Research: Double Chooz

The Search for the Third Neutrino Mixing Angle

Figure 1: The positions of the two detectors at the power plant.

The Double-Chooz experiment is investigating the remarkable property of neutrinos to convert from one type (flavour) into another. This process is called neutrino oscillations. The experiment is performed in the framework of an international collaboration at the nuclear power reactors at Chooz in northern France. At a nuclear power reactor a large number of about 10²² antineutrinos per GW thermal power are produced in nuclear processes. In order to determine the conversion probability, two practically identical detectors are constructed at different distances (0.4 km and 1.05 km) to the reactor. Neutrinos very rarely react with matter and their conversion probability is low. Therefore, it takes several years of measurements to detect a sufficient number of neutrinos to determine this probability with high accuracy.

Neutrinos

Neutrinos are elementary particles which occur as three different flavours: electron neutrino, muon neutrino and tau neutrino. For each neutrino, there is also an antineutrino. Large experimental progress has been achieved in recent years: It is known that neutrinos can convert from one flavour into another. To describe the probabilities of these conversions, besides others, the so-called mixing angles (named θ₁₂, θ₂₃, θ₁₃) are required. Two of the three mixing angles (θ₁₂, θ₂₃) were already determined experimentally in the past. The third angle (θ₁₃) is determined by the Double Chooz experiment. This angle parameterizes the admixture of the heaviest neutrino mass eigenstate to the electron neutrino and leads to an energy and baseline dependent oscillation of the electron anti-neutrino flux.

The smallness of θ₁₃ in comparison with the large values of the other two mixing angles θ₁₂ and θ₂₃ remains enigmatic. Measuring the unknown angle θ₁₃ is crucial to understand the mass and mixing pattern of leptons. Therefore, the result of Double Chooz is an important step towards a comprehensive understanding of neutrinos and the underlying theoretical models. Double Chooz is also a key experiment for future experiments with neutrino beams searching for the leptonic CP violation, as the sensitivity of those experiments strongly depend on the value of sin² (2θ₁₃).

Concept of the Double-Chooz Experiment

Figure 2: Survival probability (= 1 minus conversion probability) of electron antineutrinos assuming that the value of the sought-after mixing angle is 10 degrees.

The radioactive decay of fission products in a nuclear reactor produces electron antineutrinos as a by-product, escaping in all directions. One of two detectors is placed relatively close (at 0.4 km) to the reactor. On their way to this near detector, the anti-neutrinos have not yet the possibility to convert into another flavour. In contrast, the second detector is placed at a larger distance (at 1.05 km), where conversions become more probable. The detectors are able to measure exclusively the rate and the energy spectra of the electron antineutrinos produced in the reactor. If less neutrinos than expected due to the distance-induced dilution are observed by the far detector, it can be assumed that a part of the electron antineutrinos have converted into another flavour. The oscillation amplitude sin²(2θ₁₃) can then be derived from the event rate and energy spectrum of the far detector relative to the near detector after correcting for 1/d² (d: distance source – detector). In this way, uncertainties related to the reactor neutrino flux and energy spectra, as well as for detection efficiencies are minimized.

Principle of the Detector

Figure 3: Schematics of the design of the Double-Chooz detectors.

The detector consists of several parts, which fulfill special requirements. In the innermost of the detector, the neutrino reactions will be detected using a liquid scintillator. A neutrino hits a proton generating a neutron and a positron (the antiparticle of the electron).

Both reaction products are recorded in order to obtain a clearly defined neutrino signal: The positron annihilates together with an electron from the surroundings thereby producing two highly energetic photons. To capture the neutron, the scintillator contains gadolinium (Gd). Highly energetic photons are also emitted in this capture process, but somewhat delayed with respect to the positron signal. In the liquid scintillator contained in the two inner compartments of the detector, the energy of the photons is successively converted to finally obtain visible light. This light is detected by photomultiplier tubes. Photomultipliers are devices which can convert single photons in the visible region into an electrical signal.

The outer part of the detector serves as a shielding against radioactive radiation from the environment. The fourth compartment is used to suppress the background. Particularly cosmic muons, which could interfere with the measurement, are recorded in this part of the detector.

Results

The Double Chooz far detector was filled by the end of 2010 and first results with one detector only were published in November 2011. For the first time a reactor neutrino experiment presented an indication of electron antineutrino disappearance at short baselines consistent with neutrino oscillations and a non-vanishing value for the neutrino mixing angle θ₁₃. With the start of the second detector in early 2015 a sizeable improvement in terms of systematics and statistics could be achieved. The updated analysis can exclude a zero value for sin²(2θ₁₃) and the no-oscillation hypothesis with Double Chooz data at a probability of more than 5 standard deviations. In the analysis the neutrino rate as well as the shape information of the measured neutrino energy spectrum are included.

