Improved Measurement of Electron-antineutrino Disappearance at Daya Bay

With 2.5× the previously reported exposure, the Daya Bay experiment has improved the measurement of the neutrino mixing parameter sin^2 2θ_(13) = 0.089 ± 0.010(stat) ± 0.005(syst). Reactor anti-neutrinos were produced by six 2.9 GW_(th) commercial power reactors, and measured by six 20-ton target-mass detectors of identical design. A total of 234,217 anti-neutrino candidates were detected in 127 days of exposure. An anti-neutrino rate of 0.944±0.007(stat)±0.003(syst) was measured by three detectors at a flux-weighted average distance of 1648 m from the reactors, relative to two detectors at 470 m and one detector at 576 m. Detector design and depth underground limited the background to 5 ± 0.3% (far detectors) and 2 ± 0.2% (near detectors) of the candidate signals. The improved precision confirms the initial measurement of reactor anti-neutrino disappearance, and continues to be the most precise measurement of θ_(13).