FASERν and SND@LHC: The First Collider Neutrinos

The Large Hadron Collider produces neutrinos. In every proton-proton collision, the secondary hadrons — pions, kaons, and especially the heavy charm and beauty mesons — decay through processes that include neutrino emission. The neutrinos are typically forward-going, concentrated along the beam direction, and emerge from the collision with energies that can exceed a TeV.

Until 2022, no experiment had ever detected them. The reason was straightforward: the conventional LHC detectors at ATLAS, CMS, ALICE, and LHCb sit at large polar angles relative to the beam axis. They are designed for high-$p_T$ physics — Higgs bosons, top quarks, B-physics — where the interesting final states have substantial transverse momentum. Forward neutrinos, escaping along the beam pipe, are essentially invisible to these detectors. They pass through, undetected, and continue down the beamline.

In 2022, two small experiments installed during the LHC’s Long Shutdown 2 began taking data at locations 480 metres downstream of the ATLAS interaction point. FASER$\nu$ is a small dedicated emulsion detector. SND@LHC is a complementary scintillating-fibre and emulsion hybrid. Both are designed specifically to catch the forward neutrino flux that the conventional LHC detectors miss.

By 2023, both had reported their first observations: hundreds of neutrino interaction candidates, at energies ranging from 100 GeV to several TeV, collected in just a few inverse-femtobarns of integrated LHC luminosity. By 2026, with Run 3 well underway, both experiments are accumulating much larger samples and have started measuring the energy-differential interaction cross-section in the TeV regime — physics that no other detector technology can currently match.

This post is about the LHC neutrino program: what FASER$\nu$ and SND@LHC measure, why these measurements are unique, and what they enable for the next decade.

Where the neutrinos come from

In a proton-proton collision at $\sqrt{s}=13.6$ TeV (the current LHC energy), the dominant production of forward-going neutrinos is through the decay of secondary hadrons:

• Pion decay (${\pi }^{\pm }\to {\mu }^{\pm }{\nu }_{\mu }$) — produces muon neutrinos at GeV-scale energies. Kinematically the dominant source of ${\nu }_{\mu }$ at lower energies.
• Kaon decay ($K^{\pm }\to {\mu }^{\pm }{\nu }_{\mu }$, $K_L^0\to {\pi }^{\mp }{e}^{\pm }{\nu }_e$) — produces both ${\nu }_{\mu }$ and ${\nu }_e$ at higher energies.
• Charm decay ($D^{\pm }\to e^{\pm }{\nu }_e+X$, $D_s\to {\tau }^{\pm }{\nu }_{\tau }$) — produces electron and tau neutrinos at energies of tens to hundreds of GeV.
• Beauty decay (B-meson semileptonic decays) — produces all three flavors at higher energies.

The forward neutrino flux at LHC energies is dominated by pions and kaons at lower energies, and by charm and beauty at higher energies. The relative composition is approximately 70% ${\nu }_{\mu }$, 20% ${\nu }_e$, and 10% ${\nu }_{\tau }$ — though the ${\nu }_{\tau }$ fraction increases at higher energies as charm and beauty become more important.

The forward neutrino flux is enormous. In a single year of LHC operation at design luminosity, approximately $10^{14}$ neutrinos pass through any given square metre at 480 metres downstream of the interaction point. Most have energies above 100 GeV.

The challenge is that the cross-section for neutrino interactions on a typical detector scales like $\sigma \propto E$, so even at TeV energies the interaction rate per kg of target material per year is only of order tens to hundreds. Both FASER$\nu$ and SND@LHC are limited primarily by their target masses — a few hundred kg each — and accumulate samples of hundreds of events per year.

FASERν: the dedicated emulsion experiment

FASER$\nu$ — the Forward Asymmetric Experiment for Rare processes, with a $\nu$ subscript indicating its neutrino program — is installed in the TI12 service tunnel, 480 metres downstream of the ATLAS interaction point along the beam direction.

The detector is a 1.1-metre-long box containing 730 nuclear emulsion plates interleaved with 1-mm tungsten plates. Total mass: 1.1 tonnes of tungsten. The emulsion provides micrometre vertex resolution; the tungsten provides the target mass for neutrino interactions.

