Hyper-K is designed to have 1 megaton size, 25 times the fiducial mass of Super-K, to be used as neutrino target and proton decay source. A letter of intent was published in Arxiv:1109.3262, and more recently a study of LBL was published in Arxiv:1502.05199. A sketch of the underground cavern hosting the proposed detector is shown below.
Recently a proto-collaboration was formed. 240 collaborators so far, 106 of which come from Europe (47 from the UK alone). A design report is being prepared this year. In it an optimal detector design will be described, along with construction time and costs. Construction should start in 2018, and operation in 2025.
The currently envisioned design includes first of all a survey of the site. Based on it the cavern has been designed. The tank liner matetrial should be 5-mm high-density polypropilene. Photosensor candidates could be the ones of SK, but a new high-quantum-efficiency sensor is being developed. This might be fit with an avalanche photodetector granting a higher collection efficiency. A performance test shows a high QE in a broad range of wavelength, a 40% increase over the SK sensors. Timing resolution is also being reduced by a factor of two.
The physics potential of Hyper-K is very high. With a megaton detector you can do a lot of things. We already know what is possible with water cherenkov technique from SuperK and T2. There is good ring-imaging capability at 1 GeV, when you can really distinguish the originating particle ID.
For a proton decay search of p to positron and neutral pion, one can make a reconstruction of the proton mass at 5%, while removing atmospheric neutrino backgrounds completely. The discovery reach at 3-sigma is at 5×10^34 years. For proton going to neutrino and positive Kaons, it is trickier as the kaon is invisible in water, but positive muons from kaon decay are monochromoatic, so they yield a nice signal. A 3-sigma reach at 10^34 y is possible, with 10 years of data collection.
For CP violation a delta precision of 20% is expected. Hyper-K can also be sensitive to supernovae in nearby galaxies, and relic supernova neutrinos.