Cronin, James Watson

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James Watson Cronin

American astrophysicist James Watson Cronin (born 1931) is a pioneer in ultrahigh-energy gamma ray astronomy as well as a professor emeritus at the University of Chicago. Cronin won the 1980 Nobel Prize with colleague Val Logsdon Fitch for their discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons. Through their work, Cronin and Fitch proved that the reactions of subatomic particles are not indifferent to time. In addition to his research in the physical sciences, Cronin taught for many years at the University of Chicago.

Born into World of Academia

Cronin was born on September 29, 1931, in Chicago, Illinois. His father, James Farley Cronin, was a graduate student in classical languages at the University of Chicago, and his mother, the former Dorothy Watson, attended Northwestern University. After receiving his degree, the senior Cronin moved his family briefly to Alabama to take a teaching job but soon settled in Dallas, Texas, to teach Greek and Latin at Southern Methodist University.

As a child Cronin attended local public elementary and high schools in the Highland Park system near Dallas and recalled that his natural interest in science was guided to physics by an outstanding high school teacher. He enrolled as a physics major at Southern Methodist University in 1947, receiving his bachelor's degree in science in 1951. However, it was not until he began graduate studies at the University of Chicago in September 1951 that what Cronin considered his "real education" began. His professors were among the most stellar in the field of physics and included Enrico Fermi, Murray Gell-Mann, Edward Teller, and Maria Mayer.

Cronin earned a master's degree in 1953, writing his thesis on experimental nuclear physics under the guidance of Samuel K. Allison. Meanwhile, a class he was taking from Gell-Mann, who was even then developing his theory of Strangeness, proved crucial in Cronin's eventual decision to study in the new field of particle physics. Also in 1953, the young scientist met Annette Martin. The couple had a whirlwind romance and were married less than a year later. The scientist later credited his wife with providing an oasis of calm and encouragement during the chaos of difficult experiments and looming deadlines.

Began Career as Physicist

The completion of Cronin's doctoral studies led to his receiving a Ph.D. from the University of Chicago in 1955. He immediately accepted a job as a research physicist at the Brookhaven National Laboratory on Long Island, New York. His work at Brookhaven made use of one of the world's most powerful particle accelerators, the three-billion-electron-volt (GeV) Cosmotron. Cronin later recalled these days as some of the most exciting of his long career. His work was not performed without difficulty, however. In early 1958 the Cosmotron had to be shut down after a catastrophic magnet failure rendered it useless. Disappointed and frustrated, the team moved their experiment to the University of California at Berkeley, which owned the Bevatron particle accelerator. Meanwhile, one of Cronin's colleagues at Brookhaven, Val Fitch, had invited him to join him in a teaching position at elite Princeton University in New Jersey. Cronin accepted and was appointed assistant professor of physics at Princeton in the fall of 1958.

At Princeton, Cronin was delighted to find an enthusiastic sponsor for his esoteric atomic research, which, prior to his teaming with Fitch, initially focused on hyperon decays. Laboratory Director George Reynolds assented to Cronin's request to work independently, and over the next decade he strongly supported the physicist's work. During this time, Cronin quickly became involved in the research that would win him and Fitch the Nobel Prize. The roots of that research can be found in a classic experiment suggested in 1956 by physicists Tsung-Dao Lee and Chen Ning Yang on the conservation of parity during certain nuclear reactions. One of the most fundamental laws adhered to by physicists of the mid-20th century was the principle of conservation. Students of high-school physics are familiar with laws dealing with the conservation of mass, energy, charge, momentum, and other qualities. Such laws state that there is a symmetry between the amount of each property prior to and following any change within in a closed system. In 1956 Lee and Yang found reason to believe that a property known as parity (P)—a kind of "left-handedness" versus "right-handedness"—is not conserved in certain types of nuclear changes. Reactions might be possible, they theorized, in which an excess of left-handed or right-handed particles might be observed. Shortly after this theory was announced, another researcher, Chien-Shiung Wu, found the precise violation of parity which had been anticipated by Lee and Yang.

This revolutionary discovery raised a number of new issues for theoretical physicists. Was it possible that other types of symmetry could also be violated? Were there ways of "explaining away" the failure of parity symmetry in the Shiung experiment? Lee and Yang themselves suggested one such system. Perhaps it is possible, they said, that the combination of parity and another property, charge conjugation (C), is conserved even if each alone is not. (The term "charge conjugation" refers to the balance between positively and negatively charged particles in a reaction.) Specifically, the combination CP might remain symmetrical, Lee and Yang said, even if neither C nor P did in a particular reaction.

