The question of origins – how the stuff we are made of came to be – is a focus of research spanning many types of science. In physics, the origin of matter is an overarching question: Why is the universe made of matter while antimatter is very rare? To answer this question, the idea of “baryogenesis,” or the creation of more matter than antimatter in the earliest stages of the universe, was developed by Andrei Sakharov in 1964. Sakharov’s recipe for baryogenesis requires forces that also change the shape of spinning particles in a specific way that can be measured in the lab. One particular property used to probe the change in shape is the electric dipole moment (EDM), which measures how positive and negative charge is distributed along the axis around which a particle spins.
A decade before Sakharov’s idea appeared, two other giants of twentieth century physics, Edward Purcell and Norman Ramsey, suggested that studying the shape of the neutron could reveal new features of the forces that govern elementary particle interactions. Neutrons, the neutral particles that reside in the atomic nucleus along with protons, sometimes break free from the nucleus and their properties can be measured. Neutrons are especially convenient for measuring EDM because very strong electric fields are required, and neutral particles remain motionless in an electric field. Purcell and Ramsey were also instrumental in the development of exquisitely sensitive experimental techniques used to measure EDMs, for which both won Nobel Prizes.
Even after decades of research and the completion of scores of experiments, evidence of an EDM has remained elusive. Despite this, the experimental advances made to probe EDMs are now at the center of the search for new physics. The current model of particle physics, called the Standard Model, describes elementary particles and their interactions, but it’s possible that other interactions not described by the Standard Model exist and could have implications for baryogenesis.
A new comprehensive review of EDMs led by Professor T. Chupp has recently been published in Reviews of Modern Physics. The article aims to make the motivations, theoretical issues, and experimental techniques in the field accessible at the graduate student level. “One of the most important messages to take home,” according to Chupp, “is that after 60 years of not seeing an EDM in any system, the search is intensifying. This is in part because as the experiments become more sensitive they probe deeper into elementary particles, which also means they probe a higher energy scale. In fact, in many ways, EDMs are more sensitive than CERN’s Large Hadron Collider in the search for new forces. And we continue to be inspired by the implications for the origin of matter.”
The article, “Electric dipole moments of atoms, molecules, nuclei, and particles,” was co-authored with Michael Ramsey-Musolf of the University of Massachusetts, Peter Fierlinger of the Technical University of Munich, and Jaideep Singh of Michigan State University. It can be found at Rev. Mod. Phys. Vol. 91, 015001 (2019).