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Takaaki Kajita – Ghostbuster

Written By: Michael D. McClellan | Nobel laureate Takaaki Kajita is a quiet, unassuming man, and humble to a fault. The Japanese physicist, who, in 2015, was awarded the Nobel Prize in Physics for the discovery of neutrino oscillations, which shows that neutrinos have mass, isn’t particularly big on interviews and doesn’t sit down for many. When he does, his answers are usually brief and unfailingly polite, and often dominated with one-liners. Consider: Shortly after the announcement that Dr. Kajita had won the Nobel Prize, he was interviewed by Adam Smith, Nobel Media’s Chief Scientific Officer, and it went something like this:


Adam Smith: How did you hear the news [winning the Nobel Prize]?

Takaaki Kajita: Well, I just, well actually, when I received the phone call I was checking my e-mails.

[AS] In your office? Right. And what was your first reaction?

[TK] Well, that was really a surprise to me.

[AS] I imagine it is still sinking in.

[TK] Yes, yes, still kind of unbelievable.

[AS] You sound as if you are alone, are there not people around you yet?

[TK] Well actually I’m in a small room so no one around.

[AS] I’m sure that very soon you will be surrounded by people.

[TK] {Laughs] Thank you.


Joe Exotic of Tiger King fame he’s not, but that suits Kajita just fine. He’d rather be anywhere else but in front of a microphone, preferably at home with his wife, Michiko, or at the University of Tokyo, where he serves both as a Principal Investigator at the Institute for the Physics and Mathematics of the Universe, and as the Director of the Institute for Cosmic Radiation Research. Meet him on the street, and there would be little to reason to suspect that his discovery had effectively rewritten the balance sheet of the universe. It’d be easier to imagine sitting down with Kajita in a Japanese bar, talking about the finer points of Kyudo over a bottle of saki, than it would be to think that his work had just turned the Standard Model on its head. How significant of an achievement are we talking? The Standard Model is only the most accurate scientific theory known to human beings. More than a quarter of the Nobel Prizes in physics of the last century are direct inputs to or direct results of the Standard Model. Many recall the excitement over the 2012 discovery of the Higgs boson, but that much-ballyhooed event didn’t come out of the blue – it capped a five-decade undefeated streak for the Standard Model. Every fundamental force but gravity is included in it. Every attempt to overturn it, or to demonstrate in the laboratory that it must be substantially reworked – and there have been many over the past 50 years – has failed.

Princess Sofia and physics laureate Takaaki Kajita arrive in the Blue Hall for the 2015 Nobel prize award banquet in Stockholm City Hall.PHOTO: REUTERS

That is, until Kajita introduced the world to something called Super-Kamiokande.

While the name might conjure images of a fire-breathing creature in a Godzilla film, Super-Kamiokande is actually a neutrino-observing facility located 1000 meters underground in Hida City, Gifu Prefecture. Inside it is a cylindrical tank which holds 50,000 tons of super-pure water. Its inner walls are lined with 11,000 photo-sensors designed to identify muon neutrinos produced by cosmic rays. In simplest terms, the sensors record flashes of light caused by debris speeding away from a neutrino hit.

At this point you may be wondering what neutrino oscillations are, so let me drop an analogy on you: Imagine you’re the driver of a truck that delivers ice cream, and the company you work for sells the three standard flavors – vanilla, chocolate, and strawberry. Now, imagine that you load your truck with all three flavors at the factory and drive it to the ice cream store across town, where you swing open the cargo door and make a shocking discovery: The vanilla and strawberry ice cream that you loaded has turned into chocolate! Welcome to the strange world of neutrino physics, a world in which these incredibly small, ghostlike particles change flavors as if by magic.

Godzilla reference, check.

Ice cream analogy, check.

Now, the backstory:  First predicted in 1930 by Wolfgang Pauli, and experimentally observed in 1950, neutrinos come in three flavors – electron, muon, and tau. The mass of a neutrino is less than 1/1,000,000th of that of an electron. If you liken the weight of an electron to that of an elephant, the weight of a neutrino would be lighter than a one yen coin. They do not hold a charge; the word “neutrino” is a combination of the Italian word “neutro,” which means “neutral or electrically neutral,” and the diminutive suffix “-ino.” Literally translated, “neutrino” would mean something like “little neutral one.” Because neutrinos are extremely light and do not hold a charge, they do not interact with most other elementary particles. Several trillion of them pass through the palms of your hands every second. In fact they pass through everything, including the Earth, unimpeded.

