Double Chooz Experiment
The Double Chooz (DC) experiment is located at the Chooz nuclear power plant in France, having two cores yielding a total thermal power of 8.54 GWth. The DC far detector (FD) is placed at 1050 m away from the cores, close to the maximal oscillation distance and providing shielding (300 m.w.e.) against cosmic rays. A second identical detector (near detector ND) is installed 400 m away from the reactor cores, in a new laboratory (115 m.w.e.). This multi-detector setup allows to drastically reduce the systematic errors mainly associated with the neutrino flux and provide a precise value of θ13.
The DC detector system is shown in the figure below. The main bulk of the detector is made of four concentric cylindrical tanks with three central volumes optically coupled. The innermost volume (target) contains 10.3 m3 of Gd-loaded (1 g/l) liquid scintillator inside a transparent acrylic vessel, where the neutrinos interact via the IBD process. The target is surrounded by a Gd-free liquid scintillator layer, Gamma-Catcher (GC), contained in a second acrylic vessel used to detect g-rays escaping from the target. The more external volumes work as passive and active shield against background signals, like environmental gammas and muons.
DC started the data taking with the far detector in April 2011. Since then, many efforts and advances have been performed by the collaboration to perform a competitive measurement of θ13 using the FD data while the ND was under construction. The best value of θ13 measured by using only FD is sin2(2θ13) =0.090 + 0.032-0.029 from a fit to the observed energy spectrum . Since January 2015 the ND is taking data together with the FD. In September 2016, DC presented at CERN its first θ13 measurement exploiting the combination of 2 years of single-detector data and 15 months of double-detector data, the measured value for sin2(2θ13) is (0.119 ± 0.016). The big improvement on the measurement resolution is mainly due to the inclusion of the second detector, reducing the uncertainty on the reactor neutrino flux from 1.7% to 0.1%. The strong cancelation of the flux error is due to the iso-flux condition of the two detectors with respect to the reactors. Additionally, the target volume has been increased including the neutrino interaction in the GC, considering inclusively the neutron captures on Gd, and H. In that way, the target mass is more than three times larger reducing considerably the statistical error. This measurement has been done, as in previous analyses, by fitting a Monte Carlo (MC) prediction to the observed energy spectra. An unexpected spectrum distortion is observed at high energy (4-6 MeV) but there is not impact on θ13 measurement because the distortion is present in both detectors and cancel out. A strong correlation between the excess rate and the reactor power is observed, excluding the possibility to an unknown background. The isotope decay responsible of this excess is still under investigation. θ13 measurement is being improved with a new implementation in the fit of some systematic errors, like the error on the electron anti-neutrino (ve)spectrum prediction and the proton number uncertainty. A publication is expected by the end of this year.
As a crosscheck, a FD data to ND data fit is also done using the 15 months of double-detector data. In this case, the value of θ13 is measured to be sin2(2θ13) =0.123 ± 0.023. This result is not affected by the spectrum distortion and it is quite compatible with the MC to data fit. The measured value for sin2(2θ13) is larger than the most precise Daya Bay measurement (0.084 ± 0.005). This new result is in better agreement with T2K and NOvA beam measurements.
In the two left plots, black points show the ratio of the data, after background subtraction, to the non-oscillation prediction as a function of the visible energy of the prompt signal. Overlaid red line is the rate of the best-fit to the non-oscillation prediction with the reactor flux uncertainty (green) and total systematic uncertainty (orange). In the right plot, black points correspond to the ratio of FD data to the ND data, the total systematic uncertainty is shown in green.
Measured energy spectrum of the prompt signal (points) superimposed on the prediction without neutrino oscillation (red line) normalized to the same number of entries. The left plot corresponds to the 2 years of single-FD detector data, the central one to the 15 months of FD data in multi-detector mode and the right plot to the ND data for the same period. The spectra corresponding to different backgrounds are also shown.
CIEMAT contribution to Double Chooz
CIEMAT neutrino experimental physics group has been an active member of the Double Chooz collaboration since March 2006. The CIEMAT group has contributed extensively in the hardware and construction of the detector as well as in the physics data analysis.
The main contributions and responsibilities of our group in the Double Chooz detectors are:
- Design, manufacture, assembly and installation of the 800 mechanical supports for the 10" photomultipliers (PMTs) of the near and far detectors
- Design, manufacture and assembly of the 800 magnetic shields for the PMTs
- Design, manufacture and installation of 800 HV splitters and high voltage cables
- Installation of the PMTs inside the tanks of the distant and near detectors
- Study of the emission of light from the base of the PMTs
The CIEMAT neutrino group has contributed very significantly to the analysis with two detectors. We are responsible of the measurement of the remaining background due to accidental coincidences in the detector. In the new analysis, which includes the neutron captures on H, this background is the most important one in the FD and is the main background below 2 MeV for both detectors, where the oscillation is maximum. The CIEMAT group is in charge of the determination of the detection efficiency for both detectors and has participated in the calibration campaign carried out in summer 2017. A second independent measurement of θ13 has been performed by DC using a different analysis technique led by the CIEMAT group: the reactor rate modulation (RRM) analysis. The rate of neutrino candidates is measured during periods of different reactor power ranging from zero power to full power. This analysis allows the measurement of θ13 without an a priori knowledge of the background. This analysis has also been applied to the 15 months of two-detector data and is currently being used as cross-check since it is not affected by the spectrum distortion.