DUNE: Firing Neutrinos 1,300 Kilometres Through the Earth

One Beam, Two Detectors, 1,300 Kilometres Apart

DUNE is not a single instrument but a system with three parts. At Fermilab near Chicago, the Long-Baseline Neutrino Facility will produce the most intense high-energy neutrino beam ever built, starting at 1.2 megawatts of proton beam power with an upgrade path to roughly double that. A near detector, a few hundred metres from the source, measures the beam as it leaves - its composition, its energy spectrum, its intensity.

The beam then travels 1,300 kilometres through solid rock. Nothing guides it and nothing contains it; neutrinos simply pass through the mantle as if it were not there, spreading out slowly as they go. No tunnel is needed and none exists. The beam is aimed downward from Illinois and emerges, so to speak, in South Dakota.

There the far detector waits, 1.5 kilometres below the surface at the Sanford Underground Research Facility. It measures what arrives. Because the near detector has already characterised what was sent, the difference between the two is the physics: how many muon neutrinos vanished, how many electron neutrinos appeared, and at which energies.

The two-detector design is what makes the measurement robust. Most systematic uncertainties - beam intensity, flux modelling, cross-section assumptions - affect both detectors similarly and largely cancel in the comparison. This is the same principle T2K uses in Japan with Super-Kamiokande as its far detector, scaled up in baseline, mass and beam power.

Why Liquid Argon

A water Cherenkov detector sees a ring of light and infers a particle from its shape. A liquid argon time projection chamber does something closer to photography. When a charged particle crosses the liquid argon, it ionises atoms along its path. A strong electric field drifts those liberated electrons sideways, over metres, towards a plane of finely spaced sense wires or pixels, which record where and when each one arrived.

Because the drift speed is known, the arrival time gives the third coordinate. The result is a full three-dimensional reconstruction of every track in the event, at a resolution of a few millimetres, together with the energy deposited along each track. Particles can be identified by how they lose energy, not only by the shape of a light cone.

That matters for the physics DUNE is after. Distinguishing an electron from a photon, or resolving the individual particles emerging from a complicated interaction, is far easier with an image than with a ring. Argon is used because it is dense, chemically inert, transparent to its own scintillation light, and - being about one percent of the atmosphere - available in the required quantity at industrial cost.

The engineering price is cryogenics. Seventy thousand tonnes of argon must be held at about minus 186 degrees Celsius, and at a purity where contaminants such as oxygen are measured in parts per trillion, because a few stray oxygen molecules would capture the drifting electrons before they ever reached the readout.

The Experiment in Numbers

DUNE is under construction. The figures below describe the design as approved and, where excavation and prototyping are complete, as built.

• Baseline: 1,300 kilometres, Fermilab (Batavia, Illinois) to Lead, South Dakota
• Far detector: four modules, roughly 17,000 tonnes of liquid argon each, about 70,000 tonnes total
• Depth: approximately 1.5 kilometres underground at the Sanford Underground Research Facility
• Beam: Long-Baseline Neutrino Facility, 1.2 megawatts initially, upgradeable towards 2.4 megawatts
• Technology: liquid argon time projection chamber, millimetre-scale three-dimensional track imaging
• Collaboration: more than 1,400 scientists from over 200 institutions in more than 30 countries
• Site history: the former Homestake gold mine, where Raymond Davis Jr. ran the first solar neutrino experiment
• Status: far-site excavation completed 2024; first modules expected around the end of the decade

The Question DUNE Was Built to Answer

The universe is made of matter. The Big Bang should have produced matter and antimatter in equal amounts, which would have annihilated completely, leaving radiation and nothing else. Something tipped the balance, and the imbalance required is small - roughly one part in a billion - but it is the reason anything exists.

One candidate explanation is CP violation: a difference in how particles and their antiparticles behave. A small amount has been observed among quarks since the 1960s, but it is far too little to account for the asymmetry. The remaining hope is that the effect is much larger in the lepton sector, and the neutrino is where it would show.

