Double Chooz: Results for Theta_13
[Anatael Cabrera gave a nice presentation on the measurement of oscillations of reactor neutrinos in double Chooz. He speaks way too fast, but his talk was clear enough that I can offer the following writeup… – TD]
Using reactor neutrinos, one can use a near and a far detector to get signals of the first few oscillation periods. Chooz has reactors with a power of 8.5 GW, which means 10^21 neutrinos per second. The near detector is at 400 meters, the far one is at 1050 meters from the source. Both detect a prompt signal from the inverse beta decay, and then the neutron finds a Gadolinium atom which produces a delayed signal at 8 MeV.
The CHOOZ detector has many layers. An outer muon vito with plastic scintillator strips. Inside there is the neutrino target made of liquid scintillator doped with 0.1% Gadolinium. There is inside a “gamma catcher” in liquid scintillator, and an inner muon veto also made of liquid scintillator. Finally, an inert gamma shield reduces the radioactivity from the outside.
To well calibrate the detector, the redundancy of the device is exploited. Light sources in the detector and sources can “illuminate” the active areas. A calibration of the energy starts with the non-linear gain, then a uniformity calibration is performed. Stability is checked as it changes versus time. An absolute energy calibration is also done with sources.
The flux is known to 1.7% thanks to Bugey4 measurements. The data and MC agree well. To get their signal candidates, first of all they tag a muon for an efficient veto; then they try to identify a prompt and delayed signal. A multiplicity condition is also applied: they only want a pair of signals (prompt and delayed). The analysis using the Gadolinium signal has a large (18) signal to noise ratio. The hydrogen capture candidates occupy a wider fiducial region in the detector.
Backgrounds are essentially three: the best known is accidental, from radioactivity. Then there is one from isotopes in the active material, which cause a signal when a muon spallates. It is the worst-known spectrum. Then there is the correlated background: fast neutrons.
Of course in the reactor-off time one can study backgrounds very nicely. The oscillation analysis results are obtained in a rate and shape fit to the Gadolinium candidates. This yields a measurement of theta_13 and a background estimation. They can fit two samples, one is background-rich and allows for more control of systematics. Their result is sin^2(2 theta_13) = 0.109 +-0.040. Systematics are not the dominant source of error (0.03 stat, 0.025 syst), and the largest component of the systematic uncertainty comes from the “ugly” background from spallating isotopes correlated with a muon.
Finally, Cabrera discussed the prospects of the facility. With the near detector in three years they can shrink the measurement errors by a factor of three. This assumes that the error on the detection goes down from 1 to 0.2%, and that the background rejection can reduce the related systematics by a half. He mentioned that for theta_13 determinations reactor experiments will dominate the proceedings for a long time, mainly because of the multi-detector technology.