Why is there nearly no antimatter in the Universe?

Why is there something rather than nothing in the observable universe? The answer to this profound question is thought to be rooted in the nature of forces between subatomic particles that are not the same when the arrow of time is reversed. Such forces result in a property of particles, such as atomic nuclei, called a permanent electric dipole moment (EDM).  In the presence of an electric field, an ultra-stable atomic clock with a nonzero EDM will run slightly faster or slower. For over 60 years, such an effect has been searched for with ever increasing precision but has never been observed. On the other hand, various theories of particle physics generically predict large and measurable EDMs. For this reason there is a worldwide effort searching for EDMs in a variety of complementary systems such as neutrons, molecules, and atoms. A review article recently published in Review of Modern Physics gives a broad overview of this exciting field.

One example of an experiment is the ongoing search for the permanent EDM of radium-225, in a collaboration between Argonne National Lab, Michigan State University, and the University of Science and Technology of China. EDMs of atoms such as radium are primarily due to forces originating within the nuclear medium. The best limits on these types of forces are presently derived from the mercury-199 atom. Radium-225 is an attractive alternative because its “pear-shaped” nucleus amplifies the observable EDM by orders of magnitude as compared to the nearly spherical nucleus of mercury-199.  The Facility for Rare Isotope Beams will produce an abundance of pear-shaped nuclei such as radium-225 and protactinium-229 which will enable a search for an EDM with unprecedented sensitivity.

image showing spinnung nuclei