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There is, on average, one supernova explosion every second in the known universe. The energy released in each of these explosions is more than the total output of our sun over the course of its life, which will span some 10 billion years.

Each explosion emits neutrinos, one of the fundamental particles of nature, which carry off about 98 percent of the total energy produced. But while neutrinos are fundamental to our understanding of supernovas—and, indeed, the universe—little is known about them. That is slowly changing.

In 2015, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Japanese physicist Takaaki Kajita and Canadian physicist Arthur B. McDonald for their “key contributions to the experiments which demonstrated that neutrinos change identities.”

This year, Kajita, Yoichiro Suzuki, and his colleagues at Gifu Prefecture’s Super-Kamiokande (Super-K), the world’s largest underground neutrino detector, won the Breakthrough Prize in Fundamental Physics. The prize was awarded to five international teams investigating neutrino oscillation.

In an exclusive interview with The Journal, Kajita, who is director at the University of Tokyo’s Institute for Cosmic Ray Research and a principal investigator at the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), touched on a number of topics, including the research that led to the breakthrough.

His colleague at Kavli IPMU, US scientist Mark R. Vagins, also spoke to The Journal. A fellow Breakthrough Prize winner for his work at the Super-K, Vagins outlined the decades-long hunt for neutrinos and shed light on potential avenues for the application of this new knowledge.

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Professor Kajita, what can you tell us about neutrinos?
Neutrinos are one of the elementary particles. You can think of them as something like an electron, but without an electric charge. Moreover, that they have almost no mass is the most important thing about this discovery.

What is the significance of neutrinos having such small mass?
If neutrinos had no mass, then—like photons—they would travel at the speed of light, which means time would not advance for them. They would have no time to change their time, and therefore they would not oscillate. That is a simple way to put it.

Can you say something more about neutrino oscillation?
If neutrinos travel some distance, some of them change into another type or flavor state. Each neutrino has three flavor states, which we have named electron, muon, and tau.

Is oscillation similar to the way water changes from a liquid to a gas to a solid?
Not really, because that process involves changes in energy. Perhaps this analogy helps. If you take two tuning forks of different frequencies and hit them, the sound waves they produce will interfere with each other, and the resulting sound will alternate its loudness. In an analogous way, neutrinos change between their three flavor states—or “types”—as they propagate in the atmosphere and the Earth. When you sample them, their flavor state will be one of the three.

Was there a eureka moment in your research into neutrino oscillation?
Yes, about 30 years ago. At that time, I had just completed my PhD. I was trying to improve the analysis software at the Kamiokande experiment, the predecessor of the Super-K. The motivation then was to improve the proton decay searches of the experiment, and that involved improvements in various software. I checked the results against existing data, and found that the number of muon neutrino events at Kamiokande was almost half of the expected number. That was the eureka moment—when I thought oscillation might be the explanation.

What other questions are there to be asked about neutrinos?
For every particle, there is an antiparticle. Electrons, for instance, have anti-particles called positrons—electrons have a plus-one charge, while positrons have a minus charge. Now, we think of neutrinos as having zero charge, so an anti-neutrino must also have zero charge. If this is the case, one can ask: Are they really different? This suggests there is more beyond the Standard Model of particle physics that we are yet to understand.

What can you tell us about Japan–US collaboration in your research?
The Super-K experiment initially started via a collaboration between Japan and the United States. We signed an agreement in 1992, and we have been collaborating since then. From the beginning, the ratio of Japan–US collaborators has been about 60–40 percent. So we have been working closely together, and that has been a good experience for both countries.

Were you always into science?
Well, before I finished my undergraduate studies, I was not extremely serious about science. When I decided to pursue graduate studies, I accidentally joined Professor Masatoshi Koshiba’s group, which was at that time beginning preparation for the Kamiokande experiment.

For the first two years, my work involved planning for the experiment; there was no physics analysis involved at all. But I somehow found this kind of preparatory work really exciting, and began to think that I may be making a contribution to future science.

