Takaaki Kajita

Takaaki Kajita
Kajita in 2017
Takaaki Kajita
Born	9 3, 1959
Birthplace	Higashimatsuyama, Saitama, Japan
Nationality	Japanese
Occupation	Physicist
Title	Director, Institute for Cosmic Ray Research; President, Science Council of Japan (2020–)
Employer	University of Tokyo
Known for	Discovery of neutrino oscillations at Super-Kamiokande
Education	PhD, University of Tokyo
Spouse(s)	Michiko
Awards	Nobel Prize in Physics (2015), Fundamental Physics Prize (2016), Julius Wess Award (2013)

Takaaki Kajita (梶田 隆章, Kajita Takaaki; born 9 March 1959) is a Japanese physicist whose experimental work on neutrinos fundamentally changed the understanding of particle physics. Working at the Kamioka Observatory in Japan, Kajita led research at the Kamiokande and Super-Kamiokande detectors that provided the first compelling evidence that neutrinos oscillate between different types, or "flavors" — a phenomenon that implies neutrinos possess mass, contrary to the predictions of the Standard Model of particle physics. For this discovery, Kajita was awarded the 2015 Nobel Prize in Physics, jointly with Canadian physicist Arthur B. McDonald, who independently confirmed neutrino oscillations using a different detection method at the Sudbury Neutrino Observatory in Canada. A student of Nobel laureate Masatoshi Koshiba, Kajita has spent his career at the University of Tokyo, where he serves as director of the Institute for Cosmic Ray Research (ICRR) and as a principal investigator at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU). On 1 October 2020, he became the president of the Science Council of Japan. In recent years, Kajita has also been involved in gravitational wave research, particularly in connection with the KAGRA detector and the proposed Einstein Telescope project.

Early Life

Takaaki Kajita was born on 9 March 1959 in Higashimatsuyama, a city in Saitama Prefecture, Japan. Situated in the central part of the Kantō plain, northwest of Tokyo, Higashimatsuyama is a modest-sized city that provided the setting for Kajita's formative years. Details about his family background and childhood remain largely private, though Kajita has described his early years as unremarkable, with little indication that he would go on to a career at the forefront of fundamental physics.

Kajita attended Saitama Prefectural Kawagoe High School, one of the oldest and most academically distinguished public high schools in Saitama Prefecture. Kawagoe High School, which has a long tradition of producing graduates who go on to prominent careers in science, the civil service, and the arts, provided Kajita with a strong educational foundation. It was during his high school years that Kajita developed an interest in the natural sciences, though he has spoken modestly about his abilities as a student and did not consider himself exceptional. This self-effacing nature would become a hallmark of Kajita's public persona throughout his career, including after he achieved international recognition as a Nobel laureate.

Growing up in the Saitama region, Kajita was within reach of Tokyo's major research universities, a geographic proximity that would prove significant in shaping the trajectory of his academic career. After completing his secondary education at Kawagoe High School, Kajita chose to remain in his home prefecture for his undergraduate studies before moving to the University of Tokyo for graduate work.

Education

Kajita enrolled at Saitama University, where he completed his Bachelor of Science degree in physics. Saitama University, a national university located in the capital city of Saitama Prefecture, offered Kajita a solid grounding in physics that prepared him for advanced graduate study.

For his graduate education, Kajita moved to the University of Tokyo, one of Japan's most prestigious research institutions. He earned a Master of Science degree and subsequently completed his Doctor of Philosophy (PhD) at the university. His doctoral research was supervised by Masatoshi Koshiba, a physicist who would himself go on to win the Nobel Prize in Physics in 2002 for his pioneering contributions to astrophysics, including the detection of cosmic neutrinos. Kajita also worked closely with Yoji Totsuka, another prominent physicist at the University of Tokyo who served as an academic advisor and collaborator throughout Kajita's early career. The mentorship of Koshiba and Totsuka placed Kajita at the center of Japan's neutrino physics program, which was then in an early but rapidly developing stage. It was under their guidance that Kajita became deeply involved in the Kamiokande experiment, which would form the foundation of his most significant scientific contributions.

Career

Kamiokande and Early Neutrino Research

Kajita's career in experimental particle physics began with his involvement in the Kamiokande experiment, a large water Cherenkov detector located deep underground in the Kamioka mine in Gifu Prefecture, Japan. The Kamiokande detector was originally designed to search for proton decay, a hypothetical process predicted by certain grand unified theories of particle physics. However, the detector also proved to be a powerful instrument for detecting neutrinos — nearly massless subatomic particles that interact only weakly with matter and are produced in vast quantities by nuclear reactions in the Sun and by cosmic ray interactions in Earth's atmosphere.

