Hyper-Kamiokande in 2026: where the giant detector stands

The successor to Super-Kamiokande has been quietly being dug out of a mountain in Gifu prefecture for several years. Hyper-Kamiokande — Hyper-K for short — is the Japanese next-generation neutrino observatory, designed around a single 260,000-tonne tank of ultrapure water under about 600 metres of rock. By May 2026 the major civil-engineering phase is essentially done, and the project has shifted into the long phase of building the detector itself inside the cavern that has been waiting for it. This article describes where the project stands.

What Hyper-K is for

Hyper-Kamiokande is the centrepiece of Japan’s long-baseline neutrino programme for the 2030s. Built on the foundation laid by Super-Kamiokande, which discovered atmospheric neutrino oscillations in 1998, Hyper-K aims to do everything Super-K does — but at five times the scale.

Its primary goal is the discovery and measurement of CP violation in neutrino oscillations: the question of whether neutrinos and antineutrinos oscillate slightly differently as they travel, an asymmetry that would tie the neutrino sector to one of the leading explanations for why the universe is dominated by matter rather than antimatter. To do that, Hyper-K will receive an upgraded neutrino beam from the J-PARC accelerator complex in Tokai, 295 kilometres away — a project sometimes called T2HK, for Tokai-to-Hyper-Kamiokande.

Beyond CP violation, Hyper-K’s enormous fiducial volume makes it a flagship instrument for atmospheric neutrinos, solar neutrinos, galactic supernova bursts, proton decay searches, and a new attempt at the diffuse supernova neutrino background.

The wider design background sits in our companion piece on the Hyper-Kamiokande detector concept.

The cavern: done

The civil-engineering phase began in earnest in the early 2020s. The site is Mount Nijuugo in Gifu prefecture, about 8 kilometres from the existing Super-Kamiokande and Kamioka Observatory complex — close enough to share infrastructure and personnel, far enough to allow new excavation without interfering with the running experiment.

The cavern excavation was completed in the mid-2020s after several years of careful tunnelling and rock-stability work. The result is a single underground space about 69 metres tall and 68 metres across — large enough to house the cylindrical water tank with its inner detector, outer veto region, and access infrastructure. The amount of rock removed was substantial: tens of thousands of cubic metres of granite and gneiss, hauled out in a continuous campaign.

By early 2026, the cavern itself is essentially complete, with rock support, lining, and access infrastructure in place. The project has moved into the next phase.

Building the detector inside the cavern

The construction work happening inside the cavern in 2026 has three major tracks.

The first is the water-tank vessel. Hyper-K’s design uses a cylindrical stainless-steel-lined tank to hold 260,000 tonnes of ultrapure water. Building the tank in situ — supporting it, lining it, sealing it, and integrating it with the cavern — is a significant undertaking in its own right. This is the structural skeleton that everything else mounts onto.

The second is the photomultiplier tube installation. Hyper-K relies on tens of thousands of photomultipliers lining the inner walls of the tank to record the Cherenkov light produced when neutrinos interact in the water. The flagship design uses next-generation 20-inch PMTs developed in collaboration with Hamamatsu, with significantly improved photon detection efficiency and timing resolution compared with the original Super-Kamiokande PMTs. A small additional set of smaller multi-PMT modules complements the main array. Each PMT has to be cleaned, tested, mounted, connected to electronics, and characterised in situ — work that takes years even with a large international collaboration on site.

The third is the ultrapure water system. The water in Hyper-K has to be cleaner than nearly any industrial water on Earth, because the detector’s sensitivity to faint Cherenkov signals depends critically on absorbing as little light as possible during its journey across the tank. Building, installing, and commissioning the multistage filtration, deionisation, and degassing infrastructure is a serious engineering project. So is filling the tank: 260,000 tonnes of water doesn’t arrive overnight.

The upgraded J-PARC beam

In parallel, the J-PARC accelerator complex in Tokai is being upgraded to deliver the beam Hyper-K needs. The current T2K beam, which has been running into Super-Kamiokande, will be upgraded to higher proton power — roughly 1.3 MW of beam — which dramatically improves the statistical reach for CP-violation measurements at Hyper-K’s distance.

Beam upgrades and detector commissioning have to be synchronised so that Hyper-K is ready to receive a high-intensity beam when its calibration and water-filling campaigns conclude. Coordinating the two campaigns is one of the larger programmatic challenges of the late-2020s neutrino calendar.

Why the field is watching

Hyper-K matters for two reasons that go beyond its own physics goals.

The first is statistical reach for CP violation. A water-Cherenkov detector this large, with a 1.3 MW beam, has the integrated event rate to make a meaningful CP-violation measurement on a much shorter timescale than predecessor experiments. Combined with the parallel measurements at DUNE in the United States, the late-2020s and early-2030s are likely to be when this question is settled.

The second is astrophysical reach. A 260,000-tonne water Cherenkov detector is the world’s largest and most sensitive instrument for catching the neutrino burst from a galactic core-collapse supernova. The current rate of galactic supernovae makes such an event uncertain on any individual timescale, but the longer Hyper-K runs, the better the odds. A nearby supernova during Hyper-K’s lifetime would deliver a dataset orders of magnitude richer than SN 1987A.

It will also be the leading instrument for proton decay searches — a key test of grand-unified theories — and a major contributor to atmospheric and solar neutrino programmes.

Where it fits

Coverage of Hyper-Kamiokande on neutrino-times sits alongside our explainers on the Hyper-K detector concept, the DUNE vs Hyper-K comparison, and the parallel JUNO commissioning programme.

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For related coverage, see Hyper-Kamiokande: the next giant water tank, DUNE vs Hyper-Kamiokande, and Super-Kamiokande’s 1998 oscillation discovery.

Neutrino Times Editorial Team. (2026, May 21). Hyper-Kamiokande in 2026: where the giant detector stands. Neutrino Times. https://neutrino-times.com/articles/hyper-kamiokande-2026-construction-status/

Neutrino Times Editorial Team. "Hyper-Kamiokande in 2026: where the giant detector stands." Neutrino Times, May 21, 2026. https://neutrino-times.com/articles/hyper-kamiokande-2026-construction-status/.

Neutrino Times Editorial Team. "Hyper-Kamiokande in 2026: where the giant detector stands." Neutrino Times, 21 May. 2026, https://neutrino-times.com/articles/hyper-kamiokande-2026-construction-status/.

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