The quantum realm of particles has never ceased to amaze us, and the recent measurement of the electron's magnetic moment is a further testament to its majesty. This astounding feat of metrology is 2.2 times more precise than the previous best, recorded 14 years ago, and provides us with an incredibly accurate test of the Standard Model of particle physics. While this theory has both intrigued and perplexed physicists for decades, the incredible precision of this measurement brings us closer to understanding the fundamental nature of our universe.
greed with observations, but this method of testing the SM has become increasingly difficult. As a result, physicists have been looking for new theories that can explain the phenomena that the SM can’t. These theories, such as supersymmetry and grand unified theories, are still being developed and tested and could potentially provide answers to some of the unsolved mysteries in physics.
What is the Standard Model?
The Standard Model of particle physics makes a very precise prediction for the magnetic moment of an electron, which is equal to – µ/ µB. Here, µ (pronounced mew) is the electron’s magnetic moment (measured in amperes sq.-metres) and µB is a physical constant called the Bohr magneton. This calculation yields a dimensionless number, which can be used to measure the electron’s willingness to align with a magnetic field.
How does the electron’s magnetic moment matter?
Recently, researchers in the U.S. achieved a new level of precision in measuring this number. By suspending a single electron in a magnetic field at an ultra-cold temperature inside a vacuum chamber, and measuring currents induced in nearby electrodes by the electron’s movement, the team was able to determine the value of – µ/ µB to be 1.00115965218059, within 0.13 parts-per-trillion (ppt). To achieve such a precise result, the team carefully controlled the electric fields that held the electron in place, stabilised the magnetic field, and finely adjusted the physical properties of the hardware, eliminating sources of uncertainty that could have affected the data.
Is the result good for the SM?
Second, a series of mathematical calculations connect the data that physicists record in an experiment and the value of the electron’s magnetic moment. One of these calculations involves the fine structure constant (α) – a universal constant that specifies the strength with which an electron couples to the electromagnetic field. If the coupling is stronger, the fine structure constant (α) is higher. But its value is still an open question, and its higher value could affect the result.
These two open questions make it impossible to be completely certain about the validity of the new result, yet it still provides a remarkable insight into the behavior of the electron.
Will we ever find evidence of beyond-SM forces?
Physicists are working hard to answer the billion-dollar question of whether the Standard Model (SM) of particle physics is complete. They are testing its predictions to the best of their abilities, looking for any cracks in its façade. Already, some leads have emerged; for instance, the SM states that neutrinos should be massless, however, they are not. The muon anomaly is another example.
To gain further insight, physicists have built detectors to search for hypothetical dark-matter particles, are analysing astronomical data to understand dark energy, and continuously double-checking their calculations. Some are also debating whether a larger supercollider could be more successful than the current Large Hadron Collider.
The team who measured the electron's magnetic moment have plans to upgrade their equipment and repeat the experiment with the electron's anti-particle, the positron.
Overall, the scientific community is hoping that at least one of these endeavors, informed by the principles they uncover in their theoretical studies, will uncover a world beyond the Standard Model.