A generation ago an experiment featuring muons found a possible problem with the Standard Model of particle physics. Muons have a quantity called "g" that a naive view of the theory says ought to be 2. Measurement showed that it is not 2. The difference between the measured value and that naive value is called "g minus 2," written g-2. This issue is called the Muon Magnetic Anomaly.
Since that time we have had, on the one hand, a series of experiments that has reduced the value of g-2, and, on the other, calculations which show that in a real-world scenario g should not be 2, since the value would be impacted by other factors. Much of the difference between experiment and theory has therefore disappeared.
But not all of it. So there was a lot of attention focused on a big experiment at Fermilab designed to measure g-2 with very high precision and accuracy. The results are now in and they confirm that the discrepancy is real.
However, the experimenters do not think this undermines the Standard Model. They think the issue is with those calculations, which are very difficult and complex, and in particular with one of the values that goes into them.
So for now the Standard Model still holds, and the hope that some new experiment will point a way beyond it fades again.
Fermilab announcement, good 5-minute video from Sabine Hossenfelder.
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