Photographing TeV Neutrinos with FASER
New muon and electron neutrino cross section measurement at the TeV scale with FASER’s Emulsion Detector at the LHC 25 March 2026 | FASER Collaboration
Despite a nearly 100-year history, postulated in 1930 and discovered in 1956, neutrinos remain one of the most elusive fundamental particles. There are three know types, or “flavours,” electron, muon and tau neutrinos, all of which have a very small mass (so small it is yet to be measure precisely) and interact very rarely with matter, giving them the nickname of “ghost particles.” Although studying them requires large efforts, physicists have developed an array of experiments to detect neutrinos produced by a large variety of sources, from accelerators and nuclear reactors (artificial) to the Sun and cosmic rays (natural). However, until recently, one missing source was from collider experiments.
When studying neutrinos, an important property of their interactions is the cross section; this represents the probability of a neutrino interacting with matter. Using accelerator-based experiments, this value has been measured for neutrinos with energies up to a few hundred GeV, and in the case of muon neutrinos originating from cosmic rays, the cross section has been measured above 6 TeV. This leaves the few hundred GeV to multi–TeV region unexplored, but accessible in colliders such as the LHC at CERN.
The LHC collides particles at the highest human-made energies, with building-sized detector arranged around these interaction points measuring the resulting particles. However, many of these travel in the forward direction, along the collision line-of-sight, and are missed by these large experiments. Many of the particles subsequently decay into neutrinos, effectively producing the highest energy human-made neutrino beam.
The ForwArd Search ExpeRiment (FASER) is perfectly designed to detect these neutrinos, probing an energy range never explore before. By alternating tungsten plates and nuclear emulsion films, the FASERν sub-detector can observed the tracks of charged particles produced from a neutrino interaction, most importantly the corresponding charged daughter lepton, allowing the interaction itself to be investigated. By achieving sub-micrometer resolution, and applying an array of dedicated analysis tools, the kinematics and topology of the events can be measured and studied in great detail. An example of this can be seen in the event displays above, in which an electron neutrino interaction is shown in both the beam view (left, from the point of view of the incident neutrino) and in a rotated view (right), where the electromagnetic shower from the daughter electron is clearly visible. In 2023, FASER measured the electron and muon cross sections using the FASERν emulsion detector without reconstructing the neutrino energy, performed on a subset of the neutrino interactions recorded during the 2022 LHC run.
Since then, a larger number of neutrino interactions have been reconstructed and investigated, with seven electron neutrino candidates and thirty-three muon neutrino candidates identified in 9.5 fb-1 of data. This allowed the team to repeat and improve the electron neutrino cross section measurement, shown in the left-hand plot of the figure below.
For muon neutrinos, a neutrino energy reconstruction tool was developed by applying machine learning techniques, tailored to the constraints of emulsion detectors and forward neutrino interactions. Thanks to the unparalleled position resolution in emulsion, the momentum and angle of the daughter muon, as well as of the other charged particles in the interaction, could be measured and used as input information to the machine learning methods to reconstruct the incident neutrino energy. The highest-energy muon neutrino was reconstructed at 2.95 TeV, the artificial neutrino with the largest measured neutrino to date! By measuring the energy of each interacting muon neutrino, the cross section as a function of neutrino energy was determined, shown in the right-hand plot in the figure below. This method is currently being extended to also reconstruct electron neutrino energies. Measuring the neutrino energy is fundamental, not only to measure the cross section, but also to compare with expected neutrino fluxes and validate theoretical models for hadron production in collisions such as the LHC, as well as to investigate other interesting processes in an energy dependent way.
As more data is collected, and analysis tools and techniques continue to develop, FASER provides an exciting and novel way to look at neutrinos in a previously unexplored energy region, allowing scientists to probe both the physics behind them as well as their place in our wider understanding of our Universe.
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