  With 2.5x the previously reported exposure, the Daya Bay experiment has
improved the measurement of the neutrino mixing parameter sin^2(2theta_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 theta_13.
