Toshihide Maskawa

Toshihide Maskawa
Maskawa in 2008
Toshihide Maskawa
Born	7 2, 1940
Birthplace	Nagoya, Japan
Died	Template:Death date and age Kyoto, Japan
Nationality	Japanese
Occupation	Theoretical physicist
Known for	CP violation, CKM matrix
Spouse(s)	Akiko Takahashi
Children	2
Awards	Nobel Prize in Physics (2008) Sakurai Prize (1985) Japan Academy Prize (1985)

Toshihide Maskawa (益川 敏英, Masukawa Toshihide; 7 February 1940 – 23 July 2021) was a Japanese theoretical physicist whose work on CP violation fundamentally reshaped the understanding of particle physics and the asymmetry between matter and antimatter in the universe. Born in Nagoya, Japan, to a family whose livelihood was upended by the Second World War, Maskawa rose through the Japanese academic system to become one of the country's most celebrated scientists. Together with his colleague Makoto Kobayashi, he proposed in 1973 that the observed violation of charge-parity symmetry in kaon decay could be explained if there existed at least three generations of quarks—a prediction that was experimentally confirmed over subsequent decades and that became a cornerstone of the Standard Model of particle physics. For this discovery, Maskawa and Kobayashi were awarded one half of the 2008 Nobel Prize in Physics, with the other half going to Yoichiro Nambu "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics." The theoretical framework they established, parametrised by the Cabibbo–Kobayashi–Maskawa matrix (CKM matrix), remains one of the most important constructs in high-energy physics. Maskawa spent the bulk of his career at Kyoto University and Nagoya University, and was also known later in life as an advocate for peace and the preservation of Japan's pacifist constitution. He died on 23 July 2021 in Kyoto from gingival cancer at the age of 81.

Early Life

Toshihide Maskawa was born on 7 February 1940 in Nagoya, a major industrial city in central Japan. His family ran a small business, but the economic devastation wrought by the Second World War and the firebombing of Nagoya in 1945 severely disrupted their livelihood. Maskawa later recalled the hardship of his childhood in war-torn Japan. His father had operated a sugar wholesale business, but the family's fortunes declined sharply during and after the war.

Growing up amid the reconstruction of postwar Japan, Maskawa developed an early interest in science and mathematics. He attended local schools in Nagoya, where he showed academic promise. His formative years were shaped both by the intellectual culture of postwar Japanese academia and by the lingering memories of wartime destruction—experiences that would later inform his outspoken advocacy for peace and his opposition to militarism.

Nagoya in the postwar period was home to a vibrant physics community, centered around Nagoya University, which would become the institution most central to Maskawa's intellectual development. The university's physics department, under the leadership of Shoichi Sakata, had become a leading center for theoretical particle physics in Japan. Sakata's group was known for its collaborative spirit and its emphasis on symmetry principles in particle physics—an intellectual environment that would prove decisive in shaping Maskawa's career trajectory.

Education

Maskawa enrolled at Nagoya University, where he studied physics as both an undergraduate and graduate student. He came under the mentorship of Shoichi Sakata, a prominent theoretical physicist known for the Sakata model, which proposed that hadrons were composed of more fundamental constituents—a precursor to the modern quark model. Sakata served as Maskawa's doctoral advisor, and under his guidance, Maskawa immersed himself in the study of elementary particle physics and symmetry principles.

Maskawa completed his doctoral thesis in 1967, titled "On the Mixing Effect of Particles and Quasi-Particles" (粒子と共鳴準位の混合効果について). The Nagoya group's emphasis on composite models and flavour symmetry provided Maskawa with the theoretical tools and intellectual framework that he would later apply to the problem of CP violation. It was also at Nagoya University that Maskawa first met Makoto Kobayashi, who joined the graduate program in 1965 and with whom Maskawa would conduct his most important work.

Career

Early Academic Career at Nagoya University

After completing his doctorate, Maskawa remained at Nagoya University as a research associate and then lecturer in the physics department. He continued to work within the tradition established by Sakata, focusing on the theoretical underpinnings of elementary particle physics. The Nagoya group during this period was engaged in extending and refining models of hadron structure and the weak interaction, and Maskawa was an active participant in these efforts.

