After learning in October 1980 that he would receive a Nobel Prize, James Cronin, SM’53, PhD’55, hung up the phone and went about his day. The UChicago physics professor was taking his colleague Subrahmanyan Chandrasekhar’s course on general relativity that morning, and instructed his secretary to hold his calls. “It was certainly a great honor to receive the Nobel Prize, … but one should not let that fact go to one’s head,” he wrote in 2014.

Cronin, who died August 25 at age 84, never did (see Deaths). He was only 33 years old in 1964, when he and Val Fitch made their Nobel discovery, which began to untangle a mystery physicists had grappled with for decades: why the universe contains more matter than its oppositely charged (but otherwise equivalent) counterpart, antimatter.

Physicists had assumed that the laws of physics could not differentiate between the two, a postulation known as charge conjugation parity (CP) symmetry. But when Cronin, then at Princeton University; Fitch; and their small team at Brookhaven National Laboratory set out to verify CP symmetry, they instead discovered that nature operates differently upon matter and antimatter, and favors the existence of matter.

The finding was met with disbelief. “Initially scientists were very skeptical,” says Cronin’s longtime colleague Angela V. Olinto, the Homer J. Livingston Distinguished Service Professor in Astronomy and Astrophysics at UChicago. “They didn’t believe nature would have this kind of preference.” But the discovery lent credence to the big bang theory, helping to explain why the explosion left more matter than antimatter, enabling the formation of the solar system and human life. “Now we have connected that experiment to the reason we are here,” Olinto says.

The finding upended scientists’ understanding of the physical world, yet 16 years elapsed before it was recognized with a Nobel Prize. In the interim, Cronin continued his research, carrying out experiments in particle physics at research centers including Fermi National Accelerator Laboratory near Batavia, Illinois, and CERN, the European Organization for Nuclear Research, in Switzerland.

But during the 1980s, the epoch of experimentation conducted in small groups—what Cronin considered the “golden age” of high-energy physics—had given way to a research world dominated by large consortiums. In 1985 Cronin shifted his focus entirely, turning to the origin of cosmic rays: high-energy atomic fragments that shower the earth’s atmosphere at nearly the speed of light.

“Jim was a driven man who liked to do things with his own hands and to have a leading role in whatever project he chose,” says Cronin’s friend and colleague Alan Watson, emeritus professor of physics at the University of Leeds. Watson recalls that Cronin approached his new endeavor with characteristic diligence, visiting several international cosmic-ray institutions, including Leeds, where the two met.

Low-energy cosmic rays are commonplace; those with higher energies are far rarer. To find and measure them calls for a giant detector. By the early 1990s, Cronin and Watson had embarked together on cofounding the Pierre Auger Observatory, the largest cosmic ray detector ever built, operated by a collaboration of more than 500 scientists from 17 countries.

For more than a decade, Cronin used his Nobelist status discreetly to leverage funds and interest in the project—from the National Science Foundation, the Argentinian government, UChicago, and even UNESCO, which sponsored scientists from developing countries to participate in its design. He and Watson traveled the world to attract physicists.Eventually a plan emerged to create a facility the size of Rhode Island, situated on a high, flat plain in rural Argentina.

The proposal seemed far-fatched, says Paolo Privitera, now a professor in UChicago’s Department of Astronomy and Astrophysics, who attended an Auger collaboration meeting in Switzerland in the late 1990s when he was at the University of Rome. “I went to the meeting with a good dose of skepticism,” says Privitera. “Encountering Jim changed my mind, and ultimately my career and life.”

He remembers Cronin as approachable and eager to hear his younger colleagues’ opinions. This “combination of a superior intellect and deep humanity was an irresistible pole of attraction, and I found myself—together with many other scientists of all nationalities—bound to push, together with Jim, for this ‘crazy’ (or better visionary) experiment.”

Since Auger reported its first results in 2004, physicists have traveled from places as far-flung as Australia and the Netherlands to the remote Argentine town of Malargüe to meet and run experiments. Cronin’s leadership was essential, Privitera says: “A collaboration of physicists is like a rebellious orchestra with too many concertmasters. Jim—an extraordinary scientist and a visionary leader—was the charismatic maestro who would make them play in unison to deliver their best performance.”

Cronin was born September 29, 1931, in Chicago, while his father, the first in his working-class family to go to college, was a graduate student in classics at the University. The family soon moved to the Dallas area, and Cronin earned a bachelor’s degree in physics from Southern Methodist University in 1951. Arriving at UChicago for graduate school, Cronin felt woefully ill prepared. “Among my fellow students were many brilliant ones who possessed a far superior knowledge of physics than I did,” he later wrote. To catch up, he audited upper-level undergraduate courses and devoted his summers to studying.

In his first year, Cronin took Nobel laureate Enrico Fermi’s class on thermodynamics and statistical mechanics. Later he would call it his favorite, and would edit Fermi Remembered (University of Chicago Press, 2004), a volume about the nuclear physicist’s life and career.

Cronin met his first wife, Annette Martin, EX’56, AM’88, while both were students at UChicago. They married in 1954 and had three children. Cronin’s career began in 1955 at Brookhaven, where he worked as an assistant physicist. Three years later he joined Princeton, staying until 1971, when he became University Professor of Physics at the University of Chicago, his scholarly home for the rest of his career.

Cronin’s busy research agenda never took him far from his wife and children. “Although my dad was very focused on physics, which was never a job for him but really a passion, he was very much a family man,” says his daughter, Emily Grothe, LAB’78. Students and colleagues were regularly drawn into the Cronins’ dinners, lively with “topical and provocative” conversations. When traveling to conferences and other professional activities, Cronin often brought the family for backpacking, canoeing, or sailing on the side.

When Annette became ill with Parkinson’s disease in 2000, Grothe says, Cronin became his wife’s primary caregiver. He learned to cook and carried on the family dinners until her death five years later. Olinto remembers running into him at the grocery store during this time. “He was buying fresh tomatoes to make her tomato sauce,” she says. “I told him, ‘You know you can buy that in a can,’ but he said, ‘Annette never used cans, and I am not going to start that tradition in my house.’”

In 2006 Cronin married Carol McDonald. He kept working on cosmic rays until his death. “Jim never stopped pushing the boundaries, attacking the next question, and furthering everybody else. He had a passion about nature and the laws of physics and how this universe behaves,” says Olinto, who worked with him on the Auger project for two decades.

This past year, he and Olinto proposed the construction of an even larger cosmic ray detector that has not yet secured funding. Now she is moving forward with a prototype telescope that will launch from New Zealand in April 2017. The instrument’s journey through space will be a tribute to Cronin, who wanted to transcend even his achievement with Auger, she says. “He told me, ‘We should have built something 10 times bigger.’”