In June and July 1963, Cronin and Fitch began a series of experiments that soon provided supporting evidence for the concept of CP violation. The original purpose of these experiments was somewhat more modest, however—namely, to investigate the behavior of elementary particles known as neutral K-mesons. The investigators wanted to know more about the process by which a beam of neutral K-mesons could be separated into two parts, one consisting of short-lived neutral K-mesons that decay into two pi-mesons and another consisting of long-lived neutral K-mesons that decay into three pi-mesons.

Hard Work Aided by Luck

The kind of experiments conducted by Cronin and Fitch in 1964 would, given the advanced technology available in later decades, be able to be analyzed at lightning speed by computers. At the time, however, the process was much more laborious and involved the careful, frame-by-frame study of dozens of rolls of film taken in spark chambers which Cronin had helped to develop. Only six months after the process had begun and the primary focus of the research on neutral K-meson decay had been completed did Cronin and Fitch suddenly realize that they also had evidence for violation of CP conservation. Ultimately, they found 45 examples of CP violation in more than 23,000 of the frames studied. Yet to make absolutely certain of their astounding assertion, Cronin and Fitch spent another six months looking for alternative explanations of their findings. Discovering none, they announced their results in Physical Review Letters on July 27, 1964. For their work, Cronin and Fitch each received half of the 1980 Nobel Prize for Physics.

Elated and exhausted, Cronin and his wife left for a year and traveled to France, during which time the physicist worked at the Center for Nuclear Studies in Saclay. He learned French and enjoyed soaking up the culture of another country. Cronin later remarked that giving a lecture on physics at the College de France was one of the "great joys" of his life.

Began Teaching but Continued

Cronin returned to the United States and Princeton University in 1965, having been promoted to associate professor in 1962 and full professor by 1964. With a fresh batch of graduate students, he began a series of experiments to investigate the neutral CP-violating modes of neutral K-meson particles. He and his team worked on these experiments until 1971, when Cronin left Princeton to accept an appointment as professor of physics at his alma mater, the University of Chicago. His decision was reportedly at least partly based on his eagerness to be near the new Fermilab 400 GeV particle accelerator, located just outside Chicago.

Once he had settled in at the university, Cronin assembled a new team of talented associates to help him carry out experiments on the production of direct leptons and particles at high transverse momentum. Far from writing off CP violation as a fait accompli, he investigated with greater accuracy some of the neutral K-meson's CP-violating parameters. Outside the laboratory Cronin was a popular and effective teacher, believing strongly that his highest purpose as a professor was to develop within his students "a sense of the value of exploring nature experimentally," as he told Paula Huff on the University of Utah College of Science Web site.

In 1997 Cronin began dividing his time between the University of Chicago and a new position as physics professor at the University of Utah. He is a professor emeritus in the University of Chicago's departments of physics and astronomy/astrophysics and holds parallel posts at the Enrico Fermi Institute in Chicago.

Made High-Energy Cosmic Rays Focus of Work

Cronin and University of Leeds Professor Alan Watson also headed the international Auger Project to study the nature and origin of rare but extremely powerful high-energy cosmic rays that periodically bombard Earth with the force of a fast-ball pitch. To do so, the scientists used a new form of astronomy based on particle physics. Argentina's Pierre Auger Observatory, which contains a giant detector array, was established in October 2003, and an area near the University of Utah was chosen as the site for a similar facility. Cronin, who became spokesperson emeritus for the Utah project in 2002, works on its behalf, declaring simply, "I want to find out the answer to cosmic rays." He published an article on the topic, "Cosmic Rays: The Most Energetic Particles in the Universe," in a 1999 issue of Review of Modern Physics.

In addition to his Nobel prize, Cronin has been awarded the 1968 Research Corporation Award, the 1975 John Price Wetherill Medal of the Franklin Institute, and the 1977 Ernest O. Lawrence Award. He and his wife have a son, David, and two daughters, Emily and Cathryn.


American Men and Women of Science, Gale Group, 2003.

Biographical Encyclopedia of Scientists, Institute of Physics Publishing, 1994.


Huff, Paula, "Cosmic Rays Keep Cronin Happy," University of Utah College of Science Web site, (December 21, 2003).

"James Cronin—Autobiography," Nobel e-Museum, (December 21, 2003).

"James W. Cronin," University of Chicago Experimental Astrophysics Department Web site, (December 21, 2003).

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James W. Cronin

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