T2K

Objective

Measure νμ → νe appearance and νμ disappearance at a 295 km baseline to extract θ₁₃, Δm²₃₁, θ₂₃, and the CP-violating phase δ_CP.

Method

A pulsed νμ beam produced at J-PARC in Tokai from 30 GeV proton collisions on a graphite target, focused by magnetic horns and directed at Super-Kamiokande. A near-detector complex (ND280, INGRID) at 280 m from the target characterizes the unoscillated beam. Super-Kamiokande serves as the far detector, identifying oscillated νe or disappeared νμ through Cherenkov ring patterns.

Key results

• 2011: first hint of νe appearance at 2.5σ, suggesting non-zero θ₁₃ shortly before Daya Bay confirmed it.
• 2014: formal discovery of νe appearance at >7σ — the first non-zero-θ₁₃ appearance measurement.
• 2020: data favoring large CP violation, with δ_CP = 0 or π disfavored at the 2σ level.
• Precision νμ disappearance measurements of θ₂₃ and Δm²₃₁.
• First direct νe ↔ ν̄e comparison constraining CP asymmetry in a lepton-sector channel.

Significance

First observation of νμ → νe appearance, demonstrating the full three-flavor oscillation structure. Continuing data-taking targets the first determination of lepton-sector CP violation; combined with NOvA and eventually DUNE and Hyper-K, will resolve δ_CP and the mass ordering.

The off-axis beam

T2K — Tokai to Kamioka — uses an off-axis beam configuration. The 30 GeV proton beam at J-PARC in Tokai produces pions that decay in flight to muon neutrinos. The neutrino beam is aimed 2.5° off the axis toward Super-Kamiokande, giving a narrower energy spectrum peaked at ~0.6 GeV — precisely at the first oscillation maximum for $\Delta m_{31}^2$ at the 295 km baseline.

The narrow-band spectrum enhances the signal-to-background ratio for oscillation measurements, at the cost of flux reduction compared to on-axis configurations. The T2K geometry was influential in the design of subsequent long-baseline experiments.

Near and far detectors

ND280 (Near Detector at 280 m) measures the unoscillated beam composition and kinematics with a tracker system, TPC volumes, fine-grained calorimetry, and a magnet. It has become a workhorse for neutrino-nucleus cross-section measurements in the 0.1–10 GeV range.

INGRID is an on-axis 7-module iron/scintillator array that monitors beam direction and stability.

Super-Kamiokande at 295 km is the far detector — 50 kt of water Cherenkov with 11,146 PMTs. Oscillated ${\nu }_e$ events are identified by their e-like ring patterns; disappeared ${\nu }_{\mu }$ by the deficit in μ-like events.

Oscillation results

νμ disappearance gave precise measurements of ${\sin }^{2}2{\theta }_{23}$ consistent with near-maximal mixing, and $|\Delta m_{31}^2|$ consistent with values from MINOS and NOvA.

νe appearance was first announced at the 2.5σ level in 2011, then at 7.3σ discovery significance in 2014. The measured appearance rate, combined with Daya Bay’s ${\theta }_{13}$, favored certain values of ${\delta }_{CP}$ and the mass ordering.

By 2020, T2K’s combined neutrino and antineutrino appearance data excluded CP conservation (${\delta }_{CP}=0$ or $\pi$) at the 2σ level and favored large values of ${\delta }_{CP}$ near $-\pi /2$. The significance remains statistics-limited; later data has strengthened the hint without achieving 3σ discovery.

Interplay with NOvA

T2K and NOvA (810 km baseline, 1-2 GeV beam) share overlapping physics goals but differ in baseline and energy. The two experiments are sensitive to different linear combinations of ${\delta }_{CP}$, the mass ordering, and ${\theta }_{23}$. Combined analyses show some tension between the two experiments, most visible in the preferred ${\delta }_{CP}$ value. Resolution awaits more statistics from both experiments and, ultimately, the definitive measurements from DUNE and Hyper-K.

T2HK and the future

The Hyper-Kamiokande experiment (first data ~2027) reuses the J-PARC beamline with an upgraded power of up to 1.3 MW. The far detector is 8× larger than Super-K. The combination is expected to discover leptonic CP violation at >5σ for a substantial fraction of possible ${\delta }_{CP}$ values.

T2K continues to run through the Hyper-K construction phase, accumulating data to narrow the current uncertainties.