That’s when I decided to carry on from the master’s course to the PhD, at which point I was fully involved in the construction of the Kamiokande detector. I worked in the mine every day, and that was an interesting experience that made me like the work more. By the time the experimental analysis started, I had decided to become a scientist.

Engineers perform an eye check of a single photomultiplier (PMT)

Engineers perform an eye check of a single photomultiplier (PMT)

NEUTRINO BUSINESS
This year marks the 60th anniversary of the discovery of neutrinos, but there has thus far been no business or technological application for the elusive particle.

Kajita’s colleague Mark Vagins is among a handful of people actively developing neutrino-based technology. In his case, the work is for national security considerations such as anti-nuclear weapons proliferation.

Through his affiliation with the University of California, Irvine, Vagins is a spokesperson for the WATer CHerenkov Monitor of ANtineutrinos (WATCHMAN) detector, a project funded by the US Department of Energy.

“The idea of WATCHMAN works like this: If we can develop a detector that sees neutrinos from a supernova halfway across the universe, then that level of sensitivity can do other tricks, such as seeing neutrinos that are emitted from a nuclear reactor sitting across national borders.”

While transborder detection of such activity has never been possible, Vagins hopes the technology that WATCHMAN seeks to develop will be able to detect neutrino emissions from a very small nuclear reactor—say one that can produce a single nuclear weapon per year. This is what rogue countries are most likely to produce, he says.

“There are two great hooks if you want to snare a nefarious nation in this situation. Firstly, neutrinos, theoretically, cannot be shielded—say by hiding the reactor underground. They can go through rock and all that. Secondly, the kind of neutrinos that emerge out of a reactor are unique; they leave a very different signature to, say, solar neutrinos.”

If such a neutrino emission were detected, it would amount to incontrovertible evidence of nuclear fission, a process whereby the nucleus of an atom is split and a large amount of energy is produced.

FUNDAMENTAL PARTICLE
Quite apart from non-proliferation, neutrinos are key to our understanding of the building blocks of nature, which is based on the idea of fundamental particles. In classical Greece, atoms were thought to be the smallest unit of matter. It turns out that atoms are composite particles containing electrons and a nucleus made of protons and neutrons.

But these three components are themselves not fundamental particles. They are made of even smaller particles, for which scientists have found imaginative names. These families of particles include photons and gluons; z-bosons and w-bosons; six flavors of quarks; and leptons, which come in six types, including three kinds of neutrinos.

Together with the fundamental interactions of nature—strong nuclear, weak nuclear, and electromagnetic—these known fundamental particles describe the Standard Model of particle physics.

“What is special about the neutrino family—and this is why Professor Kajita won his Nobel Prize—is that they are able to flow between the generations—between electron, muon, and tau,” Vagins explained. “Each neutrino is a blend of those three states, but in different proportions.”

BACK IN TIME
Kajita’s discovery can be traced back to the 1970s. At that time, there was an effort to create a theory of everything that would unify strong nuclear, weak nuclear, and electromagnetic forces.

“It was a beautiful theory, and it made a prediction,” Vagins explained. “Protons would decay at a certain rate, and that rate was testable.” This radioactive decay suggested that protons—which, together with neutrons, constitute the nucleus of an atom—would decay into lighter subatomic particles.

Why was proton decay important? “Well, if you knew the rate at which protons decayed, it would be the equivalent of knowing the fate of the universe. After all, if protons aren’t forever, then nothing else is.”

The first group that attempted experiments to detect proton decay worked at the Irvine-Michigan-Brookhaven (IMB) detector, a collaboration between the University of California, Irvine, the University of Michigan, and the Brookhaven National Laboratory, located in a salt mine in Ohio.

Dr. Takaaki Kajita receiving the 2015 Nobel Prize in Physics

Dr. Takaaki Kajita receiving the 2015 Nobel Prize in Physics

COSMIC NEUTRINOS
IMB started producing data in the 1980s. In the same period, Japanese scientist Masatoshi Koshiba was developing his own underground detector called Kamiokande, located in a zinc mine in central Honshu.

