Results from Daya Bay
Jun Cao discussed the status of the Daya Bay experiment.
Daya Bay is a reactor neutrino experiment designed to measure the oscillation parameter sin^2 2theta_13 to a precision of 0.01 at 90% confidence level. The strategy is again the one of comparing fluxes of a near and far detector, as discussed in the earlier post.
The facility includes six reactor cores at 17.4 GW total power. There are two near detector sites and one far site. These are multiple detector modules with good cosmic shielding: 250 mwe at the near site, and 860 mwe at the far site. The detector design has a lot of redundancy. A water veto shield is surrounded by multiple muon detectors to reduce veto efficiency uncertainties, and four layers of resistive plate chambers at the top. The target is 20 tons, gadolinium doped liquid scintillator.
Energy calibration is the most important issue of the analysis. They use low-intensity LEDs, to verify that PMT gains are stable to within 0.3%. They have 60CO sources at different positions in the detector, and they can calibrate the energy scale using neutron capture processes. A hit-pattern discriminator can reject spontaneous light from the photomultiplier tubes very efficiently.
To detect reactor antineutrino the inverse beta decay process is exploited: antineutrinos hit a proton and emit a positron and a neutron. The positron annihilates yielding prompt photons, and the neutron is captured in gadolinium, which produces a delayed 8 MeV photon.
Several cuts are applied to the energy of the prompt and delayed signal, a muon veto, a multiplicity cut. Accidental backgrounds are evaluated by the probability of coincidences, and checked by off-window coincidences and by the vertex distance distribution. The background from fast neutrons is evaluated by extrapolation to low energy of the high-energy tail. Finally there are backgrounds from americium and 13C sources; this is evaluated by simulations.
The sum of backgrounds in the near halls is known to 10%, on the far hall to 8%. They amount to 5% and 2% of the total yields, respectively. Having multiple equal units allows them to be very confident of their estimates: they eventually can “measure” their systematic uncertainties by comparing yields in the various units.
The result now is of 0.089 +- 0.010 +- 0.005 for sin^2 2 theta_13.