C. Buck: The Double Chooz Experiment
Reactors are a strong and pure source of MeV electron antineutrinos. We may classify using the baseline the experiments: short baseline gave the “reactor anomaly”; at the scale of 10 and more km, there are the “solar” parameters measured by KamLAND. In between double chooz, Daya Bay, and Reno are sensitive to theta_13.
Double Chooz has a inner detector with an acrylic vessel containing 8.3 tons of gadolinium scintillator, externally there is a photon catcher with liquid scintillator, and externally a buffer with 80 tons of oil looked at by photomultiplier tubes. Several calibration systems complement the detector, including radioactive sources, as well as LED light injection at multiple wavelengths.
Data taking started in 2011. Recently they improved the energy scale calibration, now they have a total uncertainty of 0.74%. There have also been many updates for the neutrino signal selection, reducing backgrounds.
The rate of neutrino candidates per day is in the 50 events range when both reactors are on. In total Double Chooz collected 17351 candidates. The uncertainty in detection efficiency is determined using a californium source. The total uncertainty is of 0.6%, dominated by the gadolinium fraction.
The background can of course be checked when both reactors are off. In total, the uncertainties relative to signal predictions come mainly from the flux (1.7%) and statistics (0.8%). They finally measure a sin^2 2 theta_13 =0.09+-0.03 using the data shown in the figure below.