Abstract
This article reports an improved independent measurement of neutrino mixing angle $\theta_{13}$ at the Daya Bay Reactor Neutrino Experiment. Electron antineutrinos were identified by inverse $\beta$-decays with the emitted neutron captured by hydrogen, yielding a data-set with principally distinct uncertainties from that with neutrons captured by gadolinium. With the final two of eight antineutrino detectors installed, this study used 621 days of data including the previously reported 217-day data set with six detectors. The dominant statistical uncertainty was reduced by 49%. Intensive studies of the cosmogenic muon-induced $^9$Li and fast neutron backgrounds and the neutron-capture energy selection efficiency, resulted in a reduction of the systematic uncertainty by 26%. The deficit in the detected number of antineutrinos at the far detectors relative to the expected number based on the near detectors yielded $\sin^22\theta_{13} = 0.071 \pm 0.011$ in the three-neutrino-oscillation framework. The combination of this result with the gadolinium-capture result is also reported.
Highlights
Precise measurements of neutrino mixing parameters are crucial to searches for CP-symmetry violation among neutral leptons and tests of neutrino oscillation theory
The previous analysis of neutron captured by hydrogen (nH) IBDs from Daya Bay [15] is improved in this article with 3.6 times the number of detected IBDs and with reduced uncertainties of backgrounds and the neutron-capture energy selection efficiency
The analysis presented in this article determines sin22θ13 by counting interactions of reactor antineutrinos in each antineutrino detectors (ADs) in the one far and two near experimental halls
Summary
Precise measurements of neutrino mixing parameters are crucial to searches for CP-symmetry violation among neutral leptons and tests of neutrino oscillation theory. The three reactor antineutrino experiments, Double Chooz [19], RENO [20], and Daya Bay [21], currently provide the most precise measurements of the mixing angle. They use gadolinium-doped liquid scintillator to identify electron antineutrinos through inverse β-decay (IBD) reactions (νe + p → n + e+) with the neutron capturing on gadolinium (nGd). The previous analysis of nH IBDs from Daya Bay [15] is improved in this article with 3.6 times the number of detected IBDs and with reduced uncertainties of backgrounds and the neutron-capture energy selection efficiency.
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