FASER: how a small detector finally caught neutrinos at a particle collider

The Large Hadron Collider has been running since 2008. Every year, in each of the four interaction regions where its proton beams collide, it produces an enormous number of neutrinos — by some estimates, roughly 10¹⁶ neutrinos per year above 100 GeV streaming forward from each collision point. These are the highest-energy neutrinos ever produced by human-built machines.

For more than a decade, none of them was directly detected. The reason was geometry. Neutrinos at a collider fly out predominantly in the forward direction, almost along the beam axis. The main LHC detectors — ATLAS and CMS — surround the collision points and cannot follow particles that close to the beam pipe, where backgrounds from the proton beams themselves are overwhelming. The neutrinos simply escaped into the LHC’s downstream tunnel and were lost.

In 2018, a small collaboration proposed putting a detector down that tunnel. They called it FASER — the ForwArd Search ExpeRiment — and they pitched it to CERN as a low-cost, high-impact addition to the LHC physics program. The detector was approved in 2019, built in 2020–2021, installed in a former service tunnel by 2022, and started taking physics data when the LHC restarted in 2022.

In March 2023, FASER announced the first direct detection of neutrinos at a particle collider. Its sister experiment, SND@LHC, announced confirmation a few months later. After sixty years of collider physics, neutrinos had finally been caught in the act.

Why the forward direction

Particle physics, in most circumstances, focuses on transverse momentum — the component of momentum perpendicular to the beam axis. Reactions producing new particles tend to make those particles fly at large angles to the beam, which is why detectors like ATLAS and CMS form cylindrical or barrel-shaped structures around the collision point.

But many particle production processes — including pion and kaon production from soft proton-proton scattering — overwhelmingly send their products forward, very nearly parallel to the beam direction. Pions and kaons in turn decay into muons and neutrinos. The forward direction is therefore the dominant source of LHC-produced neutrinos.

It is also, conveniently, a place where the LHC’s natural magnetic shielding does much of the cleanup. Downstream of each interaction point, the powerful dipole and quadrupole magnets that steer the beams also sweep away charged particles other than the beam itself. By the time you get a few hundred meters past the collision point, almost nothing remains except the beams, neutrinos, and a few exotic long-lived species. The shielding is essentially free.

FASER is installed about 480 meters downstream of the ATLAS interaction point, in a former LEP-era service tunnel that has been mostly empty since the previous accelerator was decommissioned. The location is shielded by hundreds of meters of rock and dozens of meters of concrete, and the natural beam-line magnetic field sweeps everything away except the rare classes of particles that ignore both magnetic fields and matter.

How FASERν actually catches a neutrino

The neutrino-detecting heart of the experiment is a sub-system called FASERν, located just upstream of the main FASER detector. It consists of a 1.1-ton block of tungsten interleaved with emulsion films. Tungsten provides the dense target material that maximizes neutrino interaction probability per unit volume; emulsion provides extraordinary spatial resolution for reconstructing the resulting particle tracks.

When a neutrino interacts in the tungsten, it converts into a charged lepton (an electron, muon, or tau lepton, depending on flavor) plus a spray of hadrons. The charged particles fly through the surrounding emulsion films, leaving microscopic silver tracks. After about a year of running, the emulsion films are removed, developed, scanned with high-precision optical microscopes, and analyzed for events.

The emulsion’s spatial resolution is on the order of a few hundred nanometers. That is fine enough to identify the decay topology of a tau lepton — a particle that travels only a few hundred microns before decaying — and therefore to distinguish tau-neutrino interactions from the other two flavors. This is the same technique that allowed DONUT to directly detect tau neutrinos in 2000.

Downstream of the emulsion target, the main FASER detector consists of magnetized tracking chambers and an electromagnetic calorimeter. This part of the experiment also looks for hypothetical long-lived particles from the LHC collision point — dark photons, dark scalars, and similar dark-sector candidates — but most of its physics output so far has been on the neutrino side.

What the first results showed

The 2023 papers reported the detection of a clear excess of muon-neutrino events in the FASERν emulsion, well above the expected backgrounds from non-neutrino sources. The total of about 150 candidate events from the first analysis period was consistent in rate, energy spectrum, and angular distribution with the predictions for forward neutrino production from LHC collisions.

