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.