Japan's Hyper-Kamiokande Targets 2028 Start to Probe Neutrinos and the Origins of Matter

A Giant Underground Detector Takes Shape in Gifu

Six hundred meters beneath the mountains of Hida in Japan's Gifu Prefecture, scientists are building a detector designed to examine some of the smallest particles in nature. Hyper-Kamiokande will hold 260,000 tons of ultra-pure water, giving researchers a vast target in which to catch the rare interactions of particles called neutrinos.

The project recently opened its underground cavern to the press for the first time. Measuring 94 meters high and 69 meters in diameter, the cavern will house an observatory intended to become the world's largest and most sensitive detector of its kind. Its scientific targets range from the reason matter dominates the universe to the possibility that protons, basic components of atoms, eventually decay.

As of June 30, 2025, the project is working toward observations in 2028, following an original target of 2027. Rising material costs and engineering challenges have pushed back the planned start. The broader timetable has been described as 2027–2028, with construction and equipment installation standing between the underground excavation and the first scientific measurements.

Hyper-Kamiokande, also called Hyper-K or HK, succeeds Super-Kamiokande, the Japanese experiment whose discoveries helped establish that neutrinos have mass. The University of Tokyo's Institute for Cosmic Ray Research (ICRR) leads the project in collaboration with the High Energy Accelerator Research Organization (KEK) and an international network of institutions.

The ambition is to collect far more neutrino interactions than earlier detectors could capture. With a larger sample, researchers can compare subtle particle behavior more precisely and increase their chances of observing extremely rare events.

Why Neutrinos Require Such a Large Observatory

Neutrinos are among the most abundant particles in the universe, yet they are exceptionally difficult to observe. They have no electric charge, are about 100,000 times lighter than an electron and interact very weakly with matter. Trillions pass through a person's body every second without leaving a trace.

These particles are sometimes called "ghost particles" because they pass through so much ordinary material unnoticed. A detector cannot simply gather them as a container collects rain. Instead, scientists provide an enormous amount of material and wait for the occasional neutrino interaction that produces a measurable signal.

Hyper-Kamiokande's water supplies that material. Its location beneath the mountain helps screen out cosmic rays and other unwanted particles that could interfere with the measurements. The Kamioka area of Hida City also offers stable rock and existing mining infrastructure, making it suitable for such a large underground facility.

Neutrinos matter because their behavior connects several unanswered questions. They could help explain why the universe contains much more matter than antimatter, reveal what happens inside exploding stars and improve understanding of the Sun. The same observatory will also search for proton decay, a possible process in which one of matter's basic building blocks breaks apart.

From Excavation to a Tank of Ultra-Pure Water

The detector itself will be a cylindrical tank 71 meters high and 68 meters in diameter. More than 20,000 highly sensitive photomultiplier tubes, or PMTs, will line it to capture faint flashes of light. Keeping the water exceptionally clean is essential so those flashes can travel to the sensors with as little interference as possible.

Hyper-Kamiokande's scale is described as about eight times the water volume used in Super-Kamiokande. That increase, combined with more advanced sensors, is intended to allow many more neutrino interactions to be observed with greater precision.

The construction schedule stretches across several years. Excavation of the cavern began after years of planning, geological surveys and international cooperation. The project's stated milestones include:

• 2021: Excavation of the main cavern began.
• Early 2022: The access tunnel, stretching 2 kilometers into the mountain, was completed.
• Late 2023: The dome section of the main cavern was finished.
• 2024: The construction schedule had put expected completion of the barrel section, excavated in stages and forming most of the detector's volume, in this year.
• 2027: The original target for the start of observations.
• 2028: The revised target following rising material costs and engineering challenges.

Once the cavern is complete, engineers must install the tank, fit the photodetectors and fill it with ultra-pure water. The facility's design focuses on reducing background noise so researchers can identify rare, faint particle signals.

How a Neutrino Leaves a Flash of Light

A neutrino occasionally collides with an electron or an atomic nucleus in the detector's water. That interaction can produce a charged particle moving faster than light travels through water. It still travels more slowly than light in a vacuum, so the process fits within the normal speed limit of physics.

The moving particle produces Cherenkov radiation, a faint cone of blue light. Photomultiplier tubes lining the tank record that light. Researchers then study the timing and pattern of the flashes to reconstruct the incoming neutrino's energy, direction and type, also known as its flavor.

