Daya Bay

Daya Bay was a reactor experiment in the southern Chinese province of Guangdong. In 2012 it determined the last unknown mixing angle of neutrino oscillation — and thereby cleared the way for the search for CP violation.

The missing angle

The PMNS matrix contains three mixing angles. Two of them were known around the turn of the millennium: θ₁₂ from solar neutrinos and KamLAND, θ₂₃ from atmospheric neutrinos.

The third, θ₁₃, was unknown and possibly zero.

A great deal hung on this number. Had θ₁₃ been zero, CP violation in the lepton sector could not have been measured at all — the corresponding phase would no longer appear in any observable quantity. The question of why the universe consists of matter would have remained unanswerable along this route.

Earlier experiments had only bounded θ₁₃ from above. All indications suggested it was small — the question was whether small or zero.

The set-up

At the site stand six nuclear reactors with a combined 17.4 gigawatts of thermal power, one of the strongest neutrino sources in the world.

Daya Bay distributed eight identically built detectors, each with 20 tonnes of scintillator, across three underground halls: two near the reactors, about 500 metres away, and one far hall at around 1,650 metres.

The scintillator was doped with gadolinium. This element captures neutrons particularly readily and emits a strong light signal in doing so — which makes the delayed coincidence of two flashes far more distinct.

Why the design was decisive

The clever part lies in the comparison. The near detectors measure what the reactors emit; the far one measures what arrives.

That removes the largest uncertainty. Exactly how many antineutrinos a reactor gives off depends on the fuel, on the burn-up and on nuclear data that are themselves known only to within a few per cent. For a measurement concerned with a deficit of a few per cent, that would be fatal.

But because all eight detectors were identically built and filled with the same scintillator from the same batch, these uncertainties cancel almost completely in the ratio.

The detectors were even swapped between the halls during operation, in order to rule out the very last remainder of instrument-related differences.

The result

In March 2012 the collaboration presented a result after only 55 days of measuring time: θ₁₃ differs from zero, with a significance of more than five standard deviations.

The value was moreover larger than many had expected — large enough to make CP violation measurable with the facilities of the next generation.

A few weeks later the RENO experiment in Korea and Double Chooz in France confirmed the finding. T2K had shortly before provided first indications from the other direction, via the appearance of electron neutrinos in a beam.

By the conclusion of the measurements in December 2020 Daya Bay had determined θ₁₃ to about three per cent — still the most precise value today.

What followed from it

Without this result there would be neither Hyper-Kamiokande nor DUNE in their present design. Both are built to measure an asymmetry between neutrinos and antineutrinos — and that this is possible at all is what Daya Bay showed.

PMNS matrix — where θ₁₃ sits
Reactor neutrinos — the source
T2K — the measurement from the other direction
JUNO — the successor in China

Sources

Daya Bay Collaboration: Observation of Electron-Antineutrino Disappearance at Daya Bay, Physical Review Letters 108, 171803 (2012).
Daya Bay Collaboration: Precision Measurement of Reactor Antineutrino Oscillation at Kilometer-Scale Baselines, Physical Review Letters 130, 161802 (2023).