Born: September 29, 1931, Chicago, Illinois [Died] August 25, 2016. Particle physicist in Saint Paul, Minnesota, USA. He received his bachelor's degree from Southern Methodist University in 1951, his master's degree from the University of Chicago in 1953, and his doctorate from the same university in 1955. He worked at Brookhaven National Laboratory from 1955 to 1958. He moved to Princeton University in 1958 and was a professor there from 1964 to 1971. He was a professor at the University of Chicago from 1971 to 1997, and became a professor emeritus at the university in 1997. In 1964, in collaboration with Val Logsdon Fitch and others, he precisely measured the decay of neutral K mesons (→ K mesons) formed in an accelerator, and discovered that there are two types of neutral K mesons, short-lived and long-lived, and that both decay into two π mesons. This means that in this elementary particle reaction, the invariance with respect to charge conjugation transformation (C transformation) and space inversion (P transformation; → parity), in other words the invariance with respect to time reversal (→ CP symmetry), is violated. Prior to this, in 1956, Tsung-Do Lee and Chen-Ning Yang had proposed that the weak interaction does not have the symmetry of the P transformation, but even after that it was thought that symmetry was maintained with respect to the CP transformation, which performs the P transformation as well as the C transformation. For his discovery of the violation of CP symmetry, he was awarded the Nobel Prize in Physics (→ Nobel Prize) together with Fitch in 1980. (→ Elementary particle physics) Cronin |
[生]1931.9.29. イリノイ,シカゴ [没]2016.8.25. ミネソタ,セントポール アメリカ合衆国の素粒子物理学者。1951年南メソジスト大学で学士号,1953年シカゴ大学で修士号,1955年同大学で博士号を取得。1955~58年ブルックヘブン国立研究所に勤務。1958年プリンストン大学に移り,1964~71年同大学教授。1971~97年シカゴ大学教授。1997年同大学名誉教授となった。1964年にバル・ログスドン・フィッチらと共同で,加速器中で形成された中性K中間子(→K中間子)の崩壊を精密に測定し,中性K中間子には短寿命と長寿命の 2種があること,また両者とも 2個のπ中間子に崩壊することを発見。これは,この素粒子反応において荷電共役変換(C変換)および空間反転(P変換。→パリティ)に対する不変性,換言すれば時間反転に対する不変性(→CP対称性)が破れることを意味する。これに先立つ 1956年,李政道と楊振寧により,弱い相互作用は P変換の対称性をもたないことが提唱されていたが,その後も C変換とともに P変換を行なう CP変換に関しては対称性が保たれると考えられていた。CP対称性の破れを発見した功績により,1980年フィッチとともにノーベル物理学賞(→ノーベル賞)を受賞した。(→素粒子物理学) クローニン
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