NOvA: how Fermilab fires neutrinos under five US states to catch CP violation

Every 1.3 seconds, a burst of 120-GeV protons slams into a graphite target deep inside the Fermilab complex outside Chicago. The collision produces a forward shower of pions and kaons, which are focused by magnetic horns and allowed to decay in a long tunnel. The decay products include the desired muon neutrinos — and a tiny, focused beam of them streams off through the rock of Illinois, Wisconsin, and Minnesota.

810 kilometers later, deep in the northern woods near the small town of Ash River, Minnesota, a 14,000-ton block of plastic and scintillator oil waits to catch a few of them. This is the NOvA far detector — and together with its 300-ton near detector at Fermilab, it forms one of the two great long-baseline neutrino experiments running today.

A different design choice

NOvA stands for NuMI Off-axis νₑ Appearance. The name encodes its core experimental strategy: a neutrino beam from Fermilab’s NuMI facility, aimed slightly off-axis from the far detector, looking for muon neutrinos that have oscillated into electron neutrinos by the time they arrive.

The off-axis trick — first deployed at T2K in Japan — narrows the neutrino energy spectrum, sharpening the oscillation signal. NOvA sits 14.6 milliradians off the beam axis, which produces a beam peaked at about 2 GeV — exactly the energy that maximizes the muon-to-electron oscillation probability over the 810 km baseline.

The two NOvA detectors are unusual. They are made of long, narrow PVC cells filled with liquid scintillator and read out by wavelength-shifting fibers. The far detector contains 344,064 such cells, arranged in alternating horizontal and vertical planes — a design optimized for tracking the long, slim electromagnetic showers that electron neutrinos produce, which are the signature NOvA was built to catch.

What NOvA measures

NOvA addresses three big questions in long-baseline neutrino physics.

The neutrino mass ordering. Because Fermilab and Ash River are separated by 810 km of Earth, neutrinos in the beam pass through enough rock for matter effects to modify their oscillation rates. The size and sign of the matter effect depends on whether the mass ordering is normal or inverted. NOvA’s data — combined with reactor experiments — provides one of the strongest current constraints.

CP violation. The CP-violating phase δ_CP determines whether neutrinos and antineutrinos oscillate at slightly different rates. NOvA runs alternately in neutrino-mode and antineutrino-mode, comparing the appearance rates of electron neutrinos versus electron antineutrinos in the far detector. The current NOvA data prefers values of δ_CP that are mildly inconsistent with T2K’s preferred values — a tension that has fuelled active discussion in the field.

Precision oscillation parameters. NOvA’s measurement of the muon-neutrino disappearance rate gives one of the most precise determinations of the mixing angle θ₂₃ and the mass-squared difference Δm²₃₂.

The Fermilab–Ash River tension

A small but persistent puzzle has emerged from comparing NOvA and T2K. Both experiments measure essentially the same parameters with similar techniques, but their preferred values of δ_CP do not quite agree. T2K leans toward maximal CP violation near −π/2; NOvA prefers values closer to 0 or +π/2.

The disagreement is not yet at the level of a real discrepancy — both data sets are statistics-limited, and the preferred regions overlap when you account for systematic uncertainties. But it is the kind of small mismatch that physicists watch closely, because if it grows it could signal something interesting about the neutrino sector.

The eventual resolution will come from the next generation of experiments — DUNE and Hyper-Kamiokande — which will have far more statistics and better systematic control. Until then, NOvA and T2K provide the most precise existing measurements, and any tensions between them are worth taking seriously.

How the far detector actually works

The Ash River laboratory sits on the surface of the Earth, about 100 meters underground inside a custom-built steel building. Because cosmic rays are abundant at that depth, the experiment relies on timing cuts to identify neutrino events: events arriving in coincidence with the NuMI beam pulse are signal candidates; everything else is background.

Inside the detector, each cell is filled with mineral oil mixed with a small amount of pseudocumene scintillator. A charged particle moving through the cell deposits a tiny flash of light, which is captured by a wavelength-shifting fiber running through the middle of the cell. The fiber pipes the light to a photodetector on one end. By recording which cells flashed and when, the readout system reconstructs the trajectory of every charged particle in the event.

The geometry — alternating horizontal and vertical planes — allows three-dimensional reconstruction. The total mass of about 14,000 tons makes the detector large enough to catch a few thousand beam-correlated neutrino events per year, of which only a small fraction are the electron-neutrino appearance events that probe CP violation directly.

What’s next for NOvA

The original NOvA program ran from 2014 through about 2025, accumulating data at increasing beam intensities. NOvA-II, the second phase of the experiment, is taking advantage of the upgraded PIP-II proton injector at Fermilab to deliver higher beam power. The data accumulated through the late 2020s will sharpen NOvA’s constraints on CP violation and the mass ordering — and provide a cleaner comparison with T2K and an early benchmark for DUNE.

After NOvA, the same Fermilab beam line will be redirected as LBNF — the Long-Baseline Neutrino Facility — feeding DUNE in South Dakota. DUNE’s 1,300 km baseline and 70-kiloton liquid-argon detector will offer about an order of magnitude more sensitivity than NOvA. But by the time DUNE is taking physics data, NOvA’s long, careful exposure will have set the most stringent existing constraints on the parameters that DUNE is being built to measure.

Why NOvA matters

Long-baseline neutrino experiments are one of the most ambitious technological feats in modern physics. They require precision proton beams, kilometer-scale tunnels, near and far detectors thousands of kilometers apart, and analysis chains that subtract every conceivable background.

NOvA has been doing this quietly and reliably for more than a decade. It has provided some of the best existing measurements of the neutrino oscillation parameters, set up the cross-check that has revealed the NOvA–T2K tension, and helped frame the questions DUNE is being built to answer.

The story of neutrino oscillation in the 2020s is being written by NOvA and T2K. The next chapter — DUNE and Hyper-K — will rest squarely on what they have established.

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For NOvA’s Japanese counterpart, see T2K. For the larger long-baseline future, see DUNE and Hyper-Kamiokande. For the underlying physics, see How neutrino oscillation works.

Neutrino Times Editorial Team. (2025, September 18). NOvA: how Fermilab fires neutrinos under five US states to catch CP violation. Neutrino Times. https://neutrino-times.com/articles/nova-experiment-fermilab-minnesota-far-detector/

Neutrino Times Editorial Team. "NOvA: how Fermilab fires neutrinos under five US states to catch CP violation." Neutrino Times, September 18, 2025. https://neutrino-times.com/articles/nova-experiment-fermilab-minnesota-far-detector/.

Neutrino Times Editorial Team. "NOvA: how Fermilab fires neutrinos under five US states to catch CP violation." Neutrino Times, 18 Sep. 2025, https://neutrino-times.com/articles/nova-experiment-fermilab-minnesota-far-detector/.

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  note         = {Accessed: 2025-09-18}
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