Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay

Abstract: This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9~GW$_{th}$ nuclear reactors with six detectors deployed in two near (effective baselines 512~m and 561~m) and one far (1,579~m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296,721 and 41,589 inverse beta decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is (1.55 $\pm$ 0.04) $\times$ 10$^{-18}$~cm$^2$/GW/day or (5.92 $\pm$ 0.14) $\times$ 10$^{-43}$~cm$^2$/fission. This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is $0.946\pm0.022$ ($0.991\pm0.023$) relative to the flux predicted with the Huber+Mueller (ILL+Vogel) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than 2$\sigma$ over the full energy range with a local significance of up to $\sim$4$\sigma$ between 4-6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.

• The paper reports a precise measurement of reactor antineutrino flux using six detectors over 217 days, yielding IBD yields of (1.55 ± 0.04)×10⁻¹⁸ cm²/GW/day.
• It identifies significant deviations in the IBD positron energy spectrum, with discrepancies exceeding 2σ and reaching up to 4σ in the 4–6 MeV range.
• The findings reinforce reactor model predictions while highlighting the need for refined nuclear physics models to guide future neutrino experiments.

Overview of "Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay"

This essay provides an expert summary and analysis of the paper "Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay,” exploring the experimental measurement of reactor antineutrino flux and spectrum by the Daya Bay collaboration. The study contributes valuable insights into the ongoing examination of reactor antineutrinos within the framework of the Standard Model of particle physics.

The Daya Bay experiment has measured the electron antineutrino flux and the associated energy spectrum from a reactor complex consisting of six nuclear reactors. The measurement utilized six antineutrino detectors positioned in two near experimental halls with baselines of 512 m and 561 m, and one far hall with a baseline of 1,579 m. The data set covered 217 days and resulted in a detection of 296,721 inverse beta decay (IBD) candidates in the near halls and 41,589 candidates in the far hall. The resulting measurement of the IBD yield is reported as (1.55 ± 0.04) × 10^-18 cm²/GW/day or (5.92 ± 0.14) × 10^-43 cm²/fission. This finding is consistent with existing short-baseline reactor antineutrino experiments, reinforcing the reliability of current empirical data in alignment with predictions from fissile antineutrino models, such as those proposed by Huber and Mueller, and ILL and Vogel.

Key Findings and Analysis

1. Flux Measurement: The measured antineutrino flux corresponds well with prior experimental results, with a measured value of $0.946\pm0.022$ and $0.991\pm0.023$ relative to predictions from the Huber+Mueller and ILL+Vogel models, respectively. This congruence provides support for the reactor antineutrino models used as benchmarks and points to limited discrepancies within the current theoretical framework.
2. Spectral Deviations: A significant reveal from this study is the observed deviation in the measured IBD positron energy spectrum from the expected spectral predictions, with variances exceeding 2σ over the full energy range and reaching local significance levels up to approximately 4σ in the 4-6 MeV region. The inference from this deviation suggests potential inaccuracies in existing reactor antineutrino spectrum models or unaccounted phenomena in neutrino physics. This finding echoes concerns encapsulated in the "Reactor Antineutrino Anomaly," compelling further investigation.
3. Implications and Future Directions: The implications are twofold: Theoretically, this study underscores the necessity for refined models to predict reactor antineutrino spectra accurately and raises questions regarding the completeness of current nuclear physics models. Practically, the outcomes are crucial for advancing next-generation neutrino experiments, seeking to elucidate aspects such as neutrino mass hierarchy and mixing parameters. Future work may encompass detailed evaluations of the nuclear database for antineutrino production and advancements in detector technology to enhance precision measurements.

In conclusion, the Daya Bay experiment's measurements bolster the comprehensive understanding of reactor antineutrinos, while also indicating areas for further inquiry and refinement. The precision of these measurements offers critical insights that sustain the momentum toward elucidating the underlying principles governing neutrinos and potential new physics phenomena that extend beyond the Standard Model.