Another “new physics” alert, this time from Fermilab

The results from the Muon g-2 experiment show that fundamental particles called muons behave in a way that is not predicted by the Standard Model of particle physics

Fermilab, the American particle accelerator, has released first results from its “muon g-2” experiment. These results spotlight the anomalous behaviour of the elementary particle called the muon. The muon is a heavier cousin of the electron and is expected to have a value of 2 for its magnetic moment, labelled “g”.

Now, the muon is not alone in the universe. It is embedded in a sea where particles are popping out and vanishing every instant due to quantum effects. So, its g value is altered by its interactions with these short-lived excitations.

The Standard Model of particle physics calculates this correction, called the anomalous magnetic moment, very accurately.

The muon g-2 experiment measured the extent of the anomaly and on Wednesday, Fermilab announced that “g” deviated from the amount predicted by the Standard Model. That is, while the calculated value in the Standard model is 2.00233183620 approximately, the experimental results show a value of 2.00233184122.


They have measured “g” to an accuracy of about 4.2 sigma, when the results are combined with those from a 20-year-old experiment, which means the possibility that this is due to a statistical fluctuation is about 1 in 40,000. This makes physicists sit up and take note, but it is not yet significant enough to constitute a discovery – for which they need a significance of 5 sigma.

The g factor


The muon is also known as the fat electron. It is produced copiously in the Fermilab experiments and occurs naturally in cosmic ray showers. Like the electron, the muon has a magnetic moment because of which, when placed in a magnetic field, it spins and precesses, or wobbles, slightly, like the axis of a spinning top. Its internal magnetic moment, the g factor, determines the extent of this wobble.

As the muon spins, it also interacts with the surrounding environment, which consists of short-lived particles popping in and out of a vacuum.

The implications of this difference in the muon’s g factor can be significant. The Standard Model is supposed to contain the effects of all known particles and forces at the particle level. So, a contradiction of this model would imply that there exist new particles, and their interactions with known particles would enlarge the canvas of particle physics. These new particles could be the dark matter particles which people have been looking out for, in a long time. These interactions make corrections to the g factor and this affects the precession of the muon.


If the measured g factor differs from the value calculated by the Standard Model, it could signify that there are new particles in the environment that the Standard Model does not account for. This observation, together with the recently observed anomalies in B decays at CERN indicates that the effect of new yet unobserved particles is being seen.

Note of caution

There have also been calculations made by a group of scientists which appeared in Nature that use the standard model itself to explain this difference. But these so-called Lattice Models have large errors and need to be substantiated further.

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