More Muon Neutrinos with FASER’s Electronic Detector

FASER Briefings

Measurement of Muon Neutrinos as a Function of Energy and Rapidity with FASER

26 March 2026 | FASER Collaboration

The FASER experiment has performed a new analysis of muon neutrino interactions using proton collisions at the LHC. Using nearly 3x more data than the previous analysis, new measurements have been made of neutrino energy and rapidity at the LHC. This work will inform future theoretical modelling of the neutrino fluxes and cross sections.

When the Large Hadron Collider (LHC) smashes protons together at near the speed of light, it produces intense, high-energy beams of neutrinos. Neutrinos are ghostly particles that hardly interact with matter and are produced when unstable particles decay.

The LHC produces the highest energy of all human-made neutrinos and so presents a unique opportunity to study them. By counting the number of neutrinos that we see in our detector, we can deduce how likely they are to interact with matter at high energies or infer information about the flux of unstable particles that produced them.

The ForwArd Search ExpeRiment (FASER) is a detector situated in an old service tunnel 100 m underground and 480 m from where the LHC collides protons at the ATLAS experiment. The detector is placed such that it sits right on the line of sight (LOS) with the ATLAS interaction point, where the neutrino beam is most intense. Because there is 100 m of rock and concrete between FASER and the interaction point, the only particles that can reach FASER are muons and neutrinos.

FASER consists of two complementary parts: the FASER spectrometer, which is an electronic detector that measures the momentum of charged particles; and FASERν,an emulsion-based neutrino detector. FASERν, produces pictures of neutrino interactions with extraordinary spatial resolution, but the process is time consuming and does not distinguish neutrinos from anti-neutrinos.

To achieve neutrino measurements on a faster timescale, in parallel with the emulsion measurements, we can make use of the FASER spectrometer. When a muon neutrino interacts with material in FASER, it produces a muon that is reconstructed as a track in the spectrometer. By using the high-efficiency scintillator system in front of FASERν, we can look for muons that do not come from ATLAS, but emerge from FASERνinstead. Since we can also work out the charge of the muon from the direction it bends in the magnetic field we can also work out if it was a muon or anti-muon neutrino that interacted. A sketch of a muon neutrino interacting in FASER is shown below.

briefing 4 picture 2

FASER first did this in 2023 with the first observation of collider neutrinos at the LHC with great success. The Collaboration then produced their first muon neutrino flux and cross section measurements in 2025. Now with 2.8 times more data recoded between 2022-2024, FASER how now released results simultaneously measuring the neutrino energy and rapidity. Rapidity, for those unfamiliar, is a convenient way of representing the direction of a particle with respect to the LHC beam direction at the interaction point. A particle with a rapidity of zero would fly perpendicular to the beamline, whereas a particle with a rapidity of infinity would travel parallel to the beamline.

To account for the fact that the detector is imperfect, analysers perform a technique called unfolding which attempts to undo the detector specific effects. The unfolded result can then be easily compared directly to new theoretical predictions; making the lives of our theorist friends much easier. The unfolded muon neutrino flux is shown below in bins of rapidity, y, and minus-lepton-number-over-neutrino energy, -L/E.

briefing 4 picture 1

The bars in this histogram are broken up into different muon neutrino production modes. The red bar represents neutrinos that came from light meson decays, the orange bar represents strange meson decays, and the blue bar represents charm meson decays.

FASER also interpreted the results as a cross section, represented by red and orange points in the plot below. Here, you can really see how FASER fills the gap in measurements between the low-energy accelerator neutrino experiments, like NuTeV, and the ultra-high energy astrophysical neutrino experiments, like IceCube.

Photographing Neutrinos Cross Sections

This, however, is still an intermediary result. FASER still has two more years’ worth of data to analyse, and new upgrades planned for when the high-luminosity LHC arrives in the 2030s! FASER is also working on matching the tracks from muon neutrino interactions in the FASERν emulsion detector with the tracks in the electronic spectrometer – giving analysers the best of both worlds!

Learn More

July 25, 2026