Observation of Reactor Electron Antineutrino Disappearance in the RENO Experiment (1204.0626v2)

Abstract: The RENO experiment has observed the disappearance of reactor electron antineutrinos, consistent with neutrino oscillations, with a significance of 4.9 standard deviations. Antineutrinos from six 2.8 GW${th}$ reactors at the Yonggwang Nuclear Power Plant in Korea, are detected by two identical detectors located at 294 m and 1383 m, respectively, from the reactor array center. In the 229 day data-taking period between 11 August 2011 and 26 March 2012, the far (near) detector observed 17102 (154088) electron antineutrino candidate events with a background fraction of 5.5% (2.7%). The ratio of observed to expected numbers of antineutrinos in the far detector is $0.920 \pm 0.009({\rm stat.}) \pm 0.014({\rm syst.})$. From this deficit, we determine $\sin^2 2 \theta{13} = 0.113 \pm 0.013({\rm stat.}) \pm 0.019({\rm syst.})$ based on a rate-only analysis.

• The paper reports a precise measurement of reactor electron antineutrino disappearance with sin²2θ13 measured as 0.113 ± 0.013 (stat.) ± 0.019 (syst.).
• The experiment employed dual detectors positioned at 294 m and 1383 m using inverse beta decay in gadolinium-doped scintillators to capture antineutrino events.
• The results, which reject no oscillation at 4.9σ significance, enhance our understanding of neutrino oscillations and inform future reactor neutrino experiments.

Observation of Reactor Electron Antineutrino Disappearance in the RENO Experiment

The reported work details a high-precision measurement of reactor electron antineutrino disappearance conducted by the RENO (Reactor Experiment for Neutrino Oscillation) collaboration. Utilizing six reactor cores at the Yonggwang Nuclear Power Plant in Korea, the experiment observed electron antineutrinos over a 229-day data collection period. This data was instrumental in exploring the characteristics of neutrinos, specifically the mixing angle denoted as $\theta_{13}$ within the framework of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix.

Experimental Setup and Observations

The experimental design includes two identical detectors situated at different distances—294 meters and 1383 meters—from the reactor array center. These detectors operate based on the principle of inverse beta decay (IBD) in a liquid scintillator medium enriched with gadolinium. The detected events were then scrutinized by robust statistical methodologies to discern their energy spectra and temporal coincidences essential for identifying antineutrino events.

The RENO collaboration reported a significant discrepancy in the observed and expected rates of antineutrinos at the far detector, attributable to neutrino oscillation effects. Quantitatively, the ratio of the observed number of antineutrino events to the expected number was $$0.920 \pm 0.009 (\text{stat.}) \pm 0.014 (\text{syst.})$$. By employing a rate-only analysis, the collaboration proceeded to measure $$\sin^2 2\theta_{13} = 0.113 \pm 0.013 (\text{stat.}) \pm 0.019 (\text{syst.})$$.

Analysis and Uncertainties

The measurement involved significant statistical and systematic error mitigation strategies. Among these, uncorrelated systematic uncertainties related to reactor cores' thermal power output, fission fractions, and the reactor antineutrino detection cross-section were assessed. The experiment also accounted for correlated uncertainties, including antineutrino flux models and calibration of the detector response.

In total, both the systematic and statistical tests negated the possibility of no oscillation at a significance level of 4.9 standard deviations, thereby empirically establishing the non-zero values of $\theta_{13}$ and corroborating recent findings from the Daya Bay and Double Chooz experiments.

Implications and Future Directions

The precision achieved in measuring $\theta_{13}$ enhances the current understanding of fundamental neutrino properties. It further informs models of leptonic unitarity, prompting theorists to revisit existing paradigms of neutrino mass and mixing. Practically, the findings of this paper will be vital for experiments aiming to resolve the neutrino mass hierarchy, as precise knowledge of $\theta_{13}$ is crucial for interpreting neutrino oscillation data in broader contexts, such as those involving atmospheric and solar neutrinos.

Future research directives will likely focus on refining the uncertainties associated with reactor models and improving detector technologies to further decrease the statistical margin of error. Ongoing collaborations may foster a synergistic approach among different experimental modalities to establish a more nuanced understanding of neutrino oscillations, and their role within the Standard Model framework and beyond into potential new physics.