Lifetime Achievement
Takaaki Kajita receiving the 2015 Nobel Prize in Physics

Neutrinos have long been of interest to physicists, who, for decades, couldn’t explain why the number of electron neutrinos measured in experiments on Earth did not match the amount predicted by the best solar models. Chemist and physicist Raymond Davis was the first to encounter this phenomenon, which later came to be known as the Solar Neutrino Problem. Davis, who in 1967 built a neutrino collector – a tank holding 100,000 gallons of cleaning fluid nearly a mile underground in South Dakota’s Homestake mine – was as perplexed as anyone. Calculations predicted that of the 10 million billion neutrinos passing through the tank every day, roughly one would interact with a chlorine atom and change it to argon. But the detector, operated all the way until 1994, recorded only about one-third the expected number of neutrinos. Over time, a prevailing theory emerged – that the neutrinos were oscillating from one flavor to another and avoiding detection.

The Problem:  How do you prove it?

Enter Dr. Kajita and Super-Kamiokande.

Neil deGrasse Tyson on a boat inside the Super-Kamiokande

The giant detector, which went live on April 1, 1996, is able to observe atmospheric neutrinos coming from all directions. Two years later, Kajita and other members of the Super-Kamiokande Group made a groundbreaking discovery:  The number of neutrinos made on the opposite side of the Earth that had flown the long distance to the Super-Kamiokande detector was only about half as high as the number of neutrinos that came down from the atmosphere directly above the detector.

This could only mean one thing:  The discrepancy in numbers was due to “neutrino oscillation,” a phenomenon in which neutrinos change into other types of neutrinos while in flight. As the muon neutrinos created on the opposite side of the Earth passed through the Earth’s interior, they transformed into tau neutrinos, which is why observations showed fewer muon neutrinos than expected.

So, they oscillate. What’s the big deal?

2015 Nobel laureate Takaaki Kajita

Like photons, neutrinos were thought to be massless. Einstein’s Theory of Relativity states that objects with mass will never reach the speed of light, and that objects without mass will always travel at the speed of light. Also, the faster an object travels, the slower time passes, and once an object reaches the speed of light, time will stop for the object (the object will cease to experience time). Since neutrinos oscillate, they are experiencing time. The fact that neutrinos oscillate while they travel at a speed that does not reach the speed of light proves that they have mass. This, in turn, tells us that there is something missing. The Standard Model cannot be complete.

Kajita, whose responses can be as elusive as the ghostlike particles he’s chased for decades, has taken his newfound scientific superstardom in stride. He gets it. The Nobel Prize is a pretty big deal. His place in the pantheon of great neutrino ghost hunters is secure. The Standard Model will fundamentally change, and he is directly responsible. You’ll just have to forgive him if he doesn’t go all Tiger King over the hubbub, because he’s checked his ego at the door a long time ago. With Takaaki Kajita, its all about the pursuit of science. Maybe its dark matter. Maybe its something else. Maybe proving that neutrinos have mass is lightning in a bottle. It doesn’t really matter, because, at the end of the day, doing good science is good enough for him.

Please tell me about your childhood; what are some of your earliest memories?

I grew up in the Japanese countryside in Higashi-Matsuyama, a small city located about an hour’s train ride north of Tokyo. I grew up in such a peaceful environment. My house was surrounded by rice fields on the north, east, and south, so I was surrounded by nature. I think that this was very important to me in terms of becoming a scientist.


At what point do you remember becoming interested in science?

To be honest, I became most interested in science when I got involved in my graduate courses. Almost accidentally, I entered Professor [Masatoshi] Koshiba’s group, just as he was starting the Kamiokande Experiment. While I didn’t know much about it, I quickly realized that it was really a very wonderful opportunity for me. I really enjoyed working on the Kamiokande Experiment. At that point, I essentially decided to become a physicist.

The 2015 Nobel physics laureate professor Takaaki Kajita (center) poses with his gold medal together with wife Michiko (left) and daughter (right) after the 2015 Nobel prize award ceremony. PHOTO: REUTERS

In what ways did your family help lay the foundation for such a successful career?

I think that this is a very difficult question. I’m not sure how much influence I received from my parents. When I was a child, Japan was still a rather poor country, but I think my parents were already thinking that I should go away to a university so that I could work in an exciting field.


Was there a particular teacher or class that help to fuel your interest in science and mathematics?

In high school, one of my teachers was a physicist, and from his lectures I discovered my interest in astronomy. Certainly, through his lectures, I thought that astronomy was quite interesting.


What was high school like for you?

I went to Kawagoe High School, a rather typical small-town school. This school had a tradition of allowing students to do whatever they liked rather freely. Therefore, I spent a lot of time practicing Kyudo (Japanese archery). I was not particularly good at Kyudo, but I liked it. During one’s time as a high school student, you have to decide what you intend to study as an undergraduate once you are admitted to a university. Since I was interested in physics as a high school student, my choice was rather clear: I decided to learn physics in the undergraduate course at Saitama University, a local university near Tokyo.

Takaaki Kajita

How would you describe yourself?

