L. Zhan: Recent Results from Daya Bay
Daya bay offers a precision measurement of the theta_13 angle, thanks to a large thermal power (6×3 GW) and taget mass (8×20 kTons). The combination of near and far detectors reduces reactor related issues.
The detector has a three-zone structure: a target with 20T scintillator gadolinium-loaded, a photon catcher with 20T scintillator, and a buffer shielding made with mineral oil. The top and bottom optical reflectors double the photon coverage. The 8 identically designed detectors reduce systematic uncertainties.
A muon veto system consists in a water cherenkov detector in two layers, plus a four-layer system of RPC modules above the pool.
The energy is calibrated to sub-percent level. The non-linearity, which can be due to scintillator response and electronics response deficits or distorsions, are modeled and fitted, then validated with beta decays from thorium and bismuth, plus the Michel electron spectrum. Also a scintillator quenching measurement using neutron beams and compton scattering electrons is used.
Data taking started in 2011. The latest analysis uses 621 days of data, plus reactor flux and spectrum measurement performed with 217 days of live time. More than 1 million antineutrino interactions have been detected!
Backgrounds are accounted carefully, and come from accidentals (calculated from uncorrelated single hits), fast neutrons (measured from RPC and outer water tagged muon events), Americium Carbonium sources are estimated with monte carlo.
The oscillation result is a sin^2 2 theta_13 = 0.084+-0.005, and a delta_m^2_ee = (2.44+-0.11) x 10^-3 (eV^2) (see figure). The theta_13 measurement is at 6% and is the most precise, as is the delta m^2 one. An absolute reactor flux measurement finds the data in good agreement with predictions from previous short baseline experiments. Yet the absolute flux shows an excess in the 4-6 MeV region, also reported by Reno (see figure below).
Daya Bay performed also a search for light sterile neutrinos, which would introduce an additional oscillation mode. It is a relative measurement at multiple baselines, ranging from 350 to 500 to 1600 meters from the sources. The region of delta_m^2(41) between 10^-3 eV^2 and 0.1 is investigated and limits are set.
In summary, 621 days of data have brought very precise measurements of the oscillation parameters. The precision on sin^2 2 theta_13 and delta_m(ee)^2 will be brought to 3% by the end of 2017.