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.”