During the 1980s, Kajita and his colleagues at Kamiokande began studying atmospheric neutrinos — neutrinos produced when high-energy cosmic rays strike atomic nuclei in the upper atmosphere, creating showers of secondary particles, including pions and kaons, that decay into muons and neutrinos. In 1988, Kajita presented evidence at a conference that the number of muon neutrinos detected by Kamiokande was significantly less than the number predicted by theoretical models, while the number of electron neutrinos was roughly consistent with expectations. This discrepancy, which became known as the "atmospheric neutrino anomaly," hinted at a profound new physical phenomenon: the possibility that neutrinos could transform, or "oscillate," from one type to another during their flight from the atmosphere to the detector.

The atmospheric neutrino anomaly was an early and tantalizing clue, but the statistics from Kamiokande were not sufficient to establish the result conclusively. Confirming or refuting neutrino oscillations would require a much larger and more sensitive detector.

Super-Kamiokande and the Discovery of Neutrino Oscillations

The Super-Kamiokande detector, which began operations in 1996, was the successor to Kamiokande and represented a massive increase in scale and sensitivity. Located in the same Kamioka mine, Super-Kamiokande contained 50,000 tons of ultra-pure water and was lined with approximately 11,200 photomultiplier tubes — exquisitely sensitive light detectors capable of registering the faint flashes of Cherenkov radiation produced when neutrinos interact with water molecules. Kajita played a central role in the design, construction, and scientific program of Super-Kamiokande.

In 1998, the Super-Kamiokande collaboration, with Kajita as a leading figure, presented definitive evidence for neutrino oscillations at the Neutrino '98 conference held in Takayama, Japan. The key observation was a striking asymmetry in the flux of muon neutrinos arriving at the detector from different directions. Neutrinos produced in the atmosphere directly above the detector (traveling a short distance) arrived in approximately the expected numbers. However, neutrinos produced on the opposite side of the Earth (traveling a distance of roughly 13,000 kilometers through the planet before reaching the detector) arrived in significantly reduced numbers. This zenith-angle dependence was precisely the pattern predicted by the theory of neutrino oscillations: muon neutrinos, during their long journey through the Earth, were transforming into tau neutrinos, which the detector was not designed to observe as efficiently.

The 1998 announcement was a landmark in particle physics. If neutrinos oscillate between flavors, they must possess mass — a conclusion that directly contradicted the Standard Model of particle physics, which had assumed neutrinos to be massless. The discovery opened a new chapter in fundamental physics, providing the first concrete evidence for physics beyond the Standard Model. The result was met with widespread recognition in the scientific community and was subsequently confirmed by numerous independent experiments around the world, including the Sudbury Neutrino Observatory (SNO) led by Arthur B. McDonald, which demonstrated that solar neutrinos also undergo oscillations.

Kajita's contribution was recognized by the Nobel Committee as a "key contribution" to the experiments that demonstrated neutrino oscillations, describing the discovery as one that "changed our understanding of the innermost workings of matter."

Leadership at the University of Tokyo

Throughout his career, Kajita has been based at the University of Tokyo, where he rose through the academic ranks to become one of the institution's most prominent scientists. He serves as a director of the Institute for Cosmic Ray Research (ICRR), a major research institute within the university that operates the Super-Kamiokande detector and other large-scale physics experiments. He is also a principal investigator at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), an international research institute affiliated with the University of Tokyo that brings together physicists, mathematicians, and astronomers to address fundamental questions about the universe.

Gravitational Wave Research and KAGRA

In addition to his work on neutrino physics, Kajita has been involved in gravitational wave research. He has played a significant role in the development of KAGRA (formerly the Large-scale Cryogenic Gravitational wave Telescope), an underground gravitational wave detector located in the Kamioka mine — the same site as the Super-Kamiokande detector. KAGRA is notable for being the first gravitational wave observatory to operate underground and to use cryogenic cooling of its mirrors, design features intended to reduce seismic and thermal noise and thereby improve sensitivity.

Kajita has also been involved in international discussions about the proposed Einstein Telescope (ET), a next-generation gravitational wave detector that is being planned in Europe. In July 2025, Kajita participated in an event at Expo 2025 in Osaka, where the Einstein Telescope project — for which Italy has proposed Sardinia as a candidate site — was highlighted. This involvement reflects Kajita's broader interest in using large-scale underground detectors to explore fundamental physics beyond neutrino oscillations.

