Measurement of the Electron Antineutrino Oscillation with 1958 Days of Operation at Daya Bay

We report a measurement of electron antineutrino oscillation from the Daya Bay Reactor Neutrino Experiment with nearly 4 million reactor ${\overline{\nu }}_e$ inverse $\beta$ decay candidates observed over 1958 days of data collection. The installation of a flash analog-to-digital converter readout system and a special calibration campaign using different source enclosures reduce uncertainties in the absolute energy calibration to less than 0.5% for visible energies larger than 2 MeV. The uncertainty in the cosmogenic ${}^9\mathrm{Li}$ and ${}^8\mathrm{He}$ background is reduced from 45% to 30% in the near detectors. A detailed investigation of the spent nuclear fuel history improves its uncertainty from 100% to 30%. Analysis of the relative ${\overline{\nu }}_e$ rates and energy spectra among detectors yields ${\sin }^{2}2{\theta }_{13}=0.0856\pm 0.0029$ and $\mathrm{\Delta }{m}_{32}^2=({2.471}_{-0.070}^{+0.068})\times {10}^{-3}{\mathrm{eV}}^2$ assuming the normal hierarchy, and $\mathrm{\Delta }{m}_{32}^2=-({2.575}_{-0.070}^{+0.068})\times {10}^{-3}{\mathrm{eV}}^2$ assuming the inverted hierarchy.

Figure 1

Relationship between the reconstructed and true prompt energy, which is a combination of positron kinetic energy and energies of the annihilation $\gamma$ rays. The updated model and its uncertainty (red) contains improvements described in the text. The previous model [7] is shown for comparison.

Figure 2

Time from the IBD candidate to the preceding muon with $1<E_{\mu }^{\text{rec}}<1.8\mathrm{GeV}$ in EH1. Due to the high muon rate and high IBD rate, the ${}^9\mathrm{Li}$ component (red curve) cannot be accurately measured with a prompt energy cut of 3.5 MeV (top panel). Applying a higher prompt energy cut of 8 MeV reduces the IBD component (green curve), and ${}^9\mathrm{Li}$ yields can be directly determined (bottom panel). The ${}^{12}\mathrm{B}$ component is due to a coincidence of two ${}^{12}\mathrm{B}$ decays produced in the muon shower (blue curve).

Figure 3

The background-subtracted spectrum at the far site (black points) and the expectation derived from near-site measurements excluding (red line) or including (blue line) the best-fit oscillation. The bottom panel shows the ratios of data over predictions with no oscillation. The shaded area is the total uncertainty from near-site measurements and the extrapolation model. The error bars represent the statistical uncertainty of the far-site data. The inset shows the background components on a logarithmic scale. Detailed spectra data are provided as Supplemental Material [14].

Figure 4

The 68.3%, 95.5%, and 99.7% C.L. allowed regions in the $\mathrm{\Delta }{m}_{ee}^2\text{−}{\sin }^{2}2{\theta }_{13}$ plane. The one-dimensional $\mathrm{\Delta }{\chi }^2$ for ${\sin }^{2}2{\theta }_{13}$ and $\mathrm{\Delta }{m}_{ee}^2$ are shown in the top and right panels, respectively. The best-fit point and one-dimensional uncertainties are given by the black cross.