OPERA: how Gran Sasso caught five tau neutrinos appearing out of a muon beam

In the late 1990s, the Super-Kamiokande experiment discovered that atmospheric muon neutrinos disappear as they travel through the Earth — strong evidence that they were oscillating into another flavor. The most plausible candidate was the tau neutrino, because oscillation parameters worked out to favor that channel and because the alternative — oscillation into a hypothetical sterile state — was disfavored by other measurements.

But Super-K’s measurement was a disappearance experiment. It saw that muon neutrinos vanished. It did not directly identify what they became.

The next step in the story required catching the appearance of tau neutrinos in a muon beam. This is technically demanding: tau neutrinos at oscillation-friendly energies produce tau leptons through charged-current interactions, and tau leptons travel only a few hundred microns before decaying. To detect them you need extraordinary spatial resolution.

That was the mission of OPERA — the Oscillation Project with Emulsion-tRacking Apparatus — installed in the Gran Sasso underground laboratory in central Italy. Between 2008 and 2012, OPERA recorded neutrino interactions from a beam fired by CERN 730 kilometers away. By the time the analysis was complete in 2018, the collaboration had found five tau-neutrino events with a combined statistical significance of 6.1σ. The direct observation of muon-to-tau oscillation was confirmed.

The CNGS beam

The OPERA beam was called CNGS, the CERN Neutrinos to Gran Sasso. It was produced by extracting 400-GeV protons from the CERN Super Proton Synchrotron, sending them onto a graphite target, focusing the produced pions and kaons with magnetic horns, and letting them decay in flight in a 1-kilometer tunnel aimed at Italy.

The resulting beam was a nearly pure stream of muon neutrinos with energies peaked around 17 GeV — higher than most long-baseline beams, because OPERA needed enough kinematic energy to produce a tau lepton from a tau-neutrino interaction. (The tau lepton’s mass is about 1.78 GeV, so the neutrino energy has to be well above that threshold.)

The geometry of the beam was straightforward. CERN sits on the French-Swiss border. Gran Sasso sits in central Italy. The straight-line distance through the Earth’s crust is 730 kilometers, with the beam dipping a few kilometers below the surface during its trip. The whole journey takes about 2.4 milliseconds.

Over the four years of data-taking, CNGS delivered roughly 18 × 10¹⁹ protons on target to OPERA — enough to produce a small but expected number of tau-neutrino appearance events at the far end.

The emulsion-brick detector

OPERA’s detector was unusual. It was built around the use of nuclear emulsion — a fine-grained photographic-style detector that records charged-particle tracks with submicron precision.

The detector consisted of about 150,000 emulsion bricks, each a sandwich of 56 lead plates (1 millimeter thick) interleaved with 57 emulsion films. Each brick weighed about 8.3 kilograms and was about the size of a small book. Together, the bricks formed a target with about 1,250 tons of lead — enough mass to produce a meaningful number of neutrino interactions over the lifetime of the experiment.

The bricks were arranged in two giant supermodules, each about 8 meters long and 7 meters wide, instrumented with electronic trackers and a muon spectrometer to identify the broad characteristics of each event. When the electronics identified a likely neutrino interaction in a brick, that brick was removed for emulsion analysis. The relevant films were developed, scanned with high-precision optical microscopes, and analyzed in detail.

The advantage of this hybrid approach was that emulsion can resolve the short flight of a tau lepton before it decays, while the electronics can identify the relevant brick out of 150,000 without requiring the analysis of all of them. The disadvantage was the enormous human and scanning effort required for each candidate event.

What a tau-neutrino signature looks like

When a tau neutrino interacts in OPERA, it can convert into a tau lepton via the charged-current weak interaction. The tau lepton, with a mass of 1.78 GeV, decays within about 290 fs into either an electron, a muon, or a hadron, plus one or two neutrinos. At OPERA’s beam energy, the tau lepton flies about 1 millimeter before decaying — long enough that the daughter track has a clear angular kink from the parent direction.

That kink — a sudden change of direction in the charged-particle track, with no observable parent track between the production vertex and the decay — is the unmistakable signature of a tau-lepton decay. Emulsion can resolve such a kink with submicron precision; no other practical detector can.

The four daughter modes the analysis tracked were:

• Tau → electron + neutrinos
• Tau → muon + neutrinos
• Tau → 1 hadron + neutrinos
• Tau → 3 hadrons + neutrinos

Each mode has different backgrounds and acceptance, but all are characterized by the same topological feature: a track that suddenly changes direction within about a millimeter of its origin.

