Lake- and Surface-Based Detectors for Forward Neutrino Physics

The Big Picture

Imagine trying to hear a single violin in a stadium full of noise. That’s the challenge facing physicists who want to study neutrinos produced at the Large Hadron Collider: elusive, nearly massless particles that pass through matter almost without a trace, buried under an avalanche of cosmic-ray background. For decades, the obvious solution was to dig underground. But a team from Harvard and the University of Wisconsin–Madison has a different idea, one that sounds almost too simple: use Lake Geneva itself as a detector.

LHC collisions don’t just produce exotic particles for underground detectors to catch. They also fire a tight, powerful beam of neutrinos straight down the collision axis, toward the French countryside and, in one case, directly through the bottom of Switzerland’s most famous lake. These “forward neutrinos” carry clues about proton structure and the long-standing mystery of why cosmic-ray air showers produce fewer muons than our models predict.

Researchers Nicholas Kamp, Carlos Argüelles, Albrecht Karle, Jennifer Thomas, and Tianlu Yuan now propose two experiments, SINE and UNDINE, that exploit local geography around the LHC to build massive detectors weighing thousands of tons and placed tens of kilometers from the collision points. During the LHC’s planned high-intensity upgrade in the late 2020s, these detectors could catch millions of neutrino interactions at a fraction of the cost of digging new underground caverns.

Key Insight: By placing detectors tens of kilometers from the LHC collision points, on the surface or submerged in Lake Geneva, these experiments sidestep overwhelming particle traffic near the collision site and open up a rich physics program at essentially civil-engineering costs.

How It Works

The geometry of the LHC campus is a gift. Neutrino beams from collider experiments are extraordinarily narrow, within about 10 meters of width even at distances of 10 kilometers. A modestly sized detector placed far away still intercepts most of the beam. The hard part is separating real neutrino interactions from cosmic-ray muons constantly raining down from above.

The SINE concept (the Surface-based Integrated Neutrino Experiment) solves this by flipping the geometry. Neutrinos from the CMS detector travel 18 kilometers through solid bedrock before exiting Earth’s surface near the French-Swiss border. SINE would place scintillator panels on the ground and look upward for muons produced by neutrino interactions in the rock just below. Upward-going muons can’t be faked by cosmic rays, which only travel downward. The bedrock becomes the interaction target; the panels serve as the detection layer.

UNDINE (the UNDerwater Integrated Neutrino Experiment) goes further. Neutrinos from the LHCb detector pass through Lake Geneva’s basin about 50 meters below the surface. By deploying a water Cherenkov detector, which captures the faint blue glow emitted when charged particles travel faster than light in water, researchers can instrument kilotons of natural water with relatively sparse optical sensors. It’s the same technique used by IceCube in Antarctic ice and KM3NeT in the Mediterranean. Lake Geneva provides a ready-made detector tank, no excavation required.

Using Monte Carlo simulations (statistical modeling that runs millions of virtual particle collisions to predict detector behavior), the team estimates the following physics reach during the high-luminosity LHC era:

• Millions of neutrino interactions in each detector
• Sensitivity to neutrino-nucleon cross sections, how likely neutrinos are to interact with atomic nuclei, at multi-TeV energies where measurements are scarce
• Constraints on charm quark production, which feeds directly into predictions for high-energy astrophysical neutrino backgrounds
• Probes of strangeness enhancement, one proposed explanation for why air shower simulations consistently under-predict ground-level muon counts (the so-called muon puzzle)

The key advantage of the medium baseline is background suppression. Short-baseline detectors near the LHC must contend with intense beams of high-energy muons produced alongside neutrinos. At 10+ kilometers, those muons have long since shed their energy. What’s left is the neutrino signal and manageable levels of cosmic-ray background.

Why It Matters

The forward neutrino program at the LHC has already produced surprises. FASER, a compact detector installed in an existing service tunnel, first detected collider neutrinos in 2023. It was the first time anyone had directly observed neutrinos from a particle accelerator at these extreme energies. The planned Forward Physics Facility would build on that success by excavating new underground space near the LHC. But excavation is expensive, slow, and logistically complex.

SINE and UNDINE offer a different answer: use what’s already there. Lake Geneva has sat next to CERN for decades. The Jura bedrock has been absorbing muons from LHC collisions since the machine turned on. These proposals turn passive geography into active physics infrastructure.

If built, they would give the forward neutrino community more than one option beyond cavern-constrained detectors, enabling cross-checks and complementary measurements. The strangeness enhancement question alone, linking LHC data to discrepancies seen at ultra-high-energy cosmic ray observatories like the Pierre Auger Observatory in Argentina, connects collider physics and astrophysics in ways neither field can address on its own.

Bottom Line: SINE and UNDINE show that natural geography around the LHC could deliver millions of TeV-energy neutrino interactions at low cost, from cross-section measurements to the cosmic-ray muon puzzle, without digging a single new tunnel.