall and Eliseo Perez-Duenas in their article in the same issue. FASERν will be installed into position shortly before the start of LHC collisions in Run 3 (scheduled in summer 2021). The collaboration is eager to analyze the first data and study the highest ever man made neutrinos, broadening the physics output from the LHC complex.
 – FASER Collaboration, “FASERν: Technical Proposal”, arXiv:2001.03073
 - FASER Collaboration, “Detecting and Studying High-Energy Collider
Neutrinos with FASER at the LHC”, Eur. Phys. J. C 80 (2020) 61, arXiv:1908.02310