It was during this period that Maskawa deepened his collaboration with Makoto Kobayashi. The two physicists shared an interest in the problem of CP violation—the phenomenon in which the combined symmetry of charge conjugation (C) and parity (P) is not conserved in certain weak interaction processes. CP violation had been experimentally observed in 1964 by James Cronin and Val Fitch in the decay of neutral kaons, but a satisfactory theoretical explanation remained elusive within the existing framework of particle physics, which at the time recognized only two generations (or "families") of quarks.

The Kobayashi–Maskawa Theory

In 1973, Kobayashi and Maskawa published their landmark paper "CP-Violation in the Renormalizable Theory of Weak Interaction" in the journal Progress of Theoretical Physics. This paper, which would become one of the most cited works in the history of particle physics, proposed a mechanism to explain the observed CP violation by extending the quark model.

At the time, the standard theory of weak interactions developed by Sheldon Glashow, Abdus Salam, and Steven Weinberg incorporated only two generations of quarks: the up/down pair and the charm/strange pair. The mixing between these quark flavours in weak decays was described by the Cabibbo angle, introduced by Nicola Cabibbo in 1963. However, with only two generations, the quark mixing matrix did not contain enough free parameters to accommodate a complex phase—the mathematical ingredient necessary to produce CP violation.

Kobayashi and Maskawa demonstrated that if one postulated the existence of a third generation of quarks—making six quarks in total—the resulting 3×3 quark mixing matrix would contain an irreducible complex phase. This complex phase would naturally give rise to CP violation in weak decays, providing a theoretical origin for the phenomenon observed by Cronin and Fitch. The 3×3 unitary matrix describing quark mixing became known as the Cabibbo–Kobayashi–Maskawa (CKM) matrix, acknowledging Cabibbo's earlier work on the two-generation case.

The proposal was bold: at the time of publication, only three quarks (up, down, and strange) had been experimentally confirmed, and even the existence of the fourth quark (charm) had not yet been established. Kobayashi and Maskawa were predicting not just one, but two additional quarks beyond what was then known. The discovery of the charm quark in 1974 (the "November Revolution"), the bottom quark in 1977, and finally the top quark in 1995 progressively confirmed the three-generation structure that Kobayashi and Maskawa had proposed.

Direct experimental verification of the CP-violating phase in the CKM matrix came in the early 2000s, when the BaBar experiment at SLAC and the Belle experiment at KEK confirmed CP violation in the decay of B mesons, consistent with the predictions of the Kobayashi–Maskawa theory. The Kobayashi–Maskawa paper became one of the most highly cited articles in the history of high-energy physics.

The CKM matrix also has profound cosmological implications. The asymmetry between matter and antimatter in the universe—the reason that matter predominates over antimatter and that the universe as we know it exists—requires, among other conditions, the violation of CP symmetry, as identified by Andrei Sakharov in 1967. While the amount of CP violation described by the CKM matrix alone appears insufficient to fully account for the observed matter–antimatter asymmetry, the Kobayashi–Maskawa mechanism established the first concrete, experimentally verified source of CP violation within the Standard Model.

Career at Kyoto University

Following the publication of the 1973 paper, Maskawa moved to Kyoto University, where he spent the main portion of his academic career. He joined the faculty of the university's physics department and continued his research in theoretical high-energy physics. Kyoto University had a distinguished tradition in theoretical physics, having been the home institution of Hideki Yukawa, Japan's first Nobel laureate in physics, who had established the Yukawa Institute for Theoretical Physics (YITP) there.

At Kyoto University, Maskawa held various positions and continued to contribute to particle physics theory, working on problems related to the weak interaction, quark mixing, and related areas. He mentored numerous graduate students and younger researchers, contributing to the development of the next generation of Japanese theoretical physicists.

Kobayashi–Maskawa Institute and Later Career

Maskawa also held a position at Kyoto Sangyo University, where he was affiliated with the Maskawa Institute for Science and Culture (originally the Research Center for Mathematical Sciences and Computing). At Nagoya University, the Kobayashi–Maskawa Institute for the Origin of Particles and the Universe (KMI) was established in his and Kobayashi's honour, reflecting the lasting impact of their work on the university's research mission. Maskawa served as the director of this institute and contributed a message articulating its research vision.

Nobel Prize

On 7 October 2008, the Royal Swedish Academy of Sciences announced that Maskawa and Makoto Kobayashi would share one half of the Nobel Prize in Physics "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature." The other half of the prize was awarded to Yoichiro Nambu, a Japanese-born American physicist, "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics."

