Why does matter exist Electron 'roundness' gives clues!
In the earliest moments of our universe, countless protons, neutrons, and electrons formed along with their antimatter counterparts. As the universe expands and cools, almost all of these matter and antimatter particles meet and annihilate each other, leaving only photons (or flashes of light) behind.
If the universe were perfectly symmetrical and had equal amounts of matter and antimatter, this would be the end of the story - we would never have existed. But there must be some imbalance: some leftover protons, neutrons, and electrons, for example, that form atoms, molecules, stars, planets, galaxies, and eventually humans.
"If the universe is perfectly symmetrical, then there is nothing but light. This was an extremely important moment in history; suddenly, there was something in the universe."
"The question is, why?" NIST/JILA researcher Eric Cornell said, "Why is there this asymmetry?"
The mathematical theories and equations that explain our universe require symmetry. Particle theorists have refined these theories to address the presence of asymmetry; but without evidence, these theories are just math, Cornell explains, so experimental physicists, including his group at JILA, have been looking for signs of asymmetry in elementary particles such as electrons.
Now, the JILA group has made record-breaking measurements of electrons, narrowing the search for the source of this asymmetry.On July 6, its findings were published in Science.
An improved bound on the electron's electric dipole moment
Electrons are made up of a negative charge, and JILA scientists have been trying to measure how evenly that charge is distributed between the electron's north and south poles: any unevenness would indicate that the electron is not perfectly round, and this would be evidence of an asymmetry that led to the existence of matter in the early universe.
One place to look for evidence of asymmetry is the electron's electric dipole moment (eEDM) - the uniformity of charge between the electron's poles. Electrons are made up of a negative charge, and the eEDM indicates how evenly that charge is distributed between the north and south poles of the electron; any measurement of the eEDM above zero will confirm the presence of asymmetry; the electron will be more egg-shaped than round. But no one knows how small that deviation might be.
"We need to fix our math to bring it closer to reality." Tanya Roussy, a graduate student in Cornell's research group at JILA, said, "We're looking for where this asymmetry might exist so we can understand where it comes from. Electrons are elementary particles, and their symmetry tells us something about the symmetry of the universe."
Cornell, Roussy and their teams at NIST and JILA recently set a record for accurately measuring the eEDM: a 2.4-fold improvement over previous measurements.
How precise is this?Roussy explains that if an electron were the size of the Earth, their research found that any asymmetry present would be smaller than the radius of the atom.
She added that making such precise measurements is very difficult, so the team needed to be smart about it. The researchers studied molecules of hafnium fluoride (HfF+). If they applied a strong electric field to the molecule, non-circular electrons would want to align themselves with the field and move within the molecule; if they were circular, then the electrons would not move.
Using an ultraviolet laser, they strip the electrons from the molecule, forming a group of positively charged ions and "trapping" them. Alternating the electromagnetic field around the trap, the molecules were forced to align/disalign with the electromagnetic field. The researchers then used a laser to measure the energy levels of the two groups: if the energy levels between them were different, this would indicate that the electrons were asymmetric.
Experimental equipment
Measurement results
Their experiment allowed them to have a longer measurement time than past attempts, which gave them greater sensitivity: the measurements used quantum projection-noise (QPN) finite spectroscopy to probe a sample of hundreds of molecular ions for up to three seconds. However, the team's measurements showed that the electrons did not move energy levels, suggesting that as far as we have been able to measure so far, the electrons are circular.
Cornell points out that there is no guarantee that anyone will be able to find non-zero measurements of the eEDM, but this level of precision from a tabletop experiment is an achievement. It shows that expensive particle gas pedals are not the only means of exploring these fundamental questions about the universe, and that there are many avenues to try. And, although the team did not find asymmetry, its results will help the field continue to search for answers to the asymmetry of the early universe.
After several years of work, the researchers finally finished measuring the electric dipole moment of electrons in the ions of hafnium fluoride molecules and got a result very close to zero. This result indicates that the scientists did not detect the presence of an electronic electric dipole moment and can only give an upper limit: this upper limit is about 2.4 times better than the previous best result and about 11 orders of magnitude smaller than the Standard Model's prediction. This result puts strict limits on some new physical theories that go beyond the Standard Model, such as supersymmetric theories, left-right symmetric models, multidimensional space models, and so on.
"We found that up to our measurements, the electron is symmetric. If we find non-zero, that will be a big problem." Roussy added: "The best bet is to get teams of scientists around the world to look at different options. As long as we all keep measuring the truth, someone will eventually find it."
Reference links:
[1]https://www.science.org/doi/10.1126/science.adg4084
[2]https://phys.org/news/2023-07-roundness-electrons-clues.html
[3]https://mp.weixin.qq.com/s/6xnz7_qwqsy4xFtZksi9Ag