Science Council of Japan

On 1 October 2020, Kajita assumed the presidency of the Science Council of Japan (SCJ), the nation's highest-level representative body of scientists. The Science Council of Japan advises the government on matters of science and technology policy and represents the Japanese scientific community in international forums. Kajita's appointment to this position reflected both his scientific stature and his broader role as a public advocate for basic research and scientific education in Japan.

Public Lectures and International Engagement

Following his Nobel Prize, Kajita has been active as a public communicator of science, delivering lectures at universities and research institutions worldwide. In November 2025, he delivered a lecture at Queen Mary University of London on his research and career, speaking to an audience of students and staff. In January 2026, he gave a public lecture on gravitational waves at Phenikaa University in Vietnam, engaging with Vietnamese students about the frontiers of physics research. In December 2025, he was invited to attend the 51st convocation of Tribhuvan University in Nepal as a guest of honor.

In September 2025, Kajita praised India's rise in the Global Innovation Index and remarked that Japan should learn from India's progress in this area. These engagements reflect Kajita's role as an international figure in the scientific community and his interest in fostering scientific exchange across national boundaries.

Personal Life

Kajita is married to Michiko. Kajita is known for his modest and self-effacing demeanor. In interviews following his Nobel Prize, he expressed surprise at receiving the award and spoke about his work in characteristically understated terms. He has described his early realization that neutrinos were behaving differently than expected as a moment of confusion rather than triumph, noting that the anomalous data initially presented more questions than answers.

Kajita continues to reside in Japan, where he maintains his positions at the University of Tokyo.

Recognition

Kajita has received numerous awards and honors for his contributions to physics. His most prominent recognition is the 2015 Nobel Prize in Physics, which he shared with Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass." The Nobel Committee credited their work with changing the understanding of the fundamental properties of matter.

Prior to the Nobel Prize, Kajita received several significant awards. In 1999, he was awarded the Asahi Prize, one of Japan's most notable awards recognizing contributions to academic and cultural fields. In 2002, he received the Panofsky Prize from the American Physical Society, one of the premier awards in experimental particle physics, for his contributions to the study of neutrino oscillations.

In 2013, Kajita was awarded the Julius Wess Award from the Karlsruhe Institute of Technology (KIT) in Germany, recognizing outstanding achievements in theoretical physics and its experimental verification.

Kajita was also among the recipients of the 2016 Fundamental Physics Prize (Breakthrough Prize in Fundamental Physics), which was awarded to the Super-Kamiokande collaboration and other neutrino oscillation experiments.

He has also received the Nishina Memorial Prize, one of Japan's most distinguished prizes in physics. In addition, he has been honored by the Japan Academy.

He was the recipient of the first Totsuka Prize (Orito-Totsuka Prize), named in part after his academic advisor Yoji Totsuka.

Kajita has also been recognized with the Bruno Rossi Prize from the American Astronomical Society's High Energy Astrophysics Division.

Legacy

The discovery of neutrino oscillations, to which Kajita made a central contribution, is considered one of the most important advances in particle physics in the late twentieth and early twenty-first centuries. By demonstrating that neutrinos have mass, the Super-Kamiokande results provided the first experimental evidence for physics beyond the Standard Model — the theoretical framework that had successfully described particle physics for decades but which assumed neutrinos to be massless. The discovery has had far-reaching implications, motivating new theoretical work on the origin of neutrino mass, the matter-antimatter asymmetry of the universe, and the structure of grand unified theories.

The experimental methodology developed at Kamiokande and Super-Kamiokande — using large underground water Cherenkov detectors to study rare particle interactions — has influenced the design of numerous subsequent experiments worldwide. The success of these detectors demonstrated the viability of underground particle astrophysics and helped establish Japan as a leading center for neutrino research. The Kamioka mine site, where both the neutrino detectors and the KAGRA gravitational wave detector are located, has become one of the world's most important underground laboratories for fundamental physics research.

Kajita's career also represents a notable lineage in Japanese physics. As a student of Masatoshi Koshiba, who shared the 2002 Nobel Prize in Physics, and a collaborator of Yoji Totsuka, Kajita is part of a tradition of neutrino physics research at the University of Tokyo that has produced multiple generations of influential physicists and two Nobel Prizes. This tradition reflects the strength of Japan's investment in large-scale experimental physics and in the long-term scientific programs that have characterized the Kamioka Observatory since its inception.

Through his presidency of the Science Council of Japan and his international lecturing activities, Kajita has continued to advocate for the importance of basic scientific research and international scientific cooperation. His engagement with students and institutions across Asia, Europe, and beyond underscores his commitment to the global scientific enterprise.