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Register a new userA Decade of Discoveries by the Daya Bay Reactor Neutrino Experiment
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David E. Jaffe
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With the end of Daya Bay experimental operations in December 2020, I review the history, discoveries, measurements and impact of the Daya Bay reactor neutrino experiment in China.
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This presentation describes a measurement of the neutrino mixing parameter, sin^2(2theta_13), from the Daya Bay Reactor Neutrino Experiment. Disappearance of electron antineutrinos at a distance of ~2 km from a set of six reactors, where the reactor flux is constrained by near detectors, has been clearly observed. The result, based on the ratio of observed to expected rate of antineutrinos, using 139 days of data taken between December 24, 2011 and May 11, 2012, is sin^2(2theta_13) = 0.089 +/- 0.010(stat.) +/- 0.005(syst.). Improvements in sensitivity from inclusion of additional data, spectral analysis, and improved calibration are expected in the future.
The last unknown neutrino mixing angle $theta_{13}$ is one of the fundamental parameters of nature; it is also a crucial parameter for determining the sensitivity of future long-baseline experiments aimed to study CP violation in the neutrino sector. Daya Bay is a reactor neutrino oscillation experiment designed to achieve a sensitivity on the value of $sin^2(2theta_{13})$ to better than 0.01 at 90% CL. The experiment consists of multiple identical detectors placed underground at different baselines to minimize systematic errors and suppress cosmogenic backgrounds. With the baseline design, the expected anti-neutrino signal at the far site is about 360 events per day and at each of the near sites is about 1500 events per day. An overview and current status of the experiment will be presented.
This work reports a precise measurement of the reactor antineutrino flux using 2.2 million inverse beta decay (IBD) events collected with the Daya Bay near detectors in 1230 days. The dominant uncertainty on the neutron detection efficiency is reduced by 56% with respect to the previous measurement through a comprehensive neutron calibration and detailed data and simulation analysis. The new average IBD yield is determined to be $(5.91pm0.09)times10^{-43}~rm{cm}^2/rm{fission}$ with total uncertainty improved by 29%. The corresponding mean fission fractions from the four main fission isotopes $^{235}$U, $^{238}$U, $^{239}$Pu, and $^{241}$Pu are 0.564, 0.076, 0.304, and 0.056, respectively. The ratio of measured to predicted antineutrino yield is found to be $0.952pm0.014pm0.023$ ($1.001pm0.015pm0.027$) for the Huber-Mueller (ILL-Vogel) model, where the first and second uncertainty are experimental and theoretical model uncertainty, respectively. This measurement confirms the discrepancy between the world average of reactor antineutrino flux and the Huber-Mueller model.
The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery of $bar{ u}_e$ oscillations over km-baselines. Subsequent data has provided the worlds most precise measurement of $rm{sin}^22theta_{13}$ and the effective mass splitting $Delta m_{ee}^2$. The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the worlds most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes (PMTs), the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking.
The Daya Bay Reactor Neutrino Experiment has measured the last unknown neutrino mixing angle, {theta}13, to be non-zero at the 7.7{sigma} level. This is the most precise measurement to {theta}13 to date. To further enhance the understanding of the response of the antineutrino detectors (ADs), a detailed calibration of an AD with the Manual Calibration System (MCS) was undertaken during the summer 2012 shutdown. The MCS is capable of placing a radioactive source with a positional accuracy of 25 mm in R direction, 20 mm in Z axis and 0.5{deg} in {Phi} direction. A detailed description of the MCS is presented followed by a summary of its performance in the AD calibration run.
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