Koshiba is an emeritus professor at the University of Tokyo and the joint winner of the 2002 Nobel Prize in Physics, together with Americans Riccardo Giaconni and Raymond Davis, Jr., for work on the detection of cosmic neutrinos. He is also Kajita’s mentor.

While both IMB and Kamiokande were created to look for proton decay—a goal that they never managed to achieve—they were also capable of looking for neutrinos.

Both detectors consisted of a massive water tank surrounded by photon detectors whose function was to sense flashes of light created when protons decayed, or when neutrinos—of which millions are flowing through the Earth every nanosecond—interacted with the atomic nuclei and electrons comprising the huge number of water molecules filling the tank.

On February 23, 1987, a burst of neutrinos reached Earth from a supernova that had gone off 170,000 years earlier in a galaxy orbiting the Milky Way. Three neutrino detectors around the world, including IMB and Kamiokande, were triggered by this distant supernova called SN1987A.

“That really reinvigorated the field because IMB, Kamiokande, and an experiment in the then-Soviet Union detected it. Suddenly, these detectors—which were mainly supposed to see proton decay—were receiving these unexpected bursts of neutrinos.” These unique data confirmed the basic model of stellar evolution.

Using a different technology, another team led by Davis had previously measured a counting rate for neutrinos, which they claimed were solar neutrinos; but they could not provide proof for the source of their counts.

“But Kamiokande had directionality,” Vagins said. “You could plot a graph that showed the angles [from which] the neutrinos came in, which pointed back to the Sun. That was the proof that these were solar neutrinos.”

TOO FEW NU MU
However, all three experiments noticed something strange. The Davis experiment observed about three times fewer solar neutrinos than expected from the fusion processes which power the Sun. This became known as “the solar neutrino problem.” Similarly, people thought they knew how many neutrinos would be made in the atmosphere by cosmic rays hitting the air. But when both IMB and Kamiokande saw too few atmospheric muon neutrinos, this became known as the problem of “too few nu mu.”

“And it is easy to calculate the ratio of the muon and electron-type neutrinos. It should be two-to-one, or two muons for every electron. So, they expected to measure this factor of two, but they realized that there weren’t enough muon-type neutrinos,” Vagins explained.

While the calculation was simple, the discrepancy between the expected result and the actual findings was perplexing. But the Japanese scientists had a guess.

“The Japanese group called it what it turned out to be. They said, ‘We believe this is a hint of neutrino oscillation. The muon type [neutrinos] are disappearing, and while they are not necessarily turning into electron-type neutrinos, there is something happening.’” They moved forward with the planning for the next generation experiment: Super-Kamiokande.

SUPER-KAMIOKANDE
The IMB group was not convinced that the anomalies in the expected number of neutrinos detected could be explained by oscillations. But the Japanese Kamiokande group was, and—after joining forces with the American IMB scientists—the two groups worked together to build the much large Super-Kamiokande detector to see who was right.

Operations began in 1996, and within two years the results were in. The 50,000-ton Super-K had collected irrefutable evidence of neutrino oscillations.

“It was a pretty major deal,” said Vagins, who joined the Super-K in 1994 and helped establish the detector. “Indeed, when the announcement was made in 1998, dark energy was not yet understood; and, in fact, it was discovered the same year as neutrino oscillations.”

For Kajita and Vagins, neutrino science—much of which was built on the foundation of the original data set of just 24 supernova neutrinos that were detected from SN1987A—is a window into a world beyond the Standard Model of particle physics.

The elusive particles also shed light on the formation of the early universe and, in turn, to the molecules that form all matter—including us as humans.

John Amari is a writer and editor from the UK who specializes in articles on startups, entrepreneurs, science, tech, and business.
While neutrinos are fundamental to our understanding of supernovas—and, indeed, the universe—little is known about them.