The Muon g-2 experiment measured the anomalous magnetic moment of muons, confirming that it does not match the calculated value. This deviation indicates the existence of particles or interactions unknown in the Standard Model of quantum mechanics.
The Standard Model of Quantum Mechanics describes very accurately the behavior of elementary particles. However, its formulas do not always work, which may indicate the incompleteness of the theory and the existence of a deeper and more fundamental "new physics". One of these anomalies is the discrepancy between the muon precession and the most accurate calculations. Physicists from the Muon g-2 collaboration measured this deviation with great accuracy, confirming that it exists and is possibly related to the influence of as yet unknown particles. Scientists write about this in an article published in the journal Physical Review Letters.
Muons are elementary particles, similar to electrons, but about 200 times heavier than them and not nearly as stable. Muons also have a negative charge and half-integer (1/2) spin, due to which they have a magnetic moment. Once in an external magnetic field, they deflect and oscillate (precess) like tiny gyroscopes. This precession depends on the mass of the particle, its charge and the g-Factor - a factor that determines the difference between the magnetic and mechanical moments of the particle.
In a vacuum, where there is a constant creation and death of virtual particles, their presence affects the magnetic moment of muons and, as a consequence, the value of the g-Factor. The Standard Model of Quantum Mechanics allows one to take into account the contribution of all known particles and calculate the g-Factor with great accuracy. However, experimental measurements of the precession of muons do not agree slightly with the predictions of the theory. This deflection is known as the muon anomalous magnetic moment problem and is believed to indicate the existence of as yet unknown massive particles or interactions.
Thus, the most accurate value of the anomalous magnetic moment of muons, obtained in 2020, is 0.00116591810. At the same time, experiments carried out at the Brookhaven National Laboratory (BNL) showed a value of 0.00116592080. New measurements of the anomalous magnetic moment were carried out by physicists from Fermilab. Curiously, for this they used a magnetic storage ring from Brookhaven, which they transported a few years ago to a new laboratory and connected to the particle accelerator at Fermilab specifically to measure the anomalous magnetic moment of muons.
During the Muon g-2 experiment, a stream of muons was directed into a magnetic ring. In a deep vacuum, the particles moved at a speed close to light, and scientists measured their precession. More than 200 experts from seven countries took part in the collaboration, and during 2018 they collected more than eight billion measurements. The analysis and statistical processing of these data took almost two years, and their results have only now been made public. Taking into account the previous and new data, the muon g-factor is 2.00233184122, and the anomalous magnetic moment is 0.00116592061.
The value obtained by combining the BNL and Fermilab measurements has a standard deviation of 4.2 sigma. The chances that it is the result of random fluctuations do not exceed one in forty thousand. Nevertheless, the deviation is already approaching five sigma - the "gold standard" of elementary particle physics, which allows us to speak with confidence about the discovery. Scientists are confident that soon they will overcome this value.
According to them, to date, only about six percent of the information that the Muon g-2 experiment should collect has been completed. The processing of the data of the second and third launches of the system continues, along with measurements of the fourth launch. “Already the first results show an intriguing deviation from the predictions of the Standard Model, - says one of the representatives of the collaboration Chris Polly (Chris Polly) - but in the next couple of years we will learn a lot of new things.”