Improved measurements of the neutrino mixing angle $θ_{13}$ with the Double Chooz detector

TL;DR

This study presents an improved measurement of the neutrino mixing angle $\theta_{13}$ using the Double Chooz far detector with 467.9 live days of data. By developing a new energy reconstruction method and implementing enhanced background suppression, the authors achieve a robust $\sin^{2}2\theta_{13}$ determination via both a Reactor Rate Modulation analysis and a Rate+Shape analysis, yielding consistent results around $0.090$. A spectral distortion above 4 MeV is observed but shown to have negligible impact on the $\theta_{13}$ extraction, while reactor-off data provide a powerful cross-check and background constraint. The paper also projects significant gains for a near detector program, estimating a sensitivity of $\sigma(\sin^{2}2\theta_{13}) \approx 0.015$ in 3 years, potentially improving to 0.010 with further improvements, underscoring the experiment’s path toward precision neutrino oscillation measurements.

Abstract

The Double Chooz experiment presents improved measurements of the neutrino mixing angle $θ_{13}$ using the data collected in 467.90 live days from a detector positioned at an average distance of 1050 m from two reactor cores at the Chooz nuclear power plant. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties with respect to previous publications, whereas the efficiency of the $\barν_{e}$ signal has increased. The value of $θ_{13}$ is measured to be $\sin^{2}2θ_{13} = 0.090 ^{+0.032}_{-0.029}$ from a fit to the observed energy spectrum. Deviations from the reactor $\barν_{e}$ prediction observed above a prompt signal energy of 4 MeV and possible explanations are also reported. A consistent value of $θ_{13}$ is obtained from a fit to the observed rate as a function of the reactor power independently of the spectrum shape and background estimation, demonstrating the robustness of the $θ_{13}$ measurement despite the observed distortion.

Figures (25)

• Figure 1: Schematic view of the Double Chooz detector.
• Figure 2: Gain as a function of integrated charge for a typical readout channel. Points show the measurements and the line shows the gain function obtained from a fit with three parameters explained in the text.
• Figure 3: Uniformity correction map for the data obtained by fitting the neutron capture peak on hydrogen.
• Figure 4: Neutron capture peak on hydrogen: points show the data taken with $^{252}$Cf neutron source deployed at the center of the detector and the histogram shows the corresponding MC simulation.
• Figure 5: Ratio of the peak energy of $^{212}$Po $\alpha$ decay (blue) and neutron captures on H (red) and Gd (black), after application of the stability correction, to their nominal value as a function of time. Error bars show the statistical uncertainty of the peak energy.