The PandaX-III collaboration proposes to determine whether the neutrino is a Majorana particle, i.e. its own antiparticle. In this purpose this international collaboration, in which the Research Institute on Fundamental laws of the Universe (IRFU) of CEA Saclay participates, aims to observe neutrinoless double-beta decays of the Xenon-136, where the emission of the two electrons is not compensated by the simultaneous emission of two anti-neutrinos. Such an observation would violate the principle of conservation of leptonic number, in opposition with the predictions of the standard model of particle physics. The search of such rare events requires an enormous quantity of Xenon-136 atoms, a deep underground laboratory protected from cosmic rays and with low radioactive levels, like the Jinping underground laboratory (CJPL, Sichuan province, China), and a very effective particle detector.

The first phase of the experiment aims to construct a first TPC module (Time Projection Chamber) of 145kg of Xenon-136, which will be followed in a second stage by four other 200kg modules. The TPC will be equipped with detectors able to measure the energy of the two beta electrons with an excellent accuracy. The first TPC module will be commissioned end of 2024. The trajectory of the two electrons emitted by the double-beta decay will be reconstructed to measure the initial energy of those electrons, and to recognize the topology of their trajectories to differentiate them from gamma backgrounds which emit only one electron. That module will be equipped with gaseous Micromegas detectors which have a good energy resolution and a very good radio-purity which limits the amount of gamma backgrounds coming from radioactive contamination.

The PandaX-III collaboration is working on the construction of the first TPC module. It will be installed at CJPL during the year 2024. Reconstruction algorithms of detector data using neural networks are being developed, in order to complete the analytical methods already implemented in the REST environment of data reconstruction and analysis, to optimize double-beta events versus gamma backgrounds discrimination, and to improve the quality of the electron energy reconstruction. These algorithms are trained and evaluated on simulated Monte-Carlo events. Data from reduced-size TPC prototype will be also used to test these algorithms in real conditions. As soon as the first module will be installed end of 2024 these algorithms will be used for detector calibrations and for being implemented in real data analysis. They will be then used to extract the first physics results on double-beta events.

The main task of the PhD student will be to contribute on the development of data reconstruction algorithms based on neural networks, in particular by taking into account the defects of the detectors (dead channels, performance inhomogeneity, gas impurities, etc...) and by implementing in REST the data correction methods needed to compensate these defects. That work will include studies of data from prototype TPC chambers, as well as Monte-Carlo simulations. Moreover, as soon as the data from the first TPC module will be available the student will participate to the data analysis and the extraction of the physics results. These studies will be presented in conferences and published in scientific journals. The student will also participate to an R&D to optimize Micromegas detectors in order to improve their energy resolution as well as their general performance in high pressure gaseous Xenon.

A Master internship of 4 to 6 months would be also possible in the IRFU/DPhN PandaX-III group before the start of the PhD thesis.