Although the properties of neutrinos have been intensively studied, we are still missing data about their behaviour in the TeV energy scale, as well as conclusive tests of lepton universality in neutrino scattering. Embracing the discussions on recently identified flavor anomalies, which hint at lepton universality violation, the study of high-energy neutrino interactions involving both heavy leptons and heavy quarks may be a powerful and complementary tool to search for new physics effects. Such interaction channels require high neutrino energy.
In 2018, Prof. Akitaka Ariga of the AEC-LHEP proposed to measure high energy neutrinos with a detector sensitive to heavy flavor particles. The idea is to employ a novel approach to exploit the CERN Large Hadron Collider (LHC) as an intense neutrino source, which allows for the study of the high energy frontier of man-made neutrinos. An emulsion hybrid neutrino detector with a target mass of 1.1 tons is employed. Such a detector structure has been well proven to be sensitive to heavy flavor particles: tau, charm and beauty.
The project, named FASERnu [1], was then approved by CERN in 2019 and has been realized within the FASER experiment [2]. FASERnu has been taking data since the beginning of the Run 3 of LHC operation (2022 - 2026), with an expected yield of ~10,000 muon neutrinos, ~1,000 electron neutrinos, and ~10 tau neutrinos. Using this data, FASERnu will measure neutrino cross sections for all three flavors in the uncharted energy range between 360 GeV and 5 TeV. Furthermore, the channels associated with heavy quark (charm and beauty) production will be studied. FASERnu will not only investigate an unexplored energy regime, but will also provide relevant inputs to future neutrino experiments at the High-Luminosity-LHC (HL-LHC) and then at the Future Circular Collider (FCC).
Through the analysis of the pilot experiment conducted in 2018 with the LHC neutrino beam and a small detector, the team reported the detection of candidate high-energy neutrino events [3]. However, the statistical significance was insufficient to claim a confirmed observation of neutrinos. Since July 2022, physics data collection has begun in Run 3 of LHC operations. The 1.1-ton detector with nanometric precision is employed to detect three neutrino flavors (see the illustration below). In 2024, we detected electron neutrinos and muon neutrinos with a significance greater than "5 sigma" and measured the neutrino cross sections at TeV energies for the first time [4] [5]. These results include the highest energy electron neutrino observed, resulting in a daughter electron with an energy of 1.5 TeV.
FASERnu receives funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program.
More reading:
- FASERnu paper: https://doi.org/10.1140/epjc/s10052-020-7631-5
- FASER experiment website: https://faser.web.cern.ch/
- First neutrino interaction candidates at the LHC: https://doi.org/10.1103/PhysRevD.104.L091101
- Neutrino interaction rates measured at unprecedented energies: https://mediarelations.unibe.ch/media_releases/2024/media_releases_2024/neutrino_interaction_rates_measured_at_unprecedented_energies/index_eng.html
- First Measurement of the νe and νμ Interaction Cross Sections at the LHC with FASER's Emulsion Detector: https://doi.org/10.1103/PhysRevLett.133.021802
