A new experiment allowing scientists to measure an electron to a previously unprecedented level of precision has offered novel insight into the quantum-scale structure of everyone’s favorite elementary particle.
The study, published October 17th in Nature and conducted by a team at Advanced Cold Molecule Electron Electric Dipole Moment (ACME) Search, reports the findings that the shape of the electric field of an electron is perfectly spherical and symmetric, contrary to recent theories that predict electron fields to be asymmetric on quantum scales.
The new finding is further confirmation of the Standard Model (SM) of particle physics, which predicts that the elementary particles of the universe are perfectly symmetric in shape and electric charge.
The experiment puts an upper bound on the electron dipole-moment at 1.1×10−29 e·cm, almost a full order of magnitude more precise than previous experiments. In other words, according to the most precise measurements we have ever taken of the electron, it still looks to be perfectly spherical. In addition to being a marvelous technical achievement in particle physics, the findings confirm that the electron still to the best of our knowledge follows the predictions of the SM, and casts some doubt on theories that go beyond the SM that predict asymmetries in the shape on an electron charge at the 10−29 e·cm level.
Standard Model, Symmetries, And Electron Dipole Moments
The Standard Model (SM) of particle physics is a crowning achievement of modern science. Perhaps the single most confirmed scientific theory in history, the SM represents modern science’s best effort to understand the classification and behavior of the fundamental constituents of our universe; particles like quarks, neutrinos, and the infamous Higgs boson.
Despite these accolades, the SM is known to be an incomplete model. One of its main shortcomings is that it does not provide a description of the fundamental force of gravity and how gravity exerts its influence on quantum particles. Theories that attempt to synthesize gravity and the other 3 fundamental forces are called theories of quantum gravity. Another pressing issue that the SM is silent on is why there is more matter than antimatter in the universe, called baryogenesis.
According to fundamental cosmological equations, equal amounts of matter and antimatter should have been created at the big bang. Since matter an antimatter completely annihilates each other on contact, the equal amounts of matter and antimatter should have reacted converting all their mass into energy in the form of photons. Of course, this obviously did not happen; the simple fact that we exist is proof that matter somehow won out at the beginning of the universe. The question of why matter dominated over antimatter in the early universe is one of the great unsolved problems in physics.
Some theoreticians have proposed possible asymmetric mechanisms to explain the abundance of matter over antimatter. All such mechanisms as of now are speculative but still operate according to known laws of physics. As it turns out, a number of these proposed mechanisms will manifest their asymmetry by creating an asymmetry in the shape of an electron’s electric field, what is known as an electron dipole-moment (EDM). Thus an EDM would result in the electron’s otherwise spherical field appearing slightly squashed. So, one way to test for any hypothetical symmetry breaking process would be to look for aberrations of the spherical shape of an electron’s field.
In order to probe for the existence of an EDM, the team of researchers observed the effect of an electron on the energy structure of thorium monoxide, a technique that has been used in experiments stretching back to the 1950s. The team fired a bean of cooled thorium oxide molecules—1,000,000 per pulse and 50 pulses per second—into a chamber the size of an average desk. Precise lasers fired into the chamber orient the molecules as they move between two charged glass plates in the presence of a strong magnetic field. Other sensors were set up in the chamber to detect light emitted by the electrons. The specific light emission tells the team whether the electrons orientation changes during movement, which would be expected if the electron sphere had been slightly squashed.
The team found that according to their observations, the electron EDM is 0. This result means that they did not observe any squashing of the electron’s electric field, as would be expected if theories positing asymmetric mechanisms were true. Thus, as far as we known, the electron is still perfectly spherical and the SM is still correct in its description of electrons. To be more precise, the team successfully demonstrated an upper bound of the electron EDM to be at 1.1×10−29 e·cm, 8 times more precise than the previously determined upper bound of 8.7×10−29 e·cm. This upper bound was calculated based on information from previous experiments, and known uncertainties in the experimental setup.
This result is somewhat a double-edged sword. On the one hand, the experiment gives further confirmation of the SM and represents a monumental technical achievement in experimental physics. The team succeeded in observing individual electrons on a 10-100 TeV scale, an unheard of level of precision for particle physics that is comparable to experiments run at the CERN particle accelerator. On the other, it is still known that the SM is incomplete, and this particular experiment seems to rule out several plausible models meant to complete the SM. Thus, the experiment seems to confirm a model that is already known to be ultimately inadequate. Theoreticians that advocate for models that go beyond the SM most likely will have to revise their theories in light of the data or propose some alternate explanation that could explain the data. Whatever the outcome may end up being, the study certainly gives some answers and suggests further directions for research.
The particular study represents the latest in a tradition of searching for EDMs that began almost 70 years ago. Further research on the matter is needed and the team at ACME is optimistic. They believe that within a few years they should be able to increase precision and once again improve the measurement of the upper bound of EDMs. The only way to keep testing the SM is by creating procedures that probe further and further into the structure of the universe.