In an interview with Nobelprize.org following the announcement, Maskawa discussed the development of his ideas and the intellectual journey that led to the 1973 paper. At the Nobel Prize ceremony in Stockholm on 10 December 2008, Kobayashi delivered a banquet speech on behalf of both laureates, expressing their gratitude to the Royal Swedish Academy of Sciences.

Maskawa delivered his Nobel Lecture titled "What Does CP Violation Tell Us?" on the occasion of the prize, outlining the theoretical reasoning behind his and Kobayashi's work and its implications for understanding the fundamental structure of matter.

Maskawa attracted media attention for his modest and unassuming demeanour, as well as for the fact that he had never held a passport before travelling to Stockholm for the Nobel ceremony. He was also noted for having conducted his entire career within Japan, never having studied or worked abroad—a rarity among Nobel laureates in physics.

Personal Life

Maskawa married Akiko Takahashi, and the couple had two children. He was known for his reserved and modest personality, characteristics that were widely noted in media coverage of his Nobel Prize. Unlike many laureates in the sciences, Maskawa had spent his entire career at Japanese institutions and had rarely travelled internationally before the Nobel ceremony.

In his later years, Maskawa was an outspoken advocate for peace and for the preservation of Article 9 of the Japanese constitution, which renounces war as a means of settling international disputes. He was associated with the "Article 9 Association of Scientists" (九条科学者の会), a group of Japanese intellectuals committed to defending Japan's pacifist constitutional provisions. His peace activism was informed by his childhood experiences during and after the Second World War, and he spoke publicly about the importance of Japan maintaining its commitment to pacifism.

Toshihide Maskawa died on 23 July 2021 in Kyoto, Japan, from gingival cancer. He was 81 years old.

Recognition

Maskawa received numerous awards and honours over the course of his career in recognition of his contributions to theoretical physics:

• Sakurai Prize (1985) – Awarded by the American Physical Society for distinguished contributions to particle physics theory. Maskawa shared this honour with Kobayashi.
• Japan Academy Prize (1985) – Awarded by the Japan Academy for outstanding research achievements.
• Asahi Prize (1994) – One of Japan's most prominent cultural and scientific awards, recognising contributions to academic and cultural life.
• Nobel Prize in Physics (2008) – Shared with Makoto Kobayashi (one half) and Yoichiro Nambu (the other half), for work on broken symmetry in particle physics.

The Kobayashi–Maskawa Institute for the Origin of Particles and the Universe (KMI) at Nagoya University was named in honour of Maskawa and Kobayashi, serving as a research center dedicated to exploring fundamental questions in particle physics and cosmology. At Kyoto Sangyo University, the Maskawa Institute for Science and Culture carries his name.

Maskawa's Nobel Lecture, "What Does CP Violation Tell Us?", has been archived by the Nobel Foundation and remains a widely referenced document in the field.

Legacy

The theoretical framework established by Kobayashi and Maskawa in 1973 has had a lasting and profound impact on particle physics. The CKM matrix remains a central element of the Standard Model, describing the mixing of quark flavours in weak decays and encoding the sole source of CP violation within the quark sector of the Standard Model. The precision measurement of CKM matrix elements has been a major objective of experimental particle physics programmes worldwide, including the B-factory experiments at SLAC (BaBar) and KEK (Belle), as well as experiments at CERN's Large Hadron Collider (LHCb).

In 2023, an international event was held to celebrate the 50th anniversary of the Kobayashi–Maskawa theory, reflecting its enduring significance in the field. The paper by Kobayashi and Maskawa has remained among the most highly cited in the history of high-energy physics, as documented by citation databases maintained by SLAC.

Maskawa's career is also notable for demonstrating the strength and depth of the Japanese tradition in theoretical particle physics, a tradition stretching from Yukawa and Sakata through to the present day. His work, conducted entirely within Japan and rooted in the collaborative intellectual culture of the Nagoya and Kyoto schools, produced results of the highest international significance. The institutions with which he was associated—Nagoya University, Kyoto University, and Kyoto Sangyo University—have continued to honour his legacy through their research programmes and named institutes.

Beyond physics, Maskawa's advocacy for peace and his public engagement with constitutional issues in Japan added a civic dimension to his legacy. He spoke from the authority of personal experience, having lived through the destruction of war as a child, and he used his public platform as a Nobel laureate to argue for the maintenance of Japan's pacifist principles.