Daya Bay Reactor Neutrino Experiment

Objective

Measure the neutrino mixing angle θ₁₃ through short-baseline reactor antineutrino disappearance, using functionally identical near and far detectors to cancel flux and detection systematics.

Method

Six 20-ton gadolinium-loaded liquid-scintillator detectors deployed in three underground experimental halls (two near halls at ~400–500 m, one far hall at ~1,600 m) near six reactor cores with combined thermal power of 17.4 GW. Inverse beta decay events tagged by delayed neutron capture on gadolinium produce a sharp 8 MeV gamma cascade.

Key results

• 2012: definitive first measurement of θ₁₃ with sin²2θ₁₃ = 0.092 ± 0.017 at 5.2σ — the last major PMNS mixing angle to be established as non-zero.
• Final precision result: sin²2θ₁₃ ≈ 0.0856 ± 0.0029 (2022), the most precise measurement of this angle.
• Precise Δm²₃₁ measurement from reactor channel, complementing long-baseline accelerator determinations.
• Upper limits on eV-scale sterile neutrino mixing in the relevant parameter space.

Significance

Daya Bay's 2012 result established the last of the three mixing angles as non-zero, enabling CP-violation searches in long-baseline neutrino experiments. The near/far identical-detector design became the template for subsequent reactor-oscillation measurements including the short-baseline sterile searches and the planned long-baseline JUNO.

Site and configuration

The Daya Bay complex comprises six reactor cores at three sites in coastal Guangdong province, with combined thermal power of 17.4 GW — among the largest reactor neutrino sources in the world. The experimental halls were excavated into granite hillsides, with three underground halls placed at carefully optimized baselines:

• EH1: 362 m from two Daya Bay cores
• EH2: 500 m from the two Ling Ao cores
• EH3 (far): 1,579 m from the Daya Bay and 1,912 m from Ling Ao cores

Six antineutrino detectors were distributed among the halls, with two detectors per hall at EH1 and EH2 and four at the far hall. The near-hall placement samples the essentially-unoscillated reactor flux and is used as a reference against the far-hall deficit.

Detector design

Each antineutrino detector is a three-zone concentric structure:

• Inner 20-ton target: 0.1% gadolinium-loaded linear-alkylbenzene scintillator
• Middle gamma catcher: unloaded scintillator
• Outer buffer: mineral oil

192 8-inch photomultipliers view the inner volumes. A muon system of water Cherenkov and RPC layers vetoes cosmic-ray events.

Gadolinium loading gives the delayed-coincidence signal a sharper time and energy profile: neutron capture on ${}^{157}$Gd releases ~8 MeV of gamma energy within ~30 μs of the prompt positron signal, well above the natural radioactive background.

The six detectors were built to be identical within tight tolerances (mass, fiducialization, photomultiplier response) so that near-to-far ratios cancel systematic uncertainties in reactor flux modeling, detection efficiency, and cross-section.

Results

The first result (2012) measured ${\sin }^{2}2{\theta }_{13}=0.092\pm 0.016\pm 0.005$ at 5.2σ significance from 55 days of data.

Subsequent data releases refined the measurement with additional exposure and systematic improvements. The final 2022 result is ${\sin }^{2}2{\theta }_{13}=0.0856\pm 0.0029$ corresponding to ${\sin }^2{\theta }_{13}\approx 0.0223\pm 0.0007$. This is the best-measured of the three neutrino mixing angles.

Daya Bay also published a precision reactor antineutrino spectrum, confirming the well-known “5 MeV bump” in the energy distribution and constraining reactor-flux evolution models.

Shutdown and legacy

Daya Bay ceased operations in December 2020 after nine years of running. The infrastructure remains in place; some detector components have been repurposed for the PROSPECT-II short-baseline experiment at Oak Ridge.

The reactor-${\theta }_{13}$ measurement — along with RENO (Korea) and Double Chooz (France) — anchors the global PMNS determination. Long-baseline accelerator experiments (T2K, NOvA) use Daya Bay’s ${\theta }_{13}$ value as an external input when extracting ${\delta }_{CP}$, effectively breaking what would otherwise be a degeneracy.

Significance

Daya Bay demonstrated that reactor neutrinos, with their clean flavor content and well-characterized spectrum, can compete with accelerator experiments for high-precision oscillation measurements. JUNO inherits this tradition at a longer baseline and larger detector mass for the mass-ordering determination.