Improved Measurement of Electron Antineutrino Disappearance at Daya Bay   (Proceeding to NuFact12)
  The Daya Bay experiment was designed to be the largest and the deepest underground among the many current-generation reactor antineutrino experiments. With functionally identical detectors deployed at multiple baselines, the experiment aims to achieve the most precise measurement of $\sin^2 2\theta_{13}$. The antineutrino rates measured in the two near experimental halls are used to predict the rate at the far experimental hall (average distance of 1648 m from the reactors), assuming there is no neutrino oscillation. The ratio of the measured over the predicted far-hall antineutrino rate is then used to constrain the $\sin^2 2\theta_{13}$. The relative systematic uncertainty on this ratio is expected to be 0.2$\sim$0.4%. In this talk, we present an improved measurement of the electron antineutrino disappearance at Daya Bay. With data of 139 days, the deficit of the antineutrino rate in the far experimental hall was measured to be 0.056 $\pm$ 0.007 (stat.) $\pm$ 0.003 (sys.). In the standard three-neutrino framework, the $\sin^2 2 \theta_{13}$ was determined to be 0.089 $\pm$ 0.011 at the 68% confidence level in a rate-only analysis. 
Xin Qian (on behalf of the Daya Bay Collaboration)
hep-ex nucl-ex