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Register a new userA new measurement of antineutrino oscillation with the full detector configuration at Daya Bay
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We report a new measurement of electron antineutrino disappearance using the fully-constructed Daya Bay Reactor Neutrino Experiment. The final two of eight antineutrino detectors were installed in the summer of 2012. Including the 404 days of data collected from October 2012 to November 2013 resulted in a total exposure of 6.9$times$10$^5$ GW$_{rm th}$-ton-days, a 3.6 times increase over our previous results. Improvements in energy calibration limited variations between detectors to 0.2%. Removal of six $^{241}$Am-$^{13}$C radioactive calibration sources reduced the background by a factor of two for the detectors in the experimental hall furthest from the reactors. Direct prediction of the antineutrino signal in the far detectors based on the measurements in the near detectors explicitly minimized the dependence of the measurement on models of reactor antineutrino emission. The uncertainties in our estimates of $sin^{2}2theta_{13}$ and $|Delta m^2_{ee}|$ were halved as a result of these improvements. Analysis of the relative antineutrino rates and energy spectra between detectors gave $sin^{2}2theta_{13} = 0.084pm0.005$ and $|Delta m^{2}_{ee}|= (2.42pm0.11) times 10^{-3}$ eV$^2$ in the three-neutrino framework.
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This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9~GW$_{th}$ nuclear reactors with six detectors deployed in two near (effective baselines 512~m and 561~m) and one far (1,579~m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296,721 and 41,589 inverse beta decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is (1.55 $pm$ 0.04) $times$ 10$^{-18}$~cm$^2$/GW/day or (5.92 $pm$ 0.14) $times$ 10$^{-43}$~cm$^2$/fission. This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is $0.946pm0.022$ ($0.991pm0.023$) relative to the flux predicted with the Huber+Mueller (ILL+Vogel) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than 2$sigma$ over the full energy range with a local significance of up to $sim$4$sigma$ between 4-6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.
A new measurement of the reactor antineutrino flux and energy spectrum by the Daya Bay reactor neutrino experiment is reported. The antineutrinos were generated by six 2.9~GW$_{mathrm{th}}$ nuclear reactors and detected by eight antineutrino detectors deployed in two near (560~m and 600~m flux-weighted baselines) and one far (1640~m flux-weighted baseline) underground experimental halls. With 621 days of data, more than 1.2 million inverse beta decay (IBD) candidates were detected. The IBD yield in the eight detectors was measured, and the ratio of measured to predicted flux was found to be $0.946pm0.020$ ($0.992pm0.021$) for the Huber+Mueller (ILL+Vogel) model. A 2.9~$sigma$ deviation was found in the measured IBD positron energy spectrum compared to the predictions. In particular, an excess of events in the region of 4-6~MeV was found in the measured spectrum, with a local significance of 4.4~$sigma$. A reactor antineutrino spectrum weighted by the IBD cross section is extracted for model-independent predictions.
A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GW$_{rm th}$ nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of $overline{ u}_{e}$s. Comparison of the $overline{ u}_{e}$ rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors ($sim$1500-1950 m) relative to detectors near the reactors ($sim$350-600 m) allowed a precise measurement of $overline{ u}_{e}$ disappearance. More than 2.5 million $overline{ u}_{e}$ inverse beta decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (Dec. 2011--Jul. 2012) with a subsequent 1013 days using the complete configuration of eight detectors (Oct. 2012--Jul. 2015). The $overline{ u}_{e}$ rate observed at the far detectors relative to the near detectors showed a significant deficit, $R=0.949 pm 0.002(mathrm{stat.}) pm 0.002(mathrm{syst.})$. The energy dependence of $overline{ u}_{e}$ disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle $sin^22theta_{13}=0.0841 pm 0.0027(mathrm{stat.}) pm 0.0019(mathrm{syst.})$ and the effective neutrino mass-squared difference of $left|{Delta}m^2_{mathrm{ee}}right|=(2.50 pm 0.06(mathrm{stat.}) pm 0.06(mathrm{syst.})) times 10^{-3} {rm eV}^2$. Analysis using the exact three-flavor probability found ${Delta}m^2_{32}=(2.45 pm 0.06(mathrm{stat.}) pm 0.06(mathrm{syst.})) times 10^{-3} {rm eV}^2$ assuming the normal neutrino mass hierarchy and ${Delta}m^2_{32}=(-2.56 pm 0.06(mathrm{stat.}) pm 0.06(mathrm{syst.})) times 10^{-3} {rm eV}^2$ for the inverted hierarchy.
A measurement of the energy dependence of antineutrino disappearance at the Daya Bay Reactor Neutrino Experiment is reported. Electron antineutrinos ($overline{ u}_{e}$) from six $2.9$ GW$_{rm th}$ reactors were detected with six detectors deployed in two near (effective baselines 512 m and 561 m) and one far (1579 m) underground experimental halls. Using 217 days of data, 41589 (203809 and 92912) antineutrino candidates were detected in the far hall (near halls). An improved measurement of the oscillation amplitude $sin^{2}2theta_{13} = 0.090^{+0.008}_{-0.009} $ and the first direct measurement of the $overline{ u}_{e}$ mass-squared difference $|Delta m^{2}_{ee}|= (2.59_{-0.20}^{+0.19}) times 10^{-3} {rm eV}^2 $ is obtained using the observed $overline{ u}_{e}$ rates and energy spectra in a three-neutrino framework. This value of $|Delta m^{2}_{ee}|$ is consistent with $|Delta m^{2}_{mumu}|$ measured by muon neutrino disappearance, supporting the three-flavor oscillation model.
The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GW$_{textrm{th}}$ reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective $^{239}$Pu fission fractions, $F_{239}$, from 0.25 to 0.35, Daya Bay measures an average IBD yield, $bar{sigma}_f$, of $(5.90 pm 0.13) times 10^{-43}$ cm$^2$/fission and a fuel-dependent variation in the IBD yield, $dsigma_f/dF_{239}$, of $(-1.86 pm 0.18) times 10^{-43}$ cm$^2$/fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the $^{239}$Pu fission fraction at 10 standard deviations. The variation in IBD yield was found to be energy-dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1$sigma$. This discrepancy indicates that an overall deficit in measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes $^{235}$U, $^{239}$Pu, $^{238}$U, and $^{241}$Pu. Based on measured IBD yield variations, yields of $(6.17 pm 0.17)$ and $(4.27 pm 0.26) times 10^{-43}$ cm$^2$/fission have been determined for the two dominant fission parent isotopes $^{235}$U and $^{239}$Pu. A 7.8% discrepancy between the observed and predicted $^{235}$U yield suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
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