Hyper-Kamiokande will receive neutrinos from several sources. Some are created when cosmic rays strike Earth's atmosphere. Others come from reactions inside the Sun, distant supernovae and other cosmic events. An artificial beam will arrive from the J-PARC accelerator in Tokai, Ibaraki Prefecture, 295 kilometers away.

Each source offers a different scientific opportunity. A controlled accelerator beam allows researchers to compare particle behavior over a known journey, while neutrinos arriving from space carry information about places that cannot be examined directly.

Professor Masato Shiozawa of the ICRR, a leading figure in the project, described the scientific ambition at the recent unveiling:

We aim to produce results that will fundamentally change our understanding of elementary particles.

Can Neutrinos Help Explain Why Matter Exists?

Neutrinos come in three flavors: electron, muon and tau. They can change from one flavor to another while traveling, a process called neutrino oscillation. Super-Kamiokande's discovery of oscillations in 1998 showed that neutrinos have mass and earned recognition through the 2015 Nobel Prize in Physics.

Hyper-Kamiokande will investigate whether neutrinos and their antimatter counterparts, antineutrinos, change flavor differently. The difference researchers are seeking is called charge-parity violation, usually shortened to CP violation.

Matter and antimatter are central to a basic puzzle: why does the universe contain so much more matter? Finding CP violation in neutrinos could help explain that imbalance. The experiment will compare neutrino and antineutrino beams sent from J-PARC, measuring how their flavors change along the 295-kilometer route.

The project's goal is a precise test using a much larger collection of interactions. After a decade of data collection, Hyper-Kamiokande is expected to reach a "gold standard" measurement of 5 sigma for CP violation across a substantial range of possible values. Five sigma describes a demanding statistical threshold used to judge whether a measured effect is convincing.

The reach of that measurement depends on the actual values governing neutrino behavior. Researchers will use the observations to determine how neutrinos and antineutrinos compare, rather than assuming that a difference will appear.

Searching for the Decay of Protons

Protons are among the building blocks of atoms. Many theories that attempt to unite nature's fundamental forces predict that protons can decay, although the process remains hypothetical. If it occurs, matter would eventually be unstable over timescales far longer than the age of the universe.

Hyper-Kamiokande's huge water volume gives researchers a large amount of matter to watch for such an event. Its sensitive light detectors would help identify the signals left by the particles produced in a decay. The project is intended to become the world's strongest laboratory for this search.

Detecting proton decay would transform understanding of particle physics and could help guide efforts toward a "Theory of Everything." Those efforts seek a more complete explanation connecting the fundamental workings of nature.

The search can also produce useful results if no decay is observed. Researchers would set new lower limits on the proton's lifetime, narrowing the range of theories that fit the observations and ruling out some proposed models.

Catching Signals From Exploding Stars and the Sun

A massive star releases a flood of neutrinos when it explodes as a supernova. Those particles can reach Earth before light from the explosion. Hyper-Kamiokande could therefore provide an early warning of a supernova while recording information from deep inside the collapsing star.

A supernova in our galaxy could produce tens of thousands of detectable neutrinos in Hyper-Kamiokande. Such a sample would give researchers a detailed view of the processes involved in stellar explosions, the creation of heavy elements and the evolution of the cosmos.

The observatory will also search for relic neutrinos, the faint, diffuse background left by supernovae throughout cosmic history. Studying that combined signal could help scientists understand past star formation and the life cycle of matter across the universe.

Closer to home, solar neutrinos will allow Hyper-Kamiokande to probe the Sun's interior and test models of how stars work. Atmospheric neutrinos will provide another way to measure oscillations and investigate neutrino mass ordering, the question of how the different neutrino masses are arranged.

The scientific program also includes geoneutrinos, particles produced by radioactive decay inside Earth. They offer a way to investigate the planet's internal structure and heat generation, with relevance to geology and the geomagnetic field. Together, these observations extend the experiment's reach from Earth's interior to distant stellar explosions.

A Global Project Built Around Better Sensors

More than 560 researchers from over 100 institutions in 21 countries are involved in Hyper-Kamiokande. Major contributions come from Japan, the United Kingdom, Canada, Italy, Mexico and other partners. The project combines Japanese leadership with international work on equipment, software and scientific analysis.

The budget is estimated at around $600 million. The Japanese government provides the main funding, supported by substantial contributions from international partners.