The five events

The first OPERA tau-neutrino candidate was announced in 2010, then a second in 2012, a third in 2013, a fourth in 2014, and the final fifth event in 2014. Each was reported with extensive checks against the most likely backgrounds, primarily charm-hadron production in muon-neutrino interactions (which can mimic the tau-decay topology) and hadron interactions in the lead plates.

The combined 2018 analysis reported the full set with a statistical significance of 6.1σ — well above the 5σ discovery threshold. The flavor distribution across the five events was consistent with the predicted decay branching ratios, and the kinematic distributions matched expectations from muon-to-tau oscillation with the parameters previously inferred from disappearance data.

For an experiment that took years of data-taking and emulsion analysis to produce five events, the result feels modest in absolute terms. But the events were unambiguous, and they delivered exactly what the experiment had been built to find.

What we learned beyond confirmation

The OPERA result is sometimes characterized as “confirming what we already believed,” and at one level that is right: the disappearance measurements had strongly suggested muon-to-tau oscillation, and the OPERA result confirmed it. But the experiment contributed several specific things beyond mere confirmation.

Direct cross-section measurement. OPERA was the first experiment to provide a sample of tau-neutrino charged-current interactions at oscillation energies. The cross-section measurement extracted from the five events, while statistically limited, is a useful input to the field.

Constraints on non-standard interactions. The kinematic distributions of the five tau-neutrino events constrain certain models in which neutrinos have additional interactions beyond the Standard Model.

A direct test of three-flavor oscillation. Before OPERA, the three-flavor mixing matrix had been mapped through disappearance measurements and electron-neutrino appearance. The tau-neutrino appearance closed the last piece: all three appearance channels (νμ → νe via T2K and NOvA; νμ → ντ via OPERA; etc.) had now been seen directly.

Sterile-neutrino constraints in the tau sector. Some sterile-neutrino models would have suppressed tau-neutrino appearance. OPERA’s result rules out a portion of the parameter space.

After OPERA

OPERA was decommissioned at the end of CNGS running in 2012, and the emulsion analysis continued for several years afterward. The experiment is no longer operating, but its emulsion-based detection approach has been inherited by other experiments — most directly by FASERν at the LHC, which uses similar tungsten-and-emulsion bricks to detect collider neutrinos in the TeV range.

The CNGS beam itself is no longer operating; CERN’s neutrino program is now focused on the Short-Baseline Neutrino Program at Fermilab and on the DUNE experiment in the United States.

The completed three-flavor picture

By the time OPERA’s final paper appeared in 2018, the three-flavor neutrino oscillation picture had been mapped in considerable detail. All six channels of the standard three-flavor mixing — three disappearance channels and three appearance channels — had been observed. The mixing angles were measured. The mass-squared differences were known. The remaining unknowns were the CP-violating phase, the mass ordering, and the absolute mass scale.

OPERA was a key piece of completing that picture. It is not as widely remembered as some of the bigger experiments — partly because its result felt incremental once the field was confident about disappearance, and partly because five events is a smaller dataset than most modern experiments aim for. But it was an important final brick in establishing that the three known neutrino flavors all mix into each other as oscillation predicts.

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For the disappearance discovery OPERA confirmed, see Super-Kamiokande 1998. For the other long-baseline experiments, see T2K, NOvA, and DUNE. For the same emulsion technology now in use at the LHC, see FASER.

Neutrino Times Editorial Team. (2025, November 7). OPERA: how Gran Sasso caught five tau neutrinos appearing out of a muon beam. Neutrino Times. https://neutrino-times.com/articles/opera-tau-neutrino-appearance-cngs/

Neutrino Times Editorial Team. "OPERA: how Gran Sasso caught five tau neutrinos appearing out of a muon beam." Neutrino Times, November 7, 2025. https://neutrino-times.com/articles/opera-tau-neutrino-appearance-cngs/.

Neutrino Times Editorial Team. "OPERA: how Gran Sasso caught five tau neutrinos appearing out of a muon beam." Neutrino Times, 7 Nov. 2025, https://neutrino-times.com/articles/opera-tau-neutrino-appearance-cngs/.

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  title        = {OPERA: how Gran Sasso caught five tau neutrinos appearing out of a muon beam},
  howpublished = {Neutrino Times},
  year         = {2025},
  month        = {nov},
  url          = {https://neutrino-times.com/articles/opera-tau-neutrino-appearance-cngs/},
  note         = {Accessed: 2025-11-07}
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