This PhD topic is about the NUCLEUS experiment, which aims to accurately measure the process of coherent elastic neutrino scattering on nuclei (CEvNS) at the Chooz nuclear power plant in the French Ardennes. Although at ~MeV energies, CEvNS is the dominant interaction process of neutrinos with matter, it has remained unobserved for a very long time because of the difficulty of measuring the weak nuclear recoils it induces. It was only 40 years after its first prediction that this process was observed for the first time with neutrinos of a few tens of MeV at the Oak Ridge laboratory accelerator facility. The first detection of CEvNS at a nuclear reactor remains to be achieved, especially because the corresponding nuclear recoils lie in an energy regime (~100 eV) which is difficult to measure with conventional detection technologies, and also because of the unfavorable background conditions nuclear power plant environments generally offer. The NUCLEUS collaboration is therefore working on the design of an innovative detection system using two cryogenic calorimeter arrays (CaWO4 & Al2O3) capable of reaching ~10 eV energy thresholds, and surrounded by a twofold system of instrumented cryogenic vetoes. This set of cryogenic detectors will be protected by an external passive shielding and by a muon veto to improve the identification and discrimination of backgrounds. With this system, NUCLEUS aims at a precise measurement of CEvNS in order to push the study of the fundamental properties of the neutrino as well as the search for beyond standard model physics towards the low energy frontier. Interestingly, CEvNS also exhibits a cross-section 10 to 1000 times larger than the usual ~MeV neutrino detection channels (inverse beta decay reaction, neutrino-electron scattering process), making it possible to miniaturize future long-range neutrino detection setups.
The experiment is currently in its initial commissioning and testing phase at the Technical University of Munich (TUM). This step will be followed in 2024 by several data acquisition runs, aiming at (i) qualifying and validating the performances of the various detectors, (ii) validating the overall background reduction strategy, and (iii) studying and mitigating the "excess", an exponential increase in the count rate of low-energy events observed in the cryogenic calorimeters, which are of unknown origin and could degrade the experiment's sensitivity to a CEvNS signal. The relocation of the experiment to the Chooz nuclear power plant will be led by our team and will take place after the summer 2024. It is in this context that the student will begin his/her PhD work, contributing to all of the integration and commissioning operations. This crucial step will require a serie of various tests and data acquisitions to set up, fine-tune and synchronize the experiment's various detection systems. She/he will focus in particular on the external cryogenic veto and on the muon veto systems, both designed and built by our team. The analysis and the exploitation of data from this on-site commissioning phase at Chooz will enable the student to get acquainted with all existing low- and high-level analysis tools for diagnosing and characterizing these detectors. One of the student's tasks will be to improve these tools, and to set up an automation chain for diagnosing and processing the large volume of daily data (~10 TB) that will be taken during the experiment's first physics run.
Extracting the CEvNS signal from data requires several preliminary studies. The first one is to characterize the energy and time response of the detectors over the data acquisition periods. The student will take charge of one of these tasks, building on the work already accomplished during the commissioning phase. This work will lead to a detailed understanding of the operation of the detectors and the identification of all the factors likely to influence their behavior. It should be noted that our team has proposed and is responsible for an innovative method for the calibration of very low energy nuclear recoils in cryogenic calorimeters, with the installation of a dedicated facility on a low-power research reactor located at the Technical University of Vienna (Austria). The student may eventually get involved in this effort, with a view to interpreting the data collected at Chooz. Building on these results, the student will then focus on the extraction and the study of a specific background component in the collected data. This work will enable the consolidation and the fine-tuning of a predictive model of the experiment's background, using a Monte Carlo simulation framework based on the Geant 4 library. Finally, the student will set up simple statistical tests to characterize the level of confidence with which a CEvNS signal can be extracted from the data after subtraction of the measured backgrounds.
Finally, the student will use the first data from the physics run at Chooz to conduct a search for new physics beyond the standard model (measurement of the weak mixing angle at low energies, search for new neutrino couplings, constraints on the electromagnetic properties of the neutrino, etc.). This work will require the implementation of advanced statistical methods for interpreting the data, in order on the one hand to understand the impact of the various sources of uncertainty on the obtained constraints, and on the other hand to guarantee the reliability of the results.