1 Multi - Megaton Water Cherenkov Detector for a Proton Decay

The discovery of the neutrino oscillations in 1998 revisited the interest of proton decay search [2]. The existence of the tiny neutrino mass indicates the existence of the large energy scale behind and suggests the connection between the small neutrino mass and proton decay. There are strong indications of the unification scale of O(10 16)GeV: the running coupling constants with super-symmetric particles merge at this large energy scale. Proton decay directly brings us to this unification scale. The search for proton decay has been conducted for more than 30 years and currently the most stringent limits are set mostly by the Super-Kamiokande experiment [3]. The limit for e + π 0 mode is now 5.3×10 33 years (for 84ktyr), and that for ¯ νK + mode is 1.9×10 33 years. The recent theoretical development by lattice QCD suggests that all the predictions on the life time of proton should be shortened from the previous calculations about an order of magnitudes [4] and therefore the search for proton decay may become much more realistic than before. The current interested region of the proton decay life time is in the range beyond about 10 35 years for p → e + π 0 mode and 10 29−35 yrs for p → K + ¯ ν mode [5]. In this report, we will show a detector which can be used to search for proton decay in the lifetime region beyond 10 35 years. We will briefly review the current experimental status and discuss the sensitivity of the future proton decay detectors, and we specifically present a possibility of a scalable multi-megaton water Cherenkov detector immersed in the shallow water. Most stringent limits for proton decay comes from the Super-Kamiokande experiment, the 50,000tons imaging water Cherenkov detector, located 1000m underground. For the details of the Super-Kamiokande experiment, see references [3]. We explain the current status of the search for the typical decay modes of p → e + π 0 and p → K + ¯ ν. The 1289 days of data which corresponds to 79.3kty were used for the searches. The event candidates are selected by applying the following selection criteria: 1) there are 2 or 3 Cherenkov Rings; 2) all rings are showering; 3) the reconstructed π 0 mass should be between 85 and 185 MeV/c 2 for 3 Ring events; 4) there are no electrons from muon decay; 5) proton mass …