Ann Nelson, Expert on Particle Physics, Is Dead at 61

Dr. Nelson was celebrated for helping to address flaws in the Standard Model, the longtime basis for explaining how particles interact.

Ann Nelson climbing Cannon Mountain in central Washington State. A physicist who helped clarify our understanding of how the fundamental particles in the universe behave, she was also an avid hiker and died in a fall.

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David Kaplan

Ann Nelson, a theoretical physicist who helped plug holes and solve contradictions in the Standard Model, the template that forms the backbone of our understanding of fundamental particles and the universe, died on Aug. 4 in the Alpine Lakes Wilderness in Washington State. She was 61.

In 2018 Dr. Nelson was jointly awarded, with Michael Dine of the University of California, Santa Cruz, the J.J. Sakurai Prize, considered the highest prize in particle physics outside the Nobel.

She also received a Guggenheim Fellowship in 2004 and was elected to the American Academy of Arts and Sciences in 2011.Particle physics focuses on the basic building blocks of everything in the universe. The fundamental particles that have so far been identified have been given esoteric names like quarks, leptons, muons and taus. Electrons, the negatively charged particles that circle the nuclei of atoms, are leptons, while protons and neutrons, which form the nuclei of atoms and therefore make up most of the visible mass in the universe, are each composed of three quarks.

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Dr. Nelson speaking at a symposium on fundamental physics at the SLAC National Accelerator Laboratory at Stanford University in 2018. “Ann was the only student I ever had,” her doctoral adviser said, “who was better than I am at what I do best.”

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Dawn Harmer/SLAC National Accelerator Laboratory

Together, all those particles form the Standard Model of particle physics, the creation of which is one of the signature accomplishments of physicists in the 20th century and underlies the field of quantum physics.

She died in a hiking accident, said her husband, David Kaplan, who is also a physicist. He said the two of them were trekking with friends when Dr. Nelson slipped and fell into a gully.

Dr. Nelson stood out in the world of physics not only because she was a woman, but also because of her brilliance.

Howard Georgi, a Harvard professor who was Dr. Nelson’s doctoral adviser and is considered one of the leading theoreticians in particle physics, wrote on a eulogy page on the website of the magazine Physics Today: “I have had many fabulous students who are better than I am at many things. Ann was the only student I ever had who was better than I am at what I do best, and I learned more from her than she learned from me.”

Though the Standard Model has proved to be consistent in predicting experimental results, it falls short of providing a complete explanation of interactions among particles, and of how the universe works. Some of those shortcomings were what Dr. Nelson addressed in her work.

One problem she tackled was explaining why there seems to be so much more matter than antimatter in the universe, a violation of a basic principle in physics called symmetry. According to physics computations and theories, they should exist in equal amounts.

(It is a good thing that they do not, as matter and antimatter cancel each other out when they are combined, which means that if they did exist in equal amounts you wouldn’t be reading this article and none of what we know as the universe would exist.)

To account for the discrepancy, Dr. Nelson and others came up with a rigorous mathematical and theoretical model that allowed for a violation of the symmetry rule during the time that the universe was expanding and matter and antimatter were being created.

She also worked on theories to extend the Standard Model to include super particles that would be a combination of fermions (quarks and leptons) and bosons (particles that, like photons, carry forces). Physicists have been anticipating their discovery for decades and working on experiments to find them.

Ann Nelson was born in Baton Rouge, La., on April 29, 1958, the oldest of three daughters of Howard and Dorothy Ann Nelson. Her father was a vice president at Kaiser Aluminum; her mother was a docent at the M. H. de Young Memorial Museum in San Francisco after the family moved to the Bay Area when Ann was young.

Ann attended Acalanes High School in Lafayette, Calif., where she was the valedictorian, and then Stanford University. It was there, during a freshman year advanced physics class, that Ann met David Kaplan, a fellow student.

She worked for one summer at the European Organization for Nuclear Research in Geneva, known as CERN, the world’s largest nuclear accelerator, before graduating from Stanford in 1980. She continued on to Harvard, earning her Ph.D. in 1984. Before receiving her doctorate, she published her first paper, without any co-authors — rare even for established theorists.

After teaching at other universities, she and Dr. Kaplan, who were married in 1987, ended up at the University of Washington in 1994. That was where she was working when she died.

Her accident was unusual, Dr. Kaplan said, as they hiked regularly and had taken on far more dangerous passages than the one on which she fell. She was a moderator of the Washington Hikers and Climbers Facebook page, which has nearly 120,000 members. On Sunday, the page’s profile photograph was still of Dr. Nelson hiking in 2018 in the Cascades in Washington State.

In addition to Dr. Kaplan, with whom she lived in Seattle, Dr. Nelson is survived by their daughter, Sierra Kaplan-Nelson; their son, Gabriel Kaplan-Nelson; her parents; and her sisters, Laura Segala and Caroline Kris.

While at the University of Washington, Dr. Nelson became well known for championing diversity and social justice in the sciences, and particularly for mentoring students from nontraditional backgrounds. As part of her efforts to reach more diverse students, she had been giving lectures in the Palestinian territories.

In particle physics, it is often difficult to create the models to explain how particles interact, partly because the results can be strange, even unsettling. Dr. Nelson knew and accepted this.

“Ann told me,” Chanda Prescod-Weinstein, a cosmologist based at the University of New Hampshire who did postdoctoral work under Dr. Nelson at the University of Washington, wrote in Quanta magazine after her death, “that to be happy as a model builder in particle physics, I had to be O.K. with something like mounting a moose head on a wall and putting a purple scarf on it and not worrying about why it was wearing a purple scarf.”