Cosmogenic neutron production at Daya Bay

Neutrons produced by cosmic ray muons are an important background for underground experiments studying neutrino oscillations, neutrinoless double beta decay, dark matter, and other rare-event signals. A measurement of the neutron yield in the three different experimental halls of the Daya Bay Reactor Neutrino Experiment at varying depth is reported. The neutron yield in Daya Bay’s liquid scintillator is measured to be Y_n=(10.26±0.86)×10^(−5), (10.22±0.87)×10^(−5), and (17.03±1.22)×10^(−5)  μ^(−1) g^(−1) cm^2 at depths of 250, 265, and 860 meters-water-equivalent. These results are compared to other measurements and the simulated neutron yield in FLUKA and GEANT4. A global fit including the Daya Bay measurements yields a power law coefficient of 0.77±0.03 for the dependence of the neutron yield on muon energy

New measurement of θ_(13) via neutron capture on hydrogen at Daya Bay

This article reports an improved independent measurement of neutrino mixing angle θ_(13) at the Daya Bay Reactor Neutrino Experiment. Electron antineutrinos were identified by inverse β-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 ^9Li 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^22θ_(13) =0.071±0.011in the three-neutrino-oscillation framework. The combination of this result with the gadolinium-capture result is also reported

Measurement of electron antineutrino oscillation based on 1230 days of operation of the Daya Bay experiment

A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of νe’s. Comparison of the νe rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors (∼1500–1950  m ) relative to detectors near the reactors (∼350–600  m ) allowed a precise measurement of νe disappearance. More than 2.5 million νe inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012–July, 2015). The νe rate observed at the far detectors relative to the near detectors showed a significant deficit, R=0.949±0.002(stat)±0.002(syst). The energy dependence of νe 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^2 2θ_(13)=0.0841±0.0027(stat)±0.0019(syst) and the effective neutrino mass-squared difference of |Δm^2_(ee)|=(2.50±0.06(stat)±0.06(syst))×10^(−3)  eV^2. Analysis using the exact three-flavor probability found Δm^2_(32)=(2.45±0.06(stat)±0.06(syst))×10^(−3)  eV^2 assuming the normal neutrino mass hierarchy and Δm^2_(32)=(−2.56±0.06(stat)±0.06(syst))×10^(−3)  eV^2 for the inverted hierarchy

Study of the wave packet treatment of neutrino oscillation at Daya Bay

The disappearance of reactor ν¯e observed by the Daya Bay experiment is examined in the framework of a model in which the neutrino is described by a wave packet with a relative intrinsic momentum dispersion σ_(rel). Three pairs of nuclear reactors and eight antineutrino detectors, each with good energy resolution, distributed among three experimental halls, supply a high-statistics sample of ν¯e acquired at nine different baselines. This provides a unique platform to test the effects which arise from the wave packet treatment of neutrino oscillation. The modified survival probability formula was used to fit Daya Bay data, providing the first experimental limits: 2.38×10^(-17) < σ_(rel) < 0.23. Treating the dimensions of the reactor cores and detectors as constraints, the limits are improved: 10^(-(14) ≲ σ_(rel) < 0.23, and an upper limit of σ_ (rel) < 0.20 (which corresponds to σ_x ≳ 10^(-11) cm) is obtained. All limits correspond to a 95% C.L. Furthermore, the effect due to the wave packet nature of neutrino oscillation is found to be insignificant for reactor antineutrinos detected by the Daya Bay experiment thus ensuring an unbiased measurement of the oscillation parameters sin^2 2θ_(13) and Δm^2_(32) within the plane wave model

New Measurement of Antineutrino Oscillation with the Full Detector Configuration at Daya Bay

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 6.9×10^5 GW_(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 2 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 2^2θ_(13) and |Δm^2_(ee)| were halved as a result of these improvements. An analysis of the relative antineutrino rates and energy spectra between detectors gave sin^2 2θ_(13)=0.084±0.005 and |Δm^2_(ee)|=(2.42±0.11)×10^(−3) eV^2 in the three-neutrino framework