If I were to describe myself, I would say that I am simply extremely lucky. I was involved in experiments that I liked, and by accident I encountered a very interesting problem. I later I found that this problem was due to neutrino oscillations. So, I was very lucky.


Do you have a sense of humor?

Well certainly, I don’t have much of a sense of humor.


But in a news conference at the University of Tokyo, shortly after the Nobel announcement, you thanked the neutrinos for winning the award. And since neutrinos are created by cosmic rays, you thanked them, too. That’s funny!

I must admit, I thought it was funny, too [laughs].


What are some of your interests and hobbies outside of the scientific world?

To be honest, I am too busy to really have hobbies, so I have no hobbies. Okay, if you insist, drinking Japanese saki would be my hobby [laughs].


How about art, music, or literature?

Yes, I enjoy each, but I do not have much time to spend on these things.


Let’s talk about your undergraduate studies. You stayed close to home and attended Saitama University.  Were you initially unsure about what you wanted to study.

Officially I was learning physics at Saitama University, but, to be honest, I continued to spend a lot of time practicing Kyudo – even more seriously than during high school. I regret that I should have learned more physics during my undergraduate studies, because these studies in undergraduate courses form the basis of everyday research. Knowing this, I continued to spend a lot of time doing Kyudo – so much so that, until the latter half of the third year, my life was really focused on Japanese archery. That changed before the final year. I decided then that I wanted to learn more about physics, so I quit Japanese archery at that time. Well, at least after that I did not take Japanese archery so seriously [laughs]. So that was my life as an undergraduate student. In any case, I found that physics was indeed interesting. So I decided to continue to studying physics at the graduate level.


What excited you most about physics then, and what excites you most about it today?

Today, I am more interested in the research related to astronomy and astrophysics – the universe, cosmic rays, and dark matter. When I was taking the undergraduate courses, I was very interested in particle physics, particularly experimental particle physics. This is because I thought that theoretical physics was too difficult to me. I really enjoyed experiments, such as going underground, constructing the detectors, and so on. I enjoyed this work very much. So, I think I made the right decision.


You pursued your graduate studies at the University of Tokyo. Please tell me about his experience.

I learned everything about experimental particle physics in the graduate course at the University of Tokyo. As I mentioned earlier, I was particularly interested in experimental particle physics. Very fortunately, Professor Masatoshi Koshiba accepted me as a graduate course student in his group at the University of Tokyo. My life as a graduate course student began in April of 1981. Katsushi Arisaka was also a student in Professor Koshiba’s group. He had just finished his Master’s thesis based on a Monte Carlo study of a nucleon decay experiment. This was the Kamioka Nucleon Delay Experiment, also known as Kamiokande. He was the only student working on Kamiokande in early 1981. Just when I started my studies, production of newly developed photomultiplier tubes – PMTs – with a diameter of 50 centimeters began. Katsushi Arisaka convinced me that Kamiokande would be a very interesting experiment and asked me to work on it, which I started to do.

Friend and Mentor
2002 Nobel laureate Masatoshi Koshiba

You mention Professor Masatoshi Koshiba, who won the 2002 Nobel Prize in Physics.

Professor Koshiba was my physics advisor, therefore he had a very significant influence on me. Well, he is a big boss [laughs]. He would not tell us many of the details of our experiments, he would leave the research up to us. But he told us how one should be a scientist, especially how to be an experimental physicist.


You applied for postdoctoral studies through the Japan Society for the Promotion of Science (JSPS) but your application was rejected. Dr. Koshiba came to your rescue.

In Japan, the Japanese Society for the Promotion of Science posts positions that are  open to every scientific field. I applied, but unfortunately I was not selected. I had no idea what I was going to do after I obtained my PhD. Then, Professor Koshiba received approval for some positions from the University of Tokyo. I do not know the details about how he managed to do this, but afterwards he hired me. The position was for a fixed time…well, he told me that my job was only for one year, but eventually, I stayed in this position for two years.

Princess Sofia and physics laureate Takaaki Kajita

Let’s talk about Kamiokande. What were the early days like?

I enjoyed the preparation work for Kamiokande. In early spring of 1983, we started the construction work on the Kamiokande proton decay detector in Kamioka. It took almost four months to finish building the detector. I liked the construction work, watching the detector being assembled slowly but steadily. After it was filled with water, data taking with the Kamiokande experiment began in early July of 1983.


In 1986, you discovered that there was a significant deficit of the muon neutrino events when analyzing the Kamiokande data.

Oh yes, this moment was really crucial for me. I had a great feeling of excitement, and I also had the feeling that I made a mistake somewhere.


Neutrinos at that time were thought to be massless – and they were also at the heart of the Solar Neutrino Problem.

