Description du sujet de thèse

The oscillations of neutrinos were first observed only 20 years ago, when the Super-Kamiokande collaboration observed the disappearance of atmospheric neutrinos. In less than two decades, the physics of neutrino oscillations has been the fertile field for tremendous progresses and opened a new era: it would allow to measure the CP asymmetry in the leptonic sector through neutrino oscillations, and with it, to possibly explain the asymmetry between matter and antimatter that is currently observed in our Universe.

This position proposes to directly tackle this question using the T2K experiment, which is the pioneer experiment in this domain, and work on its successor, Hyper-Kamiokande. T2K is a Long-Baseline neutrino oscillation experiment located in Japan. Its main goal is the precise measurement of the (muon) electron neutrino and antineutrino (dis-)appearance. Through these measurements, T2K is measuring the ν oscillation parameters with increasing precision and is providing first indications about neutrino Mass Hierarchy (MH) and CP violation phase (δCP) in the neutrino sector. The excellent results reported by T2K have motivated a second phase of the experiment, T2K-II, that is starting in 2023 for 4 years and aims at establishing CP violation at 3σ level. Moreover, in order to reach a final (5σ) conclusion on the CP violation, the Hyper-Kamiokande experiment is now being built, and will take its very first data in 2027.

The successful candidate will work on the upgrade of the existing tool to maximize the physics output of Hyper-Kamiokande, and apply them to the existing T2K data in order to test them and provide world-leading physics results on CP violation. In particular, the LLR group has taken a significant role in the development of the Hyper-Kamiokande event reconstruction algorithm, and namely, through machine learning techniques. The successful candidate is expected to reinforce this activity, first by adapting and testing the algorithms already developed by the group for low energy neutrinos (< 10 MeV) to the high energy sector (>100 MeV) in order to search for CP violation. The candidate will work in collaboration with the physicist team, as well as the software engineers at LLR who are already collaborating on this aspect. In the second phase of the project, the candidate is expected to test and apply the algorithm on existing data, namely of the so-called Water Cherenkov Test Experiment (WCTE) and Super-Kamiokande experiments, located respectively at CERN (Switzerland) and in Japan. The candidate is expected to use these data to fine tune the algorithms, and apply them to the improve the current efficiency of the Super-Kamiokand and T2K neutrino samples. The candidate will be able to evaluate the impact on the joint analysis of the T2K accelerator and Super-Kamiokande atmospheric neutrinos. This joint fit has the potential to highly improve the current constraints on MH and CP violation compared to T2K-only to ultimately provide the world-best constraint on CP violation in the lepton sector (while preparing Hyper-Kamiokande at the same time).

Moreover, the candidate is expected to contribute to the Hyper-Kamiokande detector construction. In particular, she/he is expected to reinforce the LLR team which is responsible of the calibration of the whole Hyper-Kamiokande electronics. The candidate will help in developing the test bench locally at LLR, and then, to export it at CERN where the calibration will actually happen from end of 2024.

Contexte de travail

The neutrino group in LLR has been created in 2006 by Michel Gonin, as the first historical group in France to work on the world-leading neutrino experiments in Japan. Since then, the group has joint the unique T2K experiment, which has first discovered the neutrino appearance, as well as provided the very first hints of violation of the leptonic CP symmetry. Since 2016, the group has also joined the Super-Kamiokande experiment, and have built a strong leadership inside regarding the DSNB neutrino detection and phenomenology.
The group is composed of 5 permanent researchers, 2 postdoctoral researchers and 6 PhD students, who has unique expertize in both high energy (CP violation, mass-hierarchy issue etc.) and low energy neutrinos (Supernovae, solar or reactor neutrinos). In the context of the proposed topic, the two contact members are leading the Hyper-Kamiokande reconstruction effort, as well as the T2K oscillation analysis working groups. Moreover, the group is collaborating with the LLR software engineers as well with the ILANCE laboratory (located in Japan) in order to build a strong leadership in Machine-Learning based algorithms.

Frequent travels to Japan and CERN (working at ILANCE, collaboration meeting, data taking …) are also expected.