Since the first detection of the neutrino in 1956 at the Savannah River power plant (USA), nuclear reactors keep playing a central role in the understanding of the fundamental properties of the neutrinos. Neutrinos are electrically neutral elementary particles which can be of three different flavors, each of them being associated to the electron, muon and tau particles. Neutrinos can spontaneously transition from one flavor to another. This phenomenon, called neutrino oscillations, questions the validity of the standard model of particle physics and call for new physics. Because neutrinos are weakly interacting particles, detectors larger than the ton scale are often necessary to study them. This PhD thesis focuses on the development of a new technique for the detection of reactor antineutrinos, using the coherent elastic neutrino-nucleus scattering (CEvNS) process. Depending on the target nucleus, the CEvNS cross-section can be up to a factor 1000 larger than those of the other neutrino detection channels (inverse beta decay, neutrino-electron scattering), making it an attractive process to reduce the size of neutrino detectors and to perform precision neutrino physics. CEvNS is a promising process offering the possibility to carry out a rich experimental program, ranging from the study of the fundamental properties of the neutrino (magnetic moment, existence of additional sterile species) and the nucleus internal structure (weak charge distribution) to precision tests of the standard model of particle physics at low energies (measurement of the Weinberg angle, search for new couplings, etc). Finally, exploiting CEvNS for long range neutrino detection could also lead to useful applications, such as the detection of supernovae and the surveillance of nuclear reactors.
This PhD thesis takes place within the NUCLEUS experiment, which aims at detecting and studying CEvNS for the first time at a nuclear reactor. The CEvNS experimental signature is a very low energy nuclear recoil (< 0.1-1 keV), making conventional detection techniques ineffective to study this process. The NUCLEUS collaboration is therefore developing a new concept of cryogenic detectors achieving ultra-low energy thresholds down to 10 eV. The collaboration targets their deployment and commissioning at the Chooz nuclear power plant (France) by 2022/2023. The proposed PhD work consists in leading a detailed study and characterization of the backgrounds in this yet uncharted energy regime. This work is the main challenge to address in order to guarantee the success of the experiment, and it will lead to the first detection of CEvNS at a nuclear reactor.
In further details, the PhD student will first contribute to the development a full analysis chain for the processing of the NUCLEUS data. Background data and calibration data collected during the different stages of the experiment (cryogenic detector prototyping stage, blank assembly of the full experimental setup and integration of the full experiment at the Chooz nuclear plant) will be analyzed and compared to the predictions of a background modeling using the Geant 4 MC simulation package. The ultimate goal of this work is to quantify the background rejection performances of the NUCLEUS setup, and to end up with a detailed and comprehensive background modeling in the CEvNS region of interest (E < 1 keV). In the course of this work, a special attention will be paid to neutrons. Because they can elastically recoil on nuclei and hence perfectly mimic a CEvNS signal, neutrons are the ultimate type of background to fight against. Additionally to this work, the PhD student will also be expected to contribute to various service tasks for the collaboration: blank assembly of the NUCLEUS setup in Munich, integration and commissioning of the NUCLEUS setup at Chooz, calibration and data taking shift, etc. He/she will be hence expected to regularly travel to Chooz and Munich.
The PhD student will integrate the “low energy neutrino” group, which gather physicists from the particle physics and nuclear physics departments of the Institute of Research into the Fundamental laws of the Universe (Irfu) at CEA Paris-Saclay. Over the past decades, the team acquired a solid expertise in reactor antineutrino physics (Double Chooz, Nucifer and STEREO experiments) and in low energy antineutrino physics (CUORE, CUPID and KATRIN experiments). The PhD student will also work in an international environment, as the NUCLEUS collaboration gathers foreign partners in Germany, Austria and Italy.