The test is direct in principle. Fire muon neutrinos, count how many arrive as electron neutrinos. Then fire muon antineutrinos and count how many arrive as electron antineutrinos. If oscillation treats the two cases identically, the fractions match. If they do not, CP symmetry is violated in the lepton sector.

The difficulty is statistics and control. The effect, if present, is a modest difference between two small numbers, so it demands an enormous detector, a very intense beam, a long baseline and years of running. That set of requirements is essentially the specification for DUNE.

What Else the Detector Will Do

The same 1,300-kilometre baseline settles the neutrino mass ordering. As neutrinos travel through the Earth, matter itself modifies the oscillation - electrons in the rock affect electron neutrinos differently from the other types. This matter effect grows with distance, and at 1,300 kilometres it is strong enough to reveal whether the third mass state sits above the other two or below them, a question oscillation experiments with shorter baselines struggle to resolve.

DUNE will also watch for proton decay, as Super-Kamiokande does, but with a different sensitivity. Argon imaging is particularly good at the decay channel involving a kaon, which water detectors find hard to see. The two technologies therefore cover complementary channels rather than competing on the same one.

And it will be one of the best instruments in the world for a galactic supernova. Argon is unusually sensitive to electron neutrinos through absorption on argon-40, whereas most large detectors respond mainly to electron antineutrinos. During a core collapse the earliest burst is dominated by electron neutrinos, so DUNE would see the first seconds of the event in a channel most other detectors are nearly blind to, complementing what supernova neutrino detectors elsewhere would record.

Prototype detectors called ProtoDUNE have been running at CERN for years, at a scale of several hundred tonnes, to validate the design and the reconstruction software before the full modules are built. That staged approach is why a project of this size is credible on paper before the detector exists.

What DUNE Will Not Settle

It will not measure the absolute mass of the neutrino. Like every oscillation experiment, DUNE is sensitive to differences between squared masses and to the ordering of the states, not to how much any of them weighs. That question belongs to direct-measurement experiments of a completely different design.

It will not by itself explain the matter-antimatter asymmetry either, even if it finds a large CP-violating phase. Leptonic CP violation is a necessary ingredient in the leading class of explanations, not a complete account, and connecting the two would require additional physics at energies no accelerator will reach.

And it does not bear on whether neutrino flux can be converted into usable power. It is worth restating why, because a detector this large invites the assumption that the interactions are plentiful. They are not. The scale of DUNE exists to compensate for how rarely neutrinos interact, and the beam is intense precisely because the natural interaction rate is negligible. Neutrinovoltaic research asks a different question about ambient radiation and thermal fluctuations at a material surface, and DUNE neither supports nor refutes it.

What DUNE will do is turn the neutrino from a particle we detect into a particle we measure precisely. That is a change of category, and it is the reason a dozen countries agreed to spend a decade and a great deal of money digging out a mountain in South Dakota.

Homestake, Twice

The site is not a coincidence. Homestake was chosen in the 1960s for the same reason it is being used now: it is deep, it is stable, and the rock overhead removes almost all cosmic-ray background. Davis's chlorine tank sat at the 4,850-foot level; DUNE's caverns are on the same level, a few hundred metres away.

Davis's result - a persistent deficit of solar neutrinos - was widely assumed to be an error in his chemistry or in the solar model. It was neither. It was the first sign of oscillation, though it took the Sudbury Neutrino Observatory and Super-Kamiokande to prove it, and Davis received a share of the 2002 Nobel Prize in Physics at the age of 88.

There is a lesson in that sequence worth keeping in view. An anomaly that survives thirty years of attempted explanation is not necessarily a mistake, but it is also not a discovery until an independent experiment with different systematics confirms it. Neutrino physics has been unusually disciplined about that standard, and the field's credibility rests on it.

For a broader picture of how the different detectors divide the work, see our overview of neutrino observatories and the companion pages on IceCube and Super-Kamiokande.