In the same year, SND@LHC — the Scattering and Neutrino Detector at the LHC, located on the opposite side of the ATLAS collision point — independently reported its own detection of LHC-produced neutrinos. The two experiments use slightly different geometries and different reconstruction strategies, providing cross-checks for each other’s measurements.

Subsequent analyses have pushed the data into more detail. FASER has now reported electron-neutrino events and is working toward identifying tau-neutrino candidates, which would make the LHC the first machine to produce all three neutrino flavors at TeV energies.

What is learned from the cross-section

The most physically interesting consequence of FASER’s neutrino detection is the measurement of neutrino-nucleon cross-sections at TeV energies for the first time.

At ordinary accelerator energies (a few GeV), the neutrino-nucleon cross-section is measured to good precision by experiments like T2K, NOvA, and MINERvA. At very high energies (above tens of TeV), the cross-section is constrained indirectly by astrophysical observations from IceCube. In between — from roughly 100 GeV to about 10 TeV — there was a gap. No experiment had ever measured the cross-section in that range.

FASER closes that gap. The measurements are starting to constrain the parton-distribution functions of the proton and the heavy-quark content at very high resolutions. They also calibrate physics inputs that matter for the interpretation of atmospheric neutrino measurements and for the eventual identification of the highest-energy cosmic neutrinos in observatories like IceCube and KM3NeT.

The broader forward physics program

FASER’s success has triggered active discussion about a permanent Forward Physics Facility at the LHC — an underground cavern downstream of one of the LHC interaction points, large enough to host a suite of detectors dedicated to forward physics.

The proposed facility would include enlarged successors to FASER and SND@LHC, more elaborate detectors for searching for long-lived dark-sector particles, and a much larger neutrino target capable of detecting all three flavors with high statistics. If approved, the Forward Physics Facility would start construction in the late 2020s alongside the High-Luminosity LHC upgrade.

A new corner of neutrino physics

For most of the history of neutrino physics, collider neutrinos were a theoretical curiosity. People knew they were being produced. They knew the rates. They knew the energies. They just could not catch them.

FASER changed that. With a refrigerator-sized detector, a 1-ton tungsten target, and a stack of emulsion films, the collaboration opened a new energy regime for neutrino physics — and did it on a budget far smaller than any of the field’s other recent experiments.

The result is a reminder that ambitious physics does not always require billion-dollar detectors. Sometimes it requires a small, clever apparatus placed in exactly the right corner of an existing facility. FASER is one of the best recent examples of that approach.

---

For other directions in modern neutrino observation, see Inside IceCube and KM3NeT. For the original tau-neutrino direct detection that pioneered the emulsion technique, see DONUT. For the long-baseline experiments that dominate the GeV-scale program, see DUNE and Hyper-Kamiokande.

Neutrino Times Editorial Team. (2025, October 16). FASER: how a small detector finally caught neutrinos at a particle collider. Neutrino Times. https://neutrino-times.com/articles/faser-first-lhc-collider-neutrinos/

Neutrino Times Editorial Team. "FASER: how a small detector finally caught neutrinos at a particle collider." Neutrino Times, October 16, 2025. https://neutrino-times.com/articles/faser-first-lhc-collider-neutrinos/.

Neutrino Times Editorial Team. "FASER: how a small detector finally caught neutrinos at a particle collider." Neutrino Times, 16 Oct. 2025, https://neutrino-times.com/articles/faser-first-lhc-collider-neutrinos/.

@misc{neutrino-times-faser-first-lhc-collider-neutrinos,
  author       = {Neutrino Times Editorial Team},
  title        = {FASER: how a small detector finally caught neutrinos at a particle collider},
  howpublished = {Neutrino Times},
  year         = {2025},
  month        = {oct},
  url          = {https://neutrino-times.com/articles/faser-first-lhc-collider-neutrinos/},
  note         = {Accessed: 2025-10-16}
}

TY  - GEN
TI  - FASER: how a small detector finally caught neutrinos at a particle collider
AU  - Neutrino Times Editorial Team
PY  - 2025
DA  - 2025-10-16
PB  - Neutrino Times
UR  - https://neutrino-times.com/articles/faser-first-lhc-collider-neutrinos/
ER  -