Nearly twenty years ago, obtaining a precision measurement had the effect of a small bomb in the world of physics: that of a value associated with this elementary particle that is the muon, namely its “anomalous magnetic moment” (“anomal”, as in “anomaly”; we come back to this a little below). Indeed, this one did not stick with the predictions of the standard model. In sum, theory and practice were at odds, suggesting that a possible “new physics” still needed to be discovered. For physicists, spotting a flaw in the Standard Model is exciting, confusing and… frustrating all at the same time. It is wonderful to see that we may have missed something, but we still have to understand what!
For all this time, therefore, physicists have been working to clarify the enigma of the magnetic moment of muons, in particular within the framework of an experiment bearing the name of “Muon g-2″, carried out since 2018 at the FermiLab laboratory, in the ‘Illinois, United States. In reality, this experiment is none other than the improved copy – let’s say the extension – of a previous one carried out at the Brookhaven National Laboratory, still in the United States, between 1997 and 2001. The very one which had made it possible to observe the ” hic “with the magnetic moment of the muons, the precise measurement of which was higher than expected. On Wednesday April 7, 2021, the Muon g-2 team, made up of 200 researchers from all over the world, finally delivered its… which confirm those of Brookhaven: there is indeed a difference (of 0.0025% of course, but a gap all the same) between the experimental measurement of the magnetic moment of the muons and the theory developed from the standard model.
The muon, a magnetic spinning top
Before going into a little more detail, it is undoubtedly necessary to come back to a few concepts. In mind: what is this magnetic moment, called “anomalous”, of the muon? Let us first go back in time. In 1936, the muon, a fermion-type particle which has appreciably l[…]
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