First Results of the Daya Bay Reactor Neutrino Experiment

Abstract

The neutrino mixing angle θ13 is the gateway of studying CP violation in lepton sectors and determines the trend of future neutrino experiments. The Daya Bay Reactor Neutrino Experiment aims to precisely determine θ13, with the designed sensitivity better than 0.01 in sin2 2θ13 at the 90% C.L. The experiment takes a near-far relative measurement by comparing the observed electronantineutrino rates and spectra at various baselines from the reactors. Functionally identical antineutrino detectors are deployed in water pools underground, in order to minimize the systematic errors and to suppress the cosmogenic backgrounds. The experiment started physics data taking on Dec. 24, 2011. With 55 days of data, the Daya Bay experiment observed a non-zero value of θ13 at 5.2 standard deviations. Now the Daya Bay experiment has improved the measurement of θ13 using 139 days of data. In this talk, an overview of the experiment and the improved measurement of θ13 are presented. The ratio of the observed to the expected number of antineutrinos assuming no oscillations at the far hall has been measured to be 0.944±0.007(stat.)±0.003(syst.). In a three-neutrino framework, the sin2 2θ13 is determined to be 0.089±0.010(stat.)±0.005(syst.), by an analysis of the relative anti-neutrino rates in six detectors.