New photomultiplier tubes developed by the Japanese company Hamamatsu and other partners are designed to be more sensitive and reliable than earlier generations. Their job is to turn extremely faint flashes into usable measurements. Better sensors are therefore central to making the larger tank scientifically productive.

The water purification system is also among the world's most advanced. Cleaner water reduces unwanted background signals and helps the light reach the detector walls. The combination of water quality, sensitive sensors and shielding beneath the mountain is intended to make rare interactions easier to recognize.

Machine learning and artificial intelligence will help analyze the large amounts of data the observatory generates. Tecnológico de Monterrey in Mexico is among the institutions contributing expertise in software and data processing. The challenge continues after a flash is detected: researchers must reconstruct what happened and identify which signals belong in each scientific measurement.

Building on Japan's Neutrino Discoveries

Hyper-Kamiokande follows a Japanese research tradition stretching back decades. The original Kamiokande experiment in the 1980s made the first detection of solar neutrinos and observed neutrinos from supernova SN1987A. That work led to the 2002 Nobel Prize for Masatoshi Koshiba.

Super-Kamiokande, built in the 1990s, then discovered neutrino oscillations in 1998. Its findings helped change the scientific understanding of neutrinos by showing that they have mass. Japan's work in this field has been recognized through two Nobel Prizes.

The T2K experiment supplies another direct foundation for Hyper-Kamiokande. T2K sends a neutrino beam from J-PARC to Super-Kamiokande and has produced important measurements of neutrino properties. Hyper-Kamiokande will build on that approach with a much larger detector.

The progression connects established water Cherenkov technology with a larger scale and improved instruments. It also continues a program that has trained researchers and inspired new generations of scientists while pursuing questions about particles, stars and the composition of the universe.

How Hyper-Kamiokande Fits With DUNE and JUNO

Hyper-Kamiokande is part of an international effort to understand neutrinos. Other major projects include the Deep Underground Neutrino Experiment (DUNE) in the United States and the Jiangmen Underground Neutrino Observatory (JUNO) in China.

DUNE uses liquid argon technology, while Hyper-Kamiokande relies on water and Cherenkov light. The experiments have different strengths and approaches. Their results will complement one another, allowing researchers to compare findings and develop a fuller picture of neutrino behavior.

Takaaki Kajita, a Nobel laureate and a leader in neutrino research, has highlighted the value of that complementary work for reliable scientific progress. Hyper-Kamiokande's combination of established detector technology and unprecedented scale is intended to give it strong sensitivity across several kinds of observations.

The international effort spans both competition and cooperation. A result observed through different experimental methods can be examined from more than one direction, while each observatory pursues measurements suited to its design.

What Must Happen Before Observations Begin

With excavation nearing completion and installation of detector components expected to begin soon, the next practical tasks are to build the instrument inside the cavern and prepare it for observations. Filling such a large tank with ultra-pure water and bringing its sensors into operation are essential steps toward the planned 2028 start.

The project still has to turn its construction milestones into a working scientific facility. Its intended discoveries range from differences between neutrinos and antineutrinos to proton decay and neutrino signals from supernovae. Some measurements will require years of collected data; a stellar explosion could offer an unexpected opportunity.

ICRR director Masato Shiozawa expressed his hopes for the project and encouraged public interest in its progress:

I hope to see research outcomes that would surprise everyone, so I urge people to keep up to date on the project.

Hyper-Kamiokande's promise rests on observing particles that almost always escape detection. By watching a much larger volume of water with more sensitive instruments, researchers aim to turn those rare encounters into evidence about the origins of matter, the stability of atoms and the lives of stars.

Key Points

• Hyper-Kamiokande is being built 600 meters underground in Hida, Gifu Prefecture, Japan.
• Its planned tank will hold 260,000 tons of ultra-pure water and use more than 20,000 photomultiplier tubes.
• Observations are targeted for 2028, following an original 2027 target and delays linked to costs and engineering challenges.
• A neutrino beam from J-PARC, 295 kilometers away, will support comparisons between neutrinos and antineutrinos.
• The scientific program includes proton decay searches and studies of supernova, solar, atmospheric and geoneutrinos.
• More than 560 researchers from over 100 institutions in 21 countries are involved, with an estimated budget of around $600 million.
• The observatory builds on Kamiokande, Super-Kamiokande and T2K, and will complement DUNE in the United States and JUNO in China.