The Standard Model of particle physics has reigned triumphant for nearly half a century, confirmed by observation upon observation. Nevertheless, it fails to explain why the observable universe contain virtually no antimatter. Particles of antimatter have the same mass but opposite electrical charge of their matter counterparts. But they’ve never been seen. Such gaps have inspired physicists to search from subatomic particles to galaxies for laws of nature beyond the Standard Model.
Very small amounts of antimatter can be created in the laboratory. However, hardly any antimatter is observed elsewhere in the universe. The electric dipole moment (EDM) of the electron, a quantity so infinitesimal that for all practical, everyday purposes it’s nothing, may hold the key to explaining the existence of all matter, from galaxies to grains of sand.
Physicists believe that there were equal amounts of matter and antimatter in the early history of the universe – so how did the antimatter vanish? A Michigan State University researcher is part of a team of researchers that examines these questions in an article recently published in Reviews of Modern Physics.
The answer could be rooted in the nature of forces between subatomic particles that are not the same when time is reversed. Physicists theorize that this time-reversal violation is the key ingredient needed to unravel the cosmic mystery of the missing antimatter. Such time-reversal violating forces result in a property in particles called a permanent electric dipole moment (EDM).
For over 60 years, physicists have searched for EDMs with increasing precision, but they have never observed them. However, recent theories of particle physics predict measurable EDMs. This has led to a worldwide search for EDMs in systems such as neutrons, molecules, and atoms.
Even with record-breaking sensitivity, we did not detect an EDM and couldn’t prove that it exists says Elizabeth Petrik in John Doyles group (watch Doyle’s video below) at the Harvard Department of Physics of an earlier effort back in 2014. “On the bright side” she added, “we still learned something new: we now know that the electron EDM is even smaller than anyone ever knew before. Our result can be used to set a new upper limit on the possible size of the electron EDM of about 10-28 electron charge centimeters. What this tiny number with a funny unit means is that if you were to magnify the electron to the size of our solar system, the size of the bulge on its charge distribution could not be any larger than the width of a grain of rice.”
“This is both disappointing and tantalizing,” concluded Petrik: “There are many theories that attempt to explain the predominance of matter in the universe, but many of the most promising ones are gradually being ruled out as our EDM measurements get better and better. All this gives us hope that the EDM—and the key to the mystery of our matter universe—may be just around the corner.”
Petrik was part of Harvard’s search for the electric dipole moment of the electron (eEDM). Permanent EDMs of fundamental particles violate time-reversal symmetry, which is intimately connected to the symmetry between matter and antimatter. The research team used the techniques of atomic and molecular physics to perform an extremely precise measurement of the eEDM, at scales less than 10^-28 cm. A non-zero eEDM, which has so far eluded observation, could point the way to the solution of the matter-antimatter asymmetry puzzle and to the new laws of nature that have long tantalized the physics community.
EDM searches often involve atomic clocks operating in a controlled magnetic field (uniform in space and stable in time). In an electric field, an ultra-stable atomic clock with a nonzero EDM will run slightly faster or slower. The success of such experiments depends on how well physicists can control the surrounding magnetic field and other environmental factors.
EDMs of atoms such as radium and mercury 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. Researchers at the University of Washington, Seattle, have found that their mercury-199 clock loses less than one second every 400 centuries. This experiment is impossible to improve upon unless one can build a clock less sensitive to environmental factors. A competing experiment that seeks to do just that is the search for the EDM of radium-225. It is a collaboration between Argonne National Laboratory, Michigan State University, and the University of Science and Technology of China.
The rare isotope radium-225 is an attractive alternative. Its “pear-shaped” nucleus (see figure) amplifies the observable EDM by orders of magnitude compared to the nearly spherical nucleus of mercury-199. In order to perform a competitive experiment, a radium-225 clock only needs to be stable to less than one second every two years. This is difficult but feasible. The sensitivity of this radium clock is currently limited only by the small number of atoms available (about 0.000005 milligrams per day).
In the future, using an even more “pear-shaped” nuclei, such as the rare isotope protactinium-229, may improve the sensitivity of these EDM searches by another factor of a thousand. In other words, a competitive experiment with a protactinium clock would only need to be stable to less than one second every day.
“We, everything we see, and the rest of the observable universe exists because the antimatter vanished during birth of the universe,” Singh said. “Discovering a new source of time-reversal violation, perhaps using rare pear-shaped nuclei, would begin to explain how this happened.”
FRIB will produce an abundance of pear-shaped nuclei such as radium-225 and, for the first time, protactinium-229. This will enable a search for an EDM with unprecedented sensitivity to answer the antimatter puzzle.
The image at the top of the page shows the cosmic microwave background (CMB), discovered accidentally in 1964 by Penzias and Wilson (Nobel Prize, 1978), the CMB is a remnant of the hot, dense phase of the universe that followed the Big Bang. For several hundred thousand years after the Big Bang, the universe was hot enough for its matter (predominantly hydrogen) to remain ionized, and therefore opaque (like the bulk of the sun) to radiation. During this period, matter and light were in thermal equilibrium and the radiation is therefore expected to obey the classic blackbody laws (Planck, Wien, Stefan).
The existence of the CMB is regarded as one of three experimental pillars that point to a Big Bang start to the universe. (The other two pieces of evidence that indicate that our universe began with a Bang are the linearity of the Hubble expansion law and the universal cosmic abundances of the light element isotopes, such as helium, deuterium, and lithium.)