Exploring the universe from Antarctic ice: The Next Frontier in Neutrino Astronomy

Why can’t we observe them with light?
Unraveling the origin of cosmic rays with neutrinos

Could you tell us about the IceCube project?

The IceCube is the world’s largest neutrino telescope. Located at the South Pole, it uses the thick Antarctic ice itself as a giant detector to capture high-energy neutrinos arriving from distant regions of the universe.

One of the major goals of the IceCube project is to identify the high-energy celestial objects that are the sources of cosmic rays and to unravel the long-standing mystery of how cosmic rays are produced.
When neutrinos interact with matter, they produce high-energy charged particles that emit Cherenkov light*. By detecting this light with photodetectors embedded in the ice, we can determine the direction and timing of neutrino events and investigate their astrophysical sources. By combining these observations with other types of astronomical data, including electromagnetic observations, we can identify and study the source of these cosmic neutrinos. This approach is known as multi-messenger astronomy.
I joined the project in January 2005, two months after IceCube construction began. I am currently a core member of the Gen2 management team and lead the in-ice detector subsystem, while also serving as the development/production manager for the high-precision photodetector.

*Cherenkov light: Light produced when high-energy charged particles travel through transparent materials, such as water or glass, faster than the speed of light. It appears bluish white to the human eye, but spans a wide range of wavelengths, including ultraviolet light. By observing this light, researchers can investigate the properties of high-energy particles and cosmic rays.

Why is neutrino observation necessary for understanding high-energy celestial objects?

The universe is filled with light, including the cosmic microwave background radiation, a remnant of the Big Bang, and light emitted by stars. In this sense, it is like being surrounded by a mist of light. High-energy celestial objects produce cosmic rays, gamma rays, and neutrinos. Cosmic rays are deflected by magnetic fields, while gamma rays collide with the surrounding light and disappear.
Neutrinos, however, hardly interact with matter, allowing them to travel straight to Earth even from great distances. By detecting these neutrinos, we can identify high-energy celestial objects that cannot be seen with light.

Could you tell us about the most important findings from your research so far?

First, in 2013, the IceCube Chiba University group made the world’s first observation of two high-energy neutrinos of extraterrestrial origin. Subsequent analyses by the IceCube Collaboration increased the sample to 28 such neutrinos.  This achievement was selected as the “Breakthrough of the Year” by Physics World, a British science journal, for having the most decisive impact on the advancement of physics that year.
Then, in 2018, the IceCube Collaboration identified blazars* as a source of high-energy neutrinos. The discovery was enabled by IceCube’s online real-time alert system, whose development was led in large part by the Chiba University group.  This was a major achievement because it was the first time the source of high-energy neutrinos, which had previously been unknown, was determined.
Moreover, in 2021, the IceCube Collaboration made the first observation of the Glashow resonance**, a phenomenon predicted by Sheldon Glashow in 1960. This discovery provided the first direct evidence that the astrophysical neutrino flux includes electron antineutrinos and represented another major milestone in neutrino astronomy. Furthermore, in 2023, the IceCube Collaboration observed high-energy neutrino emission from the center of the Milky Way. This result established the Milky Way as a source of high-energy neutrinos and provided a unique probe of where the high-energy cosmic rays interact with matter within our Galaxy.

*Blazars : A type of active galactic nucleus (AGN) with a supermassive black hole at its center, in which jets of matter are ejected at nearly the speed of light and directed toward Earth. Blazars are among the most energetic celestial objects and are characterized by extremely violent fluctuations in brightness.
**Glashow resonance: A rare phenomenon in which an electron antineutrino from space strikes an electron with exactly the right energy to create a W boson, a particle usually studied in high-energy accelerator experiments. First predicted by physicist Sheldon Glashow in 1960, this process was observed by IceCube in 2021, marking the first demonstration of the Glashow resonance in nature and on a cosmic scale.

What are the next steps toward unraveling the origins of cosmic rays?

To unravel the origins of cosmic rays, we need to identify many more high-energy celestial objects. That is why development of Gen2, which will have approximately eight times the volume and more than five times the sensitivity of the current detector, is underway. At the same time, we are upgrading IceCube, which will serve as a bridge to Gen2. 

Balancing three key challenges: Performance, cost, and mass production 

What are the challenges in developing Gen2?

A telescope’s performance can be improved by increasing the volume of ice monitored and the number of detectors installed. However, drilling holes 2.5-3 km deep into the Antarctic ice is extremely expensive. To keep the overall cost down, we need to reduce the number of holes and their diameter. 

Furthermore, Gen2 will use approximately 10,000 detectors, roughly twice the current number. Unlike a single, state-of-the-art device, equipment produced on this scale must be manufactured in large quantities while maintaining consistent quality. Finding the right balance between performance, cost, and the efficiency of mass production is a major challenge. 

What types of detectors are currently under development?

For the IceCube Upgrade, two types of detector modules have been developed: D-Egg, designed by our Japanese team, and mDOM, developed under German leadership. D-Egg features two photodetectors, one above the other, in a vertically elongated housing, improving sensitivity while allowing the module to fit into a narrow hole. The mDOM has 24 small photodetectors arranged in a sphere to capture light from all directions.  
Building on the strengths of both designs, we are now developing a next-generation module that combines their key advantages and serves as a common standard for the entire IceCube-Gen2 collaboration.

Could you tell us about the IceCube Upgrade project currently underway?

The IceCube Upgrade project aims to enhance the precision of the detector and minimize systematic uncertainties in future observations. As part of the project, D-Egg and mDOM units are being densely installed in the central part of the existing IceCube array, allowing us to observe lower-energy neutrinos. The project also includes the installation of various calibration devices to study the optical properties of the ice, as well as highly sensitive seismometers that will contribute to seismic research. This important project will expand the energy range accessible to IceCube and help ensure the successful realization of the future Gen2 detector. 

With the realization of Gen2, what aspects of the universe that were previously invisible will become visible?

By increasing the sensitivity of telescopes, we will be able to detect neutrinos from a wider range of sources across the universe. If we can identify five to ten more high-energy celestial objects, we expect to gain an overall picture of their distribution throughout the universe and how they accelerate particles.

A desire to explore the unknown: The spirit that led me to research

What first drew you to space research?

I’ve always been fascinated by things that are far away and beyond my reach. Even as a child in elementary school, I longed to travel to distant places. When I was in my second year of junior high school, I used the money I had saved from delivering newspapers to travel alone to Hokkaido. That same desire to reach beyond the familiar is what draws me to things I cannot see or touch, and ultimately to space.

My current goal is to identify the celestial object that produces the highest-energy emissions in the universe. Neutrinos are currently the only way to investigate this.

Such objects are expected to produce energies more than 1,000 times higher than those observed so far through electromagnetic waves. I believe the appeal of this research lies in discovering aspects of the universe that humanity has yet to explore.

Do you have a message for students who aspire to become researchers?

To build a successful career as a researcher, it is essential to discover what genuinely interests and motivates you. In my laboratory, I encourage students to explore a wide range of research topics and identify the questions that spark their curiosity.

If you have not yet found your passion, try as many different things as possible. You can never truly know what excites you until you experience it yourself.

What may seem like “wasted time” when you are young often becomes a valuable part of who you are, broadening your perspective and shaping your creativity. Student life is a rare period during which you can afford to take risks and learn from mistakes. Rather than focusing too much on efficiency or immediate results, I encourage you to pursue your curiosity, even when it leads you in unexpected directions or toward things that initially seem unproductive.