Well, as I said, I joined the Kamiokande experiment during my graduate course studies, and I received my PhD based on my research for proton decay at Kamiokande. I continued to work on Kamiokande after getting my PhD, as I was still interested in the proton decays and I wanted to improve the proton decay analysis. Through this kind of improvement study, I realized that there was something strange in the background of the proton decays in the atmospheric neutrinos. Basically, there was a substantial deficit of muon neutrinos in the atmospheric neutrino events, and that was the beginning of my interest in neutrinos.

Super-Kamiokande – Kamioka Mozumi mine in Japan – 1000m underground

Let’s talk about your research at the Super-Kamiokande Neutrino Observatory. How does it differ from the Sudbury Neutrino Observatory in Canada?

The detection principle of the neutrino interaction is similar to the Sudbury Neutrino Observatory. We detect radiation created by particles which are created by neutrino interactions. However, in terms of the details, our detectors are different. First of all, our detectors are not so deep. Our detectors are only 1 kilometer deep. Also, the structure of the detector is different. In SNO, they have kind of a cylindrical volume at the center of the detector to hold the heavy water. But in the case of Kamiokande and Super-Kamiokande, we simply use normal water. Therefore, we do not have the special container for the heavy water. We simply use a water tank to hold the water.

With heavy water, SNO is able to observe electron neutrinos from the sun. And also, SNO is also able to observe the total neutrino flux from the sun. But, in the case of normal water, we can observe these solar neutrinos by the neutrino electron scattering. So, the actual mechanism of detecting solar neutrinos are completely different.


What’s it like working with teams of other scientists and researchers?

I worked in both the Kamiokande and the Super-Kamiokande. Kamiokande was initially a small team. I think we had certainly more than 10 people, but, on the other hand, in the Super-Kamiokande, we had more than 100 people. So we had a big team.

At that time, the other main members of Kamiokande were Teruhiro Suda from the Institute for Cosmic Ray Research of the University of Tokyo, and Atsuto Suzuki and Kasuke Takahashi from the High Energy Accelerator Research Center. Soon after I joined the Kamiokande experiment, Yoji Totsuka returned from Deutsches Electron Synchroton and started to help us. Soon he too joined Kamiokande. Kazumasa Miyano from Niigata University and Tadashi Kifune from ICRR joined during the preparation stage of the experiment. Also, Masayuki Nakahata, who was an undergraduate course student, worked with us. So, as you can see, experiments like those being conducted at Kamiokande and Super-Kamiokande require the involvement of many different people with different strengths and skillsets.

The director of the University of Tokyo Cosmic Ray Research Institute, Nobel laureate winner Takaaki Kajita
Kazuyoshi Yamamoto photographing

Did you enjoy being part of a team?

Well, I do enjoy participating in teams. As far as the data analysis is concerned, I was one of the conveners of the atmospheric neutrino analysis. So, we had video meetings, which was the way to discuss and analyze the data. Also, we had collaboration meetings two or three times a year. I should mention that, in the analysis, both my Japanese and U.S. colleagues played very important roles in the initial stages of the Super-Kamiokande analysis.


Science takes creativity and creative thinking.  Do you have an example?

This is a very difficult question. It depends on how you define creativity [laughs]. Well, certainly we always tried to improve the analysis, but I don’t know how much of this activity is related to creativity. I’m not sure. I’m sorry. I don’t have a good answer to this question.


Since 1949, there have been twenty-eight Japanese winners of the Nobel Prize.  And you are only the ninth to winner the Nobel Prize for Physics. What does that mean to you?

I don’t ordinarily think about this question. Certainly, I feel that I’m very lucky because, well, for example, in 1949, it was not possible to carry out experimental research in Japan. But, in my time, I was able to conduct experimental physics. For that I feel really lucky.


In what ways did winning the Nobel Prize change your life?

Oh yes, since winning the Nobel Prize I have been very busy. One good thing is, I have opportunities to speak about the problems facing Japanese scientific systems. So, that is a good thing.


The Nobel Prize is the ultimate award, but I get the sense that your focus has always been on the science, and that there was never a preoccupation with whether or not you would win it.

Oh yes. Yes, absolutely. As I said, in Kamiokande, I accidentally found the deficit of atmospheric muon neutrinos, and that was really important for me. I really wanted to understand what was going on and that was essentially the only motivation for me to carry out the research.


It’s not every day that someone’s work challenges the Standard Model the way yours did. To me, the significance of that statement is far more profound than the actual Nobel Prize. Do you ever stop to think about that?

When we found the deficit of muon neutrinos we thought this could be due to neutrino oscillations. And, of course we realized that if it is indeed due to neutrino oscillations, this could be a very important work.

Takaaki Kajita

Final Question: You’ve achieved great success in your life. If you could offer one piece of advice to aspiring scientists, what would that be?

For many people, the reason he or she becomes a scientist is an interest in nature. So, I hope that many young people, although they may be busy, don’t forget their initial interest in nature. I believe that this is the single most important thing in order to become a good scientist.

Michael McClellan
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