Observation of Energy and Baseline Dependent Reactor Antineutrino Disappearance in the RENO Experiment

Abstract: The RENO experiment has analyzed about 500 live days of data to observe an energy dependent disappearance of reactor $\overline{\nu}e$ by comparison of their prompt signal spectra measured in two identical near and far detectors. In the period between August 2011 and January 2013, the far (near) detector observed 31541 (290775) electron antineutrino candidate events with a background fraction of 4.9\% (2.8\%). The measured prompt spectra show an excess of reactor $\overline{\nu}_e$ around 5 MeV relative to the prediction from a most commonly used model. A clear energy and baseline dependent disappearance of reactor $\overline{\nu}_e$ is observed in the deficit of the observed number of $\overline{\nu}_e$. Based on the measured far-to-near ratio of prompt spectra, we obtain $\sin^2 2 \theta{13} = 0.082 \pm 0.009({\rm stat.}) \pm 0.006({\rm syst.})$ and $|\Delta m_{ee}^2| =[2.62_{-0.23}^{+0.21}({\rm stat.}) _{-0.13}^{+0.12}({\rm syst.})]\times 10^{-3}$~eV$^2$.

• The paper demonstrates energy‐ and baseline‐dependent reactor antineutrino disappearance using dual detectors at 294 m and 1383 m from reactor cores.
• It precisely measures oscillation parameters with θ13 = 0.082 ± 0.009(stat.) ± 0.006(syst.) and |Δm_ee^2| ≈ 2.62×10⁻³ eV², significantly reducing systematic uncertainties.
• The findings prompt refinements in reactor antineutrino models and provide a benchmark for further studies on CP violation and neutrino mass hierarchy.

Observation of Energy and Baseline Dependent Reactor Antineutrino Disappearance in the RENO Experiment

The paper documents the findings of the RENO (Reactor Experiment for Neutrino Oscillation) Collaboration regarding the observation of energy-dependent disappearance of reactor antineutrinos, $\overline{\nu}_e$, and the consequent precise measurement of the neutrino mixing angle $\theta_{13}$ and mass-squared difference $|\Delta m_{ee}^2|$. The analysis is based on approximately 500 days of data collected between August 2011 and January 2013 using two identical detectors situated at different baselines from a group of six nuclear reactors.

The RENO experiment identifies electron antineutrino events through inverse beta decay (IBD) interactions, utilizing a dual detector setup at 294 meters and 1383 meters from the reactor cores. This setup provides a robust mechanism for measuring the disappearance of $\overline{\nu}_e$ due to neutrino oscillations. The near detector recorded 290775 IBD candidate events, while the far detector observed 31541 IBD events.

The energy-dependent disappearance of reactor antineutrinos was evident from the excess of $\overline{\nu}_e$ observed around 5 MeV in the prompt signal spectra, which could not be entirely explained by the existing theoretical models. This discrepancy necessitates further investigation and potential refinement of the antineutrino production models in reactors.

The parameters were extracted by analyzing the spectral data in conjunction with the far-to-near event rate ratios. The extracted oscillation parameters are $$\sin^2 2\theta_{13} = 0.082 \pm 0.009(\rm stat.) \pm 0.006(\rm syst.)$$ and $$|\Delta m_{ee}^2| = [2.62_{-0.23}^{+0.21}({\rm stat.}) _{-0.13}^{+0.12}({\rm syst.})] \times 10^{-3}$$ eV$^2$. These values were achieved with significant improvements in systematic uncertainties, notably through enhanced background estimation and event selection criteria.

The implications of this research are multifold. Practically, the accurate measurement of $\theta_{13}$ substantiates opportunities for probing CP violation in the lepton sector and advancing our understanding of the mass hierarchy of neutrinos. The analysis provides a metric for comparison and integration with other major neutrino experiments such as T2K and NO$\nu$A, potentially enriching the global understanding of neutrino oscillation phenomena.

Further prospects for research include scrutinizing the 5 MeV anomaly in antineutrino spectra and reassessing reactor antineutrino yield models. The refinement of oscillation parameters strengthens theoretical frameworks and detection methodologies that could be leveraged in future neutrino experiments targeting CP violation and neutrino mass ordering.

Thus, the RENO experiment reinforces the resolution of $\theta_{13}$ while also posing intriguing questions about reactor antineutrino production and the broader impacts on particle physics research avenues.