NOvA

Objective

Measure νμ → νe appearance and νμ disappearance at 810 km from the Fermilab NuMI beam to extract θ₂₃, Δm²₃₁, the mass ordering, and δ_CP.

Method

A 14 kt liquid-scintillator tracking calorimeter in Ash River, Minnesota, composed of extruded PVC cells filled with liquid scintillator and read out by wavelength-shifting fibers. A 300-ton functionally identical near detector sits at Fermilab 1 km from the NuMI beam target. The 14 mrad off-axis configuration gives a narrow-band 2 GeV neutrino spectrum matched to the first oscillation maximum.

Key results

• 2015: first νe appearance observation at the 3σ level from the NOvA beam.
• 2019–present: highest-statistics antineutrino-mode running of any accelerator neutrino experiment, providing the cleanest single-experiment νe/ν̄e asymmetry test.
• Mild preference for normal mass ordering, approximately 2σ significance in combined analyses.
• δ_CP best fit near 0 or 2π in normal ordering, in some tension with T2K results.

Significance

NOvA's long baseline produces substantial Earth matter effects that, combined with the ν/ν̄ asymmetry, make it particularly sensitive to the mass ordering. Together with T2K it provides the current two-experiment constraint on δ_CP, pending the much larger statistics expected from DUNE and Hyper-K.

The NuMI beam

The NuMI beam at Fermilab uses 120 GeV protons from the Main Injector at a design power of 700 kW (upgraded to 1 MW by 2020 and aiming higher). The protons strike a graphite target, producing pions and kaons that are focused by magnetic horns and decay in flight along a 675 m decay pipe.

The horn polarity determines whether the beam is neutrino-mode (selecting ${\pi }^{+}$, producing ${\nu }_{\mu }$) or antineutrino-mode (selecting ${\pi }^{-}$, producing ${\bar{\nu }}_{\mu }$). NOvA alternates between the two modes with roughly equal exposures, enabling direct CP-asymmetry measurements.

Off-axis geometry

Like T2K, NOvA is deliberately off-axis: the Ash River far-detector site is 14 mrad off the NuMI beam axis, giving a narrower spectrum peaked at ~2 GeV — matched to the first oscillation maximum at 810 km. The near detector at Fermilab sits at 1 km and at the same off-axis angle so that it samples the same spectrum as the far detector, modulo small baseline corrections.

Detector technology

Both detectors use extruded PVC cells filled with mineral-oil-based liquid scintillator. Each cell measures 3.9 × 6.6 cm and is 14 m long in the far detector; they are arranged in alternating horizontal and vertical layers to give 3D tracking. Wavelength-shifting fibers collect the scintillation light and route it to avalanche photodiodes outside the detector volumes.

The resulting detector is a fine-grained tracking calorimeter with excellent electron/muon/neutral-pion separation — essential for distinguishing ${\nu }_e$ appearance from neutral-current π⁰ backgrounds.

Physics results

νμ disappearance gives precise measurements of ${\sin }^2{\theta }_{23}$ and $\Delta m_{31}^2$. NOvA’s result slightly favors non-maximal ${\theta }_{23}$ with an octant preference that shifts modestly across analysis updates.

νe appearance has been measured in both neutrino and antineutrino modes. NOvA’s results combine with Daya Bay’s ${\theta }_{13}$ to constrain ${\delta }_{CP}$. Current combined analyses:

• Favor normal mass ordering at ~2σ
• Favor ${\delta }_{CP}$ near 0 or $2\pi$ in the normal ordering (near maximum CP conservation in this octant combination)
• Do not strongly prefer either octant of ${\theta }_{23}$

Tension with T2K

NOvA and T2K’s preferred ${\delta }_{CP}$ values differ by ~2σ in the normal ordering. Whether this is statistical fluctuation, residual systematic, or a hint of new physics is currently an open question. The tension will be resolved — or made more compelling — as more data accumulates at both experiments, and as DUNE and Hyper-K come online.

Outlook

NOvA will continue accumulating exposure through the late 2020s. Its eventual retirement is tied to the transition of the NuMI infrastructure to LBNF (Long-Baseline Neutrino Facility) that will feed DUNE at a much longer 1,300 km baseline through the Earth’s mantle. NOvA’s lessons on long-baseline calibration, neutrino cross-sections, and systematic control directly inform DUNE’s design.