The Hyper-K Underwater Electronics Assembly project

Starting in 2027, the Hyper-Kamiokande experiment in Japan will search for leptonic CP violation in long-baseline accelerator neutrino oscillations with a realistic potential of discovery within 3 to 10 years from the start of the data taking depending on the value of the CP violating phase, to be measured with a resolution better than $23^{\circ}$. The neutrino mass ordering will be determined with a significance better than four standard deviations by combining data from accelerator and atmospheric neutrinos. Beyond the physics of neutrino oscillations, Hyper-Kamiokande will achieve unprecedented sensitivities to the detection of proton decays and supernova burst and relic neutrinos and will look for other types of astrophysical neutrinos, indirect evidence of dark matter and sterile neutrinos. A key contribution is given by European institutes, in particular to the far detector underwater electronics system, that will allow to operate about 23,600 PMTs of the Hyper-Kamiokande water Cherenkov far detector, of which 20,000 20-inch PMTs of the inner detector and 3,600 3-inch PMTs of the outer detector. About 900 electronics units will be installed underwater. Each one comprises two boards for the PMT signal digitization, a data processing board, a high-voltage and a low-voltage module, all contained inside a stainless steel water tight vessel. In this Letter of Intent, we propose the SPSC to host at CERN under the program of the Neutrino Platform a new project focused on the assembly, testing and shipment to Japan of the 900 underwater electronics units. Such project is a common effort lead by the European institutes involved in Hyper-Kamiokande, that would have easy access to the experimental facilities at CERN. After a preparatory phase, the project will become fully operational at the beginning of 2025 for a duration of about 1.5 years. Space for the storage of the sub-system components, assembled units and for the test and assembly activities will be needed as well as technical expertise (mechanical engineer and technicians) and support for the shipment to Japan. The project will be fully funded by the Hyper-Kamiokande collaboration, including the technical personnel. CERN and the Neutrino Platform have been identified as the ideal framework to carry out such project. Full support has been expressed by the leaders of the Neutrino Platform.