Czundao Lee Physicist Date of Birth: 25.11.1926 Country: China

1. Early Life and Education
2. Research and Career
3. The Parity Puzzle
4. The Parity Violation Experiment
5. The Aftermath
6. Recognition and Honors

Early Life and Education

Tsung-Dao Lee, a Chinese-American physicist, was born in Shanghai in 1926. The third of six children to businessman Li Jinqun and Chang Mingchang, Lee graduated from Jiangsu Secondary School in Kunshan in 1943. He enrolled at Zhejiang University in Guizhou, but the Japanese invasion forced the university to relocate to Kunming, where it became part of the amalgamation known as the National Southwestern Associated University. Lee and fellow physicist Chen Ning Yang, who would later become his colleague, were students at the university in Kunming. Lee graduated with a Bachelor of Science in physics in 1946.

That same year, Lee traveled to the United States on a Chinese government scholarship to study at the University of Chicago, where he worked under Enrico Fermi. There, he also met Yang, who had also received a Chinese government scholarship. In 1950, Lee completed his doctoral dissertation, titled "Hydrogen Content of White Dwarf Stars."

Research and Career

After spending a few months as a research assistant in astrophysics at Yerkes Astronomical Observatory in 1950, Lee joined the University of California, Berkeley as a research assistant in physics. In 1951, Lee and Yang reunited at the Institute for Advanced Study in Princeton, New Jersey. In 1953, Lee became an assistant professor at Columbia University and was promoted to full professor in 1956. At the age of 29, he became the youngest professor in Columbia's history.

The Parity Puzzle

Lee and Yang's friendship had deepened during their two years at Princeton. After Lee returned to Columbia and Yang remained at the Institute for Advanced Study, the pair began meeting regularly for lunch to discuss scientific problems. One of these problems concerned two seemingly different types of K-mesons, unstable particles found among the fragments of high-energy bombardments of atomic nuclei. K-mesons decayed differently: one type (called the theta meson) decayed into two pions, while the other (called the tau meson) decayed into three pions.

However, experimental evidence suggested that the tau and theta mesons were actually the same particle, sharing the same mass and lifetime. The most serious challenge to considering the tau and theta mesons as different particles was the law of parity, an aspect of one of the most fundamental symmetries in physics. Parity, in essence, means that the interactions of particles and their mirror images obey the same physical laws and are indistinguishable. Nature does not favor left over right, and we should expect the outcome of any experiment to be unbiased.

Particles or energy states have definite parity and are referred to as being even (+1) or odd (-1). The law of parity states that the parity of a decaying particle is equal to the product of the parities of the particles into which it decays, so the total parity remains unchanged. Since the parity of a pion is -1, the parity of a system of two pions is (-1)(-1) = +1. Therefore, the theta meson, which decays into two pions, should have a parity of +1, while the tau meson, which decays into three pions, should have a parity of (-1)(-1)(-1) = -1. Thus, the law of parity required that the theta and tau mesons be different particles. However, the convincing experimental evidence of their similarities contradicted this conclusion. Lee and Yang set out to unravel this puzzle.

The Parity Violation Experiment

The law of parity had first been clearly formulated in 1925 and had gained widespread acceptance since then, as its application in theoretical and experimental investigations proved remarkably fruitful. Moreover, the idea of parity seemed intuitively reasonable: why should nature have a preference for one direction over another? Physicists recognize four fundamental interactions: the strong force (between nucleons – the particles that make up the nucleus), the electromagnetic force (between charged particles), the weak force (involved in the emission of particles during radioactive decay), and the gravitational force (between any masses).

In their quest to resolve the tau-theta meson problem, Lee and Yang re-examined the experimental evidence supporting parity conservation. To their surprise, they found that there was abundant data demonstrating the conservation of parity in strong or electromagnetic interactions, but none that confirmed parity conservation in weak interactions. Gravitational interaction, the weakest of the four, is usually negligible in subatomic particle interactions. Experimentalists had never directly tested the conservation of parity in weak interactions, possibly because they assumed its validity. In the decay processes of the tau and theta mesons, it was the weak interaction that played the dominant role.

Lee and Yang, primarily theorists, proposed several experiments to provide a decisive answer to the question of left-right symmetry in weak interactions. After six months of painstaking preparation, one such experiment was carried out in 1956-1957 by Columbia University researcher Wu Chien-shiung and others at the US National Bureau of Standards in Stanford. Radioactive cobalt, transformed into nickel during decay and releasing excess energy as beta radiation (an electron) and a neutrino (a particle with zero mass and zero charge), was placed into an electromagnet coil and cooled to near absolute zero, minimizing thermal effects. Since atoms and their nuclei behave in some ways like tiny magnets, most of the cobalt atoms aligned themselves parallel to the strong magnetic field within the coil, its direction serving as a reference.

Beta decay (electron emission) is a consequence of the weak interaction. If parity were conserved in the decay of cobalt, an equal number of electrons should be emitted toward the north and south magnetic poles of the source. Wu's results showed conclusively that more electrons were emitted from the south magnetic pole than from the north. Thus, parity was not conserved in weak interactions.

The experimental outcome came as a surprise to Lee and Yang themselves, despite their daring hypothesis. It was quickly corroborated by other experiments, conducted at Columbia University by Richard L. Garwin, Leon Lederman, and Marcel Weinrich. These experimenters used the decay of pions into muons, followed by the decay of muons into electrons and neutrinos (or antineutrinos). They found that the muons and electrons were emitted asymmetrically upward and downward, as would be expected if parity were conserved.

Subsequent experiments in various laboratories extended the demonstration of parity violation to the decays of other particles.

The Aftermath

The fall of the long-held law of parity conservation opened the way to a solution to the tau-theta meson puzzle: the same particle could decay via two different pathways. It also opened up new avenues of research and fueled hope for progress toward the goal set by Albert Einstein of a unified theory encompassing all four fundamental interactions.

Lee and Yang were awarded the 1957 Nobel Prize in Physics "for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding elementary particles." In the presentation speech, O.B. Klein of the Royal Swedish Academy of Sciences stated that "Your clear and unbiased thinking has enabled you to cut the Gordian knot of elementary particle physics, in which experimental and theoretical work is now proceeding at a rapid pace thanks to your brilliant achievement."

Lee's scientific interests were wide-ranging. He made significant contributions to diverse fields of physics, including field theory, statistical mechanics (the study of the atomic origin of thermal phenomena), hydrodynamics, turbulence theory, and astrophysics. In 1950, Lee married Chin Shui-chang (Jeannette), and they had two sons. Colleagues describe Lee as a modest and introverted man. For recreation, he enjoys reading detective stories and listening to music. In 1963, Lee became a naturalized citizen of the United States.

Recognition and Honors

In addition to the Nobel Prize, Lee received the 1957 Albert Einstein Prize awarded by the Yeshiva University. In 1958, Princeton University awarded him an honorary doctorate. He is a member of the National Academy of Sciences and the American Physical Society.