Figure 4: Ratio of measured prompt energy spectrum (data points) between far and near detector. The blue dashed line shows the no-oscillation scenario while the red line indicates the best-fit model when allowing for neutrino oscillations.

Several analysis cross-checks were done. The mixing parameter was also determined using a sample of neutrino interactions in which the neutron produced in the inverse beta-decay is captured on H atoms and Gd atoms in the scintillator. Although the background rate is higher in this analysis it has the advantage of adding a factor two higher statistics. The result that was found is consistent with the one of the Gd-sample. The best fit value of sin²(2θ₁₃) = 0.119+-0.016 is by 2.2 standard deviations higher than the results of other reactor antineutrino experiments in China (Daya Bay) and Korea (RENO).

Figure 5: Combined fit result of the reactor rate modulation analysis.

Moreover, an independent analysis based on rate-only information was performed. Here the observed and expected (in absence of oscillation) neutrino candidate rates are compared for different reactor power conditions, e.g. with both reactors on, one reactor off, both reactors off or intermediate configurations when the reactors are not running at full power. Plotting the observed candidate rate versus the expected rate for these different reactor power conditions allows to fit the data points with a linear model parametrized by θ₁₃ and the background rate. The advantage of this approach is that no a priori assumptions on the background need to be made, since the analysis is independent of background models. A combined fit for H and Gd data samples was performed giving a result of sin²(2θ₁₃) = 0.099+-0.021 which is in very good agreement with our rate and shape result. This proves the robustness of these antineutrino analyses in Double Chooz.

The DC systematic error is dominated by the number of protons inside the scintillator. A dedicated campaign for improved proton number estimation is foreseen during DC decommissioning in 2018.

Double Chooz and the MPIK

The MPIK is one of the initiators of the experiment. Feasibility studies were performed, a novel gadolinium-loaded liquid scintillator was developed and produced, and the unloaded scintillator was optimized. Further, a large part of the photomultipliers was tested and characterized at the MPIK and prepared for their integration into the experiment. The institute is also strongly involved in Monte Carlo simulation studies and in the data analysis.

The liquid scintillators consist of organic compounds. The main difficulty in the production of a gadolinium-containing scintillator is to find a gadolinium compound, which is soluble in an organic liquid. This can be achieved by a complex compound with special beta-diketones. Their synthesis and purification procedures were developed at the MPIK. In addition, a number of requirements must be fulfilled by the scintillator: stability for several years, compatibility with the surrounding materials, radiochemical purity, and suitable optical properties. The scintillator must produce as much as possible visible light per event. At the same time, it must be as transparent as possible for this light in order that most of the light arrives at the photomultipliers. These and further optical properties were analyzed and optimized by adjusting the chemical composition of the scintillator accordingly.

Figure 6: Tanks for the liquid scintillator and crystals of the gadolinium complex.

Figure 7: Test rig for 30 photomultipliers in the light-protected Faraday cage.

The light emitted by the scintillator is recorded by 400 photomultipliers in each detector. An accurate calibration of the photomultipliers is a prerequisite for an exact reconstruction of the energy of a neutrino event. For this purpose, at the MPIK in close cooperation with Japanese scientists, every single of the 800 photomultipliers was characterized with respect to its features like, for example, quantum efficiency, quality of the signal and time resolution. For an efficient testing of this high number of photomultipliers, a test rig in a Faraday cage was set up. In this lightprotected chamber, 30 photomultipliers were arranged in a matrix and thus be tested simultaneously in a comparative manner. Several pulsed light sources like lasers and LEDs, and a data acquisition system were installed inside the chamber.

On the simulation side the focus was on the optical modelling of the scintillation light production and propagation in our detector. The Geant4 based detector simulation includes scintillator parameters which are specific for our liquids and were obtained by dedicated measurements in our laboratories. The main input parameters are light yields, attenuation lengths, emission spectra, quenching factors, reemission probabilities, refractive indices and quenching parameters. In a final stage these parameters need to be tuned in the simulation to match detector data, in particular the data taken with calibration sources (neutron and gamma sources at various energies are used).

Concerning the analysis of detector data the key aspects of our activities are related with the understanding of the energy scale and detector efficiency studies. In particular, we analyze the position dependence of the detector response, energy non-linearity effects and the stability of the energy scale. Furthermore, edge effects related to the Gd-capture efficiency close to the acrylic vessels and efficiency studies in general using calibration data from neutron sources are investigated. Finally, we combine the results of the Double Chooz experiment with those of other experients in a global analysis.

Further information can be found on the official Double Chooz web page.

References

[1] Double Chooz Collaboration, Phys.Rev.Lett.108, 131801 (2012).
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Contact:

• Prof. Dr. Manfred Lindner :
  Tel: +49 (0) 6221 516800
  E-Mail: manfred.lindner [at] mpi-hd.mpg.de
• Dr. Christian Buck
  Tel: +49 (0) 06221 516664
  E-Mail: christian.buck [at] mpi-hd.mpg.de