The detection principle: a neutrino enters the FASER$\nu$ target and interacts with a tungsten nucleus, producing a charged lepton plus a hadronic shower. The lepton’s track is recorded with sub-micrometre precision in the surrounding emulsion. By scanning the emulsion offline, individual interaction vertices can be identified and the lepton trajectory measured.

The companion FASER spectrometer downstream of the emulsion target measures the lepton momentum, allowing flavor identification (electron vs. muon vs. hadron) and reconstruction of the interaction kinematics.

FASER$\nu$’s emulsion targets are exposed for periods of weeks at a time. Each exposure is then physically removed, processed, and shipped to scanning facilities. The first FASER$\nu$ data from Run 3 was collected in 2022; the first results were published in 2023.

The first FASERν result

The first FASER$\nu$ result, published in Physical Review Letters 131, 031801 (2023), reported the observation of LHC-produced neutrino interactions:

• 153 candidate ${\nu }_{\mu }$ + tungsten interactions in the 7 fb⁻¹ Run 3 dataset
• Energy range: 100 GeV to several TeV
• Charged-current event topology: identified muon track + hadronic shower vertex
• Statistical significance: greater than 16$\sigma$

This is the first direct observation of collider-produced neutrinos. The energy range probed (hundreds of GeV) is well above the kinematic reach of conventional accelerator-neutrino experiments and is in a regime where deep inelastic scattering dominates the cross-section.

The cross-section measurement reported in the same paper: $\sigma ({\nu }_{\mu }+N)=(2.5\pm 0.4)\times 10^{-37}{\text{ cm}}^2/N$ at average energy of approximately 600 GeV, consistent with Standard-Model predictions to within the experimental precision of about 15%. The measurement is the first ever in this energy regime from accelerator-produced neutrinos (cosmic-ray cascade observations have probed the regime, but with much larger systematic uncertainties).

SND@LHC: the complementary experiment

SND@LHC (Scattering and Neutrino Detector at the LHC) is installed in the TI18 tunnel on the opposite side of the LHC ring at 480 metres from the ATLAS interaction point. It is run by a separate but overlapping collaboration.

The detector is a hybrid: 800 kg of tungsten target mass interleaved with scintillating fibres providing real-time tracking, plus emulsion modules upstream of the tracking section for high-resolution vertex reconstruction. The scintillating fibre tracker provides the timing and energy resolution that pure emulsion lacks; the emulsion provides the spatial resolution for vertex identification.

SND@LHC also has a magnetic spectrometer downstream for momentum measurement. The combination of fibre tracker + emulsion + magnet allows full kinematic reconstruction of each neutrino interaction.

SND@LHC’s first results, also published in 2023, reported observations of ${\nu }_{\mu }$ interactions at LHC energies, consistent with the FASER$\nu$ measurement and providing independent confirmation.

The complementarity between the two experiments is intentional. FASER$\nu$ provides the cleanest emulsion-based vertex resolution and the largest target mass for raw event count. SND@LHC provides faster electronic readout (no offline emulsion scanning is required for tracking) and better real-time selection of ${\nu }_{\tau }$ candidates through topology analysis.

What’s special about TeV-scale neutrinos

The energy regime accessed by FASER$\nu$ and SND@LHC — hundreds of GeV to several TeV — is unique. Conventional accelerator-neutrino experiments (T2K, NOvA, MINOS) operate at sub-GeV to a few-GeV energies. The high-energy cosmic-ray atmospheric neutrinos at IceCube reach TeV energies but with much larger systematic uncertainties from the atmospheric production model.

LHC neutrinos give:

• Clean source kinematics. The LHC collisions are well-characterised, the secondary hadron decay chains are calculable, and the resulting neutrino flux at 480 metres can be predicted to about 10-20% precision.
• Charged-current measurements at TeV energies. The cross-section can be measured against this calculable flux, providing the first model-independent test of the deep-inelastic-scattering cross-section in this regime.
• Tau neutrino events at high rates. Charm and beauty decays produce ${\nu }_{\tau }$ in much greater abundance than any conventional accelerator beam. The LHC neutrino sample includes hundreds of ${\nu }_{\tau }$ candidates per year of running — a sample size that no terrestrial experiment can match.
• Probe of forward physics. Forward LHC physics — the production of hadrons at small angles relative to the beam — has been historically under-measured. The neutrino flux at FASER$\nu$ depends on this forward production, so the neutrino measurement constrains the underlying QCD physics.

