Recently, there have been more frequent references to gaps in the Standard Model of physics – to as yet unknown particles or forces. Measurements with muons, the heavier cousins of electrons, are now providing new evidence for such a new physics. In the muon-g-2 experiment, physicists re-measured the so-called anomalous magnetic moment of the muon. Significant deviations from the value predicted by the physical standard model occurred. With 4.2 standard deviations, these discrepancies do not yet reach the threshold of a definite discovery, but they are getting closer and closer to it.
The muon, like its “cousin”, the electron, belongs to the group of leptons – elementary particles which, together with quarks and bosons, form the building blocks of matter. Even if muons are short-lived and around 100 times heavier than an electron, like this one they have a half-integer spin, are simply negatively charged and have a magnetic moment. In an externally acting magnetic field, interactions cause the direction of this internal magnet to fluctuate slightly – similar to the slight tumbling of the earth’s axis due to precession. This so-called g-factor can be calculated exactly on the basis of the charge, mass and spin of the muon and should actually be two. In reality, however, it does not do this because the particle is influenced by quantum fluctuations during its flight by an external magnetic field. The interaction with these interfering effects changes the g-factor by a small amount – the so-called anomalous magnetic moment.
Deviations between theory and experiment
Theoretically, this anomalous magnetic moment, referred to as g-2, can also be calculated on the basis of the physical standard model. Then this anomaly should have the value of a = 116,591,810 (43) x 10-11 to have. However, a good 20 years ago measurements at the Brookhaven National Laboratory in the USA showed deviations from this theoretical target value. At that time, the discrepancies determined in the muon-g-2 experiment were around 3.7 standard deviations – too few to be considered a reliable discovery. Since then, physicists have been investigating whether this deviation is real or just the result of systematic uncertainties in theory and experiment. For this purpose, the muon-g-2 experiment was upgraded and relocated to the Fermi National Accelerator Laboratory (Fermilab). There, physicists from the muon g-2 collaboration have carried out new, more precise measurements of the anomalous magnetic moment of the muon in recent years. More than 200 researchers from 35 institutions in seven countries work together in the muon g-2 collaboration.
For the experiment, muons with a uniformly polarized spin are generated and fed into the Fermilab’s storage ring – the magnetic axis of all these particles points in the same direction. Superconducting magnetic coils in the storage ring generate a strong, uniform magnetic field that acts on the muons racing in circles at high speed and changes the alignment of their magnetic moment and spin. How strong the anomalous magnetic moment is can be measured by the direction and number of positrons that are created when the short-lived muons decay and are captured by detectors.
Discrepancies are confirmed
Now the Muon g-2 collaboration has published the first results of their measurements. They include the data that were determined on around eight billion muons during the 2018 term. According to this, the experimentally determined anomalous magnetic moment of the muon is a = 116 592 040 (54) x 10-11 and has a relative uncertainty of 460 parts in 1 billion. This value thus deviates by 4.2 standard deviations from the theoretical value calculated on the basis of the standard model. The probability that this deviation between experiment and theory is random is 0.0025 percent (1 in 40,000). The discrepancy to theory is just below the value at which physicists speak of a discovery – in this case a clear refutation of the Standard Model. For this, the sigma value would have to reach five, which corresponds to a random probability of less than 0.00005 percent.
“This is an incredibly exciting result,” says Ran Hong of the Argonne National Laboratory, a member of the Myon g-2 collaboration. Because it suggests that there is an explanation gap in the Standard Model. Apparently the muon not only interacts with the expected quantum fluctuations during its flight through the magnetic field, but is also influenced by an as yet unknown factor – possibly by as yet unknown particles or forces. “The value we measure reflects the interactions of the muon with everything else in the cosmos. But if theorists calculate the same value and take into account all known particles and forces, we don’t get the same answer, ”says Renee Fatemi of the University of Kentucky. “This is a strong indication that the muon is reacting to something that is not included in our best theory so far.” The physicists hope that the ongoing evaluations of the measurements from 2019 and 2020 will further corroborate this result.
(Video: Fermilab)
Source: Muon-g-2 collaboration, Physical Review Letters, doi: 10.1103 / PhysRevLett.126.141801