Reactor On-Off Antineutrino Measurement with KamLAND (1303.4667v2)

Abstract: The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor $\bar{nu}{e}$ flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor $\bar{nu}{e}$ oscillation analysis. The data set also has improved sensitivity for other $\bar{nu}{e}$ signals, in particular $\bar{nu}{e}$'s produced in $\beta$-decays from $^{238}$U and $^{232}$Th within the Earth's interior, whose energy spectrum overlaps with that of reactor $\bar{nu}{e}$'s. Including constraints on $\theta{13}$ from accelerator and short-baseline reactor neutrino experiments, a combined three-flavor analysis of solar and KamLAND data gives fit values for the oscillation parameters of $tan^{2} \theta_{12} = 0.436^{+0.029}_{-0.025}$, $\Delta m^{2}_{21} = 7.53^{+0.18}_{-0.18} \times 10^{-5} {eV}^{2}$, and $sin^{2} \theta_{13} = 0.023^{+0.002}_{-0.002}$. Assuming a chondritic Th/U mass ratio, we obtain $116^{+28}_{-27}$ $\bar{nu}{e}$ events from $^{238}$U and $^{232}$Th, corresponding to a geo $\bar{nu}{e}$ flux of $3.4^{+0.8}_{-0.8} \times 10^{6} {cm^{-2}s^{-1}}$ at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo $\bar{nu}_{e}$ rate.

• The paper details reactor on-off measurements at KamLAND that enhance the detection of geoneutrinos and refine neutrino oscillation parameters.
• The analysis utilizes a three-flavor neutrino oscillation framework, yielding precise estimates of tan²θ₁₂, Δm²₂₁, and sin²θ₁₃ over 2991 live days.
• The study provides updated geoneutrino flux measurements from U and Th decay, offering insights into Earth's radiogenic heat production and mantle convection.

Reactor On-Off Antineutrino Measurement with KamLAND: A Detailed Analysis

This paper presents an in-depth analysis of antineutrino measurements from the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND), with a focus on the period following the long-term shutdown of Japanese nuclear reactors. KamLAND's unique position allows for a detailed paper of electron antineutrinos ($\overline{\nu}_{e}$) from both reactor and geophysical sources. The analysis provides significant insights into neutrino oscillation parameters and the Earth's radiogenic heat production.

Key Findings and Results

Antineutrino Detection and Analysis: KamLAND utilizes inverse beta decay to detect $\overline{\nu}_{e}$, focusing on the resulting coincidence between the prompt positron and delayed neutron capture signals. The paper reports data collected over 2991 days of live time, which is subdivided into three periods based on operational changes. The reduction of reactor-generated $\overline{\nu}_{e}$ due to reactor shutdown provides a unique opportunity to enhance the sensitivity of detecting $\overline{\nu}_{e}$ from other sources, particularly geoneutrinos.

Neutrino Oscillation Parameters: The experimental data is analyzed using a three-flavor neutrino oscillation framework, integrated with constraints from other solar, accelerator, and reactor neutrino experiments. The paper yields precise measurements of the oscillation parameters, including $\tan^{2} \theta_{12}$ and $\Delta m^{2}_{21}$, with notable precision on $\theta_{13}$ due to the enhanced sensitivity from the reduced reactor backgrounds. The results are $\tan^{2} \theta_{12} = 0.436^{+0.029}_{-0.025}$, $$\Delta m^{2}_{21} = 7.53^{+0.18}_{-0.18} \times 10^{-5}$$ eV$^2$, and $\sin^{2} \theta_{13} = 0.023^{+0.002}_{-0.002}$.

Geoneutrino Flux Estimation: The paper also provides an updated measurement of the geo $\overline{\nu}_{e}$ flux resulting from the decay of $^{238}$U and $^{232}$Th within Earth's mantle and crust. The KamLAND data indicate $116^{+28}_{-27}$ events, translating to a flux of $3.4^{+0.8}_{-0.8} \times 10^{6}$ cm$^{-2}$s$^{-1}$. This measurement supports existing geological models but remains consistent with a 2$\sigma$ confidence level for differentiating among such models.

Implications and Future Directions

From a theoretical standpoint, the refined measurements enhance the understanding of neutrino oscillations, contributing to the precision testing of the three-flavor model. Practically, the results provide critical data for evaluating Earth's heat production, impacting geophysical and geological model assessments. The improved understanding of the geo $\overline{\nu}_{e}$ flux has broader implications for Earth's thermal dynamics, potentially influencing theories on mantle convection processes and the heat balance of Earth's interior.

Moving forward, the paper suggests the necessity of reducing the experimental uncertainties further to discriminate between competing Earth composition models. The deployment of larger-scale detectors with greater distance from reactors, potentially complemented by multi-site flux analyses, could yield data with improved statistical significance to refine geo $\overline{\nu}_{e}$ source contributions. Such advances could provide more stringent constraints on mantle composition and radiogenic heat generation, offering deeper insights into Earth's thermal history and dynamics.