Measurements of neutrino mixing angle θ13 by nuclear reactors

Neutrino oscillation is one of the most exciting topics in the elementary particle physics recently. Among the three mixing angles, θ13, which has been left without finite evaluation, has come to be measured by some facilities using power reactors in spite of the difficulties due to the very weak interaction of neutrinos with matter. The mixing angle is explained by the MNS (Maki, Nakagawa, Sakata) matrix. The survival provability of the reactor neutrinos along the flight path L is estimated by assuming neutrino energy and θ13. Three competing experiments have been started one after the other: (1) Double Chooz experiment using the 8.5 GWth Chooz Nuclear Power Reactor (France) with the 8.2 tons detector at 1.05 km distance by the collaboration of seven to eight countries, (2) Data Bay experiment near Hong Kong by China and US with 17.4 GWth Daya Bay Reactor with 80 tons detector at 1.6 km and (3) RENO experiment in Korea using 16.50 GWth Yong Gwang Reactor with 16 tons of detector at 1.44 km by the team of Seoul National University in the main. Their measurement principles and experimental facilities are illustrated. Experiments were started in 2011 and results were reported soon. Their results of the reactor beams and the ratio between the detected events and expected ones are shown. Numerical results of each sin22θ13 are described with systematic errors. These three results are graphically shown as the function of sin22θ13. In the same figure, accelerator results of MINOS and T2K are shown for comparison. Relation between the long L accelerator experiments and reactor ones is discussed. (S. Funahashi)