In 2011 we already thought that the electron was a perfect sphere, or rather, almost perfect, since the figure that separates this particle from perfection is less than 0.000000000000000000000000001 cm, a rather insignificant amount. Now, we know that this elementary particle is still the most perfect sphere in the universe, according to a new measurement by experts from Northwestern, Harvard and Yale universities (USA).
” If we had discovered that the shape was not round, this would be the biggest headline in physics in decades , but our discovery is still just as scientifically important because it strengthens the standard model of particle physics and excludes alternative models,” explains Gerald. Gabrielse, co-author of the study published in the journal Nature.
The electron gets its shape from the way the positive and negative charges are distributed within the particle. The best theory of how particles behave, called the Standard Model of particle physics, holds that the electron should maintain its round shape almost perfectly.
But some theories suggested that a group of hypothetical subatomic particles outside the electron could create a slight separation between the positive and negative charges, giving the electron a pear shape. That charge separation is called the electrical dipole moment or EDM. Searching for an electron EDM can reveal whether particles that do not exist in the standard model are suspended around the electron undetected.
With this goal in mind, the team fired a beam of cool thorium oxide molecules into a chamber the size of a large desk. The researchers then studied the light emitted by the molecules. A crooked light would indicate an electric dipole moment. Since the light was not bent, the research team concluded that the shape of the electron was, in fact, round, confirming the prediction of the standard model . That there is no evidence for an electric dipole moment means that there is no evidence for these hypothetical heavier particles. And if in spite of everything these particles exist, their properties differ from those predicted by the theorists.
Thus, the researchers in the new study probed the electron EDM as precisely as possible: They fired a beam of cool thorium oxide molecules into a chamber the size of a large desk. Then, they studied the light emitted by the molecules. Since the light was not twisted (there was no electric dipole moment), there was no change in this new measurement, so the experts concluded that the shape of the electron was round, again confirming the prediction of the standard model. There is no window open to new physics (at the moment).
The finding improves the team’s last best measurement by a factor of 10 to find an EDM of 10-29 centimeters of electron charge.
More doubts
“ The Standard Model as it is cannot be correct because it cannot predict why the universe exists ,” Gabrielse said. “That is a pretty big lagoon.”
It seems that, at the moment, we do not know where the kit of the standard model is.
However, this result could make it difficult for the Large Hadron Collider, located at the CERN laboratory near Geneva, to find signs of new physics beyond the standard model. The LHC knocks proton particles together at high speeds to create new particles and test their properties. Physicists want to find signs of particles that are not in the standard model because that theory cannot explain some crucial features of the universe, such as why there is more matter than antimatter. But so far, the LHC has not given us the answer. The new measurement suggests that any extra particles that exist may be permanently out of reach of the LHC.
It appears that we have exceeded what the LHC can see. It has become too small for us.
There is a way out for the LHC: The ACME Collaboration’s calculation provided some explanation for the difference between matter and antimatter in the universe. If that assumption doesn’t hold, then the new particles could be light enough for the LHC to produce. But we would also be left without an explanation of why matter dominates antimatter.
The proposed successor to the LHC, the Future Circular Collider (FCC) , could give us these answers if it is ever built. At the moment, it is in the design phase with the objective of expanding the research that is currently being carried out in the Large Hadron Collider (LHC), once the latter reaches the end of its useful life.
The goal of the FCC is to push the energy and intensity frontiers of particle colliders, with the aim of reaching collision energies of 100 TeV, in search of new physics.
Reference: ACME Collaboration, Improved limit on the electric dipole moment of the electron, Nature (2018). DOI: 10.1038 / s41586-018-0599-8
Image credit: Nicolle R. Fuller, National Science Foundation