Tau neutrino production

The most exciting target of the LHC neutrino program is the ${\nu }_{\tau }$ measurement. As noted earlier, conventional accelerator-neutrino experiments produce only tiny numbers of ${\nu }_{\tau }$, and direct measurements of tau-neutrino interactions are restricted to a handful of events from DONUT, OPERA, and IceCube.

LHC charm and beauty decays produce abundant ${\nu }_{\tau }$. The expected ${\nu }_{\tau }$ event rate at FASER$\nu$ is approximately 10-20 per year of running. This is a small but significant sample that allows the first dedicated study of tau-neutrino interactions at TeV energies.

By 2026, FASER$\nu$ has reported the first ${\nu }_{\tau }$ candidates from LHC running, with several events showing the characteristic short-track-with-kink topology in emulsion. The full Run 3 dataset (2022-2026) is expected to yield 50-100 ${\nu }_{\tau }$ events — already exceeding the all-time DONUT-plus-OPERA ${\nu }_{\tau }$ sample.

These will provide:

• The first cross-section measurement of ${\nu }_{\tau }$ + nucleon at TeV energies
• A precision test of lepton flavor universality between ${\nu }_e$, ${\nu }_{\mu }$, and ${\nu }_{\tau }$
• A search for non-Standard-Model effects specific to the tau sector
• A constraint on ${\nu }_{\tau }$ properties (cross-section, kinematics, possible mass effects) at the highest energies yet probed

Looking ahead: HL-LHC era

The High-Luminosity LHC (HL-LHC), starting in 2030, will increase the integrated luminosity per year by a factor of approximately 10. The corresponding neutrino flux will scale linearly. With dedicated upgrades, FASER$\nu$ and SND@LHC could accumulate samples of $10^4$–$10^5$ events per flavor per year by 2035.

Proposed next-generation experiments take this further:

Forward Physics Facility (FPF) — a proposed dedicated facility downstream of ATLAS, hosting larger neutrino detectors with target masses of 10-100 tonnes (compared to 1 tonne for FASER$\nu$). Event rates would scale up by factor 100-1000.

FLArE (Forward Liquid Argon Experiment) — a 100-tonne liquid-argon TPC at the FPF, providing electronic readout and full kinematic reconstruction of every neutrino event.

FASER2 — a successor to FASER with much larger acceptance, designed to fully exploit the HL-LHC era.

By 2035, the LHC neutrino program could produce $10^4$–$10^5$ ${\nu }_{\tau }$ events. This would transform the tau-neutrino sector from a series of single-event searches to a fully characterised flavor with cross-sections, kinematic distributions, and mixing parameters measured to similar precision as the electron and muon sectors.

The broader significance

The LHC neutrino program is, in many ways, a gift the field didn’t ask for and didn’t quite know how to use. The forward neutrino flux had been calculated for decades, but no detector was placed to catch it because the conventional ATLAS/CMS layout had no need for forward-going particle detection.

The success of FASER$\nu$ and SND@LHC has shown that, with very modest investment (the experiments cost approximately $10M each — small by particle-physics standards), an entirely new high-energy neutrino-physics regime can be opened. The HL-LHC era will solidify this into a major program.

The deeper physics: at TeV energies, the deep-inelastic-scattering cross-section is calculable to high precision in the Standard Model, but with significant theoretical uncertainty from the parton distribution functions and the higher-order QCD corrections. LHC neutrinos provide a direct measurement of this cross-section, which simultaneously constrains the underlying QCD inputs.

The high-energy neutrino sector is also a window into beyond-Standard-Model physics. Tau-flavor anomalies, lepton-universality violations, and possible non-standard interactions could all manifest preferentially at TeV energies. The LHC neutrino program is the first dedicated probe of these effects in a clean accelerator environment.

A small experiment, in a previously unused tunnel, has opened a window. By 2030, the window will be wide open — and what we see through it will likely surprise the field in ways that the original FASER and SND@LHC proposals did not anticipate.