Daya Bay

Background. By 2011, two of the three angles governing neutrino oscillation were well measured, but the third, θ₁₃, remained unknown and might have been zero. Its size matters enormously: only a non-zero θ₁₃ leaves room for charge-parity violation in the lepton sector. Nuclear reactors are intense, well-understood sources of electron antineutrinos, making them ideal for the search (see reactor neutrinos).

Objective. Daya Bay aimed to measure θ₁₃ by tracking how many reactor antineutrinos disappear over short baselines, where the oscillation driven by θ₁₃ reaches its first dip.

Method. Eight gadolinium-doped liquid-scintillator detectors were deployed in near and far underground halls surrounding six powerful reactor cores. By comparing the antineutrino rate and energy spectrum measured at about 360 m with those at roughly 1.6 km, the collaboration cancelled most systematic uncertainties about the reactors themselves. The far detectors should see fewer antineutrinos than a simple extrapolation predicts, and the size of that deficit fixes the angle. You can explore the physics with the oscillation calculator.

Results. In 2012 Daya Bay reported a clearly non-zero θ₁₃ at 5.2σ, with sin²2θ₁₃ near 0.092 — a comfortably large value, soon confirmed by the RENO experiment in South Korea. The measurement was unusually clean for a parameter many had feared would be vanishingly small.

Significance. A sizeable θ₁₃ opened the experimental hunt for leptonic CP violation and shaped the design of the next generation of experiments — DUNE, Hyper-Kamiokande, and JUNO — which now pursue the neutrino mass ordering and the CP-violating phase.