T2K: how Japan fires a neutrino beam 295 kilometers across its own country

Every two seconds, somewhere along the eastern coast of Japan, a small cylinder of carbon is bombarded by a burst of high-energy protons from the J-PARC accelerator complex in Tokai. The collisions produce a spray of pions. The pions are focused by giant magnetic horns and allowed to decay in flight inside a 96-meter tunnel. Most of the decay products are muons and muon neutrinos.

The muons get absorbed by a wall of iron at the end of the tunnel. The neutrinos punch through, point themselves slightly downward, and travel 295 kilometers westward through the rock of Honshu. Their destination is Super-Kamiokande — the 50,000-ton water Cherenkov tank under a mountain near Toyama — which catches a few of them in flight.

This is T2K, the Tokai-to-Kamioka experiment. It has been running since 2009 and has produced some of the most precise measurements of neutrino oscillation parameters in the field. It is also, alongside its American counterpart NOvA, a current leader in the hunt for CP violation in the neutrino sector.

The basic idea

Long-baseline neutrino experiments work on a simple principle. Produce a clean beam of one neutrino flavor at one end. Detect what flavor arrives at the other end. The difference tells you how the neutrinos have oscillated during their trip.

For T2K, the beam starts as muon neutrinos (or, alternating, muon antineutrinos). By the time they arrive at Super-Kamiokande 295 km later, some have oscillated into other flavors — most notably electron neutrinos. By counting how many electron neutrinos show up in the detector and comparing the rate for neutrinos versus antineutrinos, T2K can extract two crucial parameters:

The mixing angle θ₁₃, which governs how strongly muon and electron flavors mix.

The CP-violating phase δ_CP, which determines whether neutrinos and antineutrinos oscillate at slightly different rates.

Both quantities matter for fundamental physics. θ₁₃ was measured in the early 2010s by T2K and the reactor experiments Daya Bay and RENO. δ_CP is still being measured today, and its value remains the most-watched number in neutrino physics.

Why 295 kilometers

The choice of baseline is not accidental. At a neutrino energy of about 600 MeV — the peak of T2K’s beam — the muon-to-electron oscillation probability reaches a maximum at almost exactly that distance.

The geometry of Japan made this combination possible. J-PARC sits on the Pacific coast at Tokai. Super-Kamiokande is 295 km away in the Kamioka mountains. The line between them lies entirely within Japan, requiring no international agreements about firing particle beams under foreign territory.

The beam direction is slightly off-axis from Super-Kamiokande’s location — about 2.5 degrees. This off-axis trick produces a narrower energy spectrum than a perfectly on-axis beam would, which sharpens the oscillation measurement. T2K was the first major experiment to use this design choice, and most subsequent long-baseline programs (NOvA, DUNE, Hyper-Kamiokande) have followed the same approach.

What T2K has actually found

T2K’s results, accumulated over more than a decade, have been important.

First observation of electron-neutrino appearance. In 2011, T2K reported the first indication that muon neutrinos in their beam were turning into electron neutrinos at the Super-Kamiokande end. By 2013 this had grown to a robust 7.5σ discovery — the first direct observation of the oscillation channel that CP violation can affect.

Strong constraint on θ₁₃. T2K’s measurements of the mixing angle θ₁₃ are competitive with the reactor experiments, providing an important cross-check.

Preference for maximal CP violation. Most recent T2K results favor a CP-violating phase near δ_CP = −π/2, which would correspond to nearly maximal CP violation. The result is not yet decisive — the statistical significance hovers around 2σ — but it points in an interesting direction, and the result has aged well as more data accumulates.

Improved oscillation parameter precision. Combined fits using T2K, NOvA, and reactor data have steadily reduced the uncertainties on all three mixing angles and the mass-squared differences.

How the beam is actually produced

J-PARC stands for Japan Proton Accelerator Research Complex. It is a sprawling facility on the Pacific coast that combines a linear accelerator, a rapid-cycling synchrotron, and a main ring that produces 30-GeV protons at high intensity. A fraction of those protons are extracted toward T2K’s neutrino beam line.

The protons hit a graphite target. Pions and kaons emerge in a forward spray. Magnetic horns — pulsed at thousands of amperes — focus the positively or negatively charged secondaries into a tight beam, choosing whether the resulting neutrino beam is dominated by muon neutrinos (focusing positive pions) or muon antineutrinos (focusing negative ones).

The focused beam enters a 96-meter-long decay tunnel where the pions and kaons decay. The decay products include the desired muon neutrinos plus a contamination of muons and a small electron-neutrino component from kaon decays. A 30-meter-thick beam dump at the end absorbs everything except the neutrinos.

The intensity is staggering. About 10²¹ neutrinos pass through Super-Kamiokande per year. Of those, the experiment catches roughly a few hundred per year that actually interact in the detector.

What’s next

T2K is being upgraded. The T2K-II phase, currently underway, increases beam power and improves near-detector capabilities to push CP-violation sensitivity higher. The hardware upgrades should let T2K reach a 3σ measurement of maximal CP violation, depending on what nature actually chose.

After T2K, the same beam line will be redirected to feed Hyper-Kamiokande, the 260,000-ton successor to Super-Kamiokande, due to start in 2027. Hyper-K will be roughly eight times the size of Super-K and will receive an upgraded, higher-intensity beam. Together, the J-PARC + Hyper-K combination is one of the two great CP-violation experiments of the next decade — the other being DUNE in the United States.

Why T2K still matters

T2K is, in some ways, the prototype for everything that comes next. The off-axis beam technique, the use of a precisely tuned baseline, the combined near-and-far detector strategy, the careful systematics work — all of this was developed and refined by T2K over a decade and a half of running.

By the time Hyper-Kamiokande and DUNE produce their first major results, T2K will have spent two decades preparing the ground. The mountain near Toyama, with its 50,000 tons of ultra-pure water under a kilometer of rock, will have caught hundreds of thousands of accelerator-produced neutrinos. Each one tells a small piece of the story of how nature works at its most fundamental level.

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For T2K’s bigger sibling under construction, see Hyper-Kamiokande. For the American counterpart, see DUNE. For the underlying physics, see How neutrino oscillation works.

Neutrino Times Editorial Team. (2025, September 13). T2K: how Japan fires a neutrino beam 295 kilometers across its own country. Neutrino Times. https://neutrino-times.com/articles/t2k-experiment-tokai-to-kamioka-neutrino-beam/

Neutrino Times Editorial Team. "T2K: how Japan fires a neutrino beam 295 kilometers across its own country." Neutrino Times, September 13, 2025. https://neutrino-times.com/articles/t2k-experiment-tokai-to-kamioka-neutrino-beam/.

Neutrino Times Editorial Team. "T2K: how Japan fires a neutrino beam 295 kilometers across its own country." Neutrino Times, 13 Sep. 2025, https://neutrino-times.com/articles/t2k-experiment-tokai-to-kamioka-neutrino-beam/.

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  title        = {T2K: how Japan fires a neutrino beam 295 kilometers across its own country},
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