Print Friendly, PDF & Email

Insights into Editorial: Muon g–2: landmark study challenges rulebook of particle physics


Newly published results of an international experiment hint at the possibility of new physics governing the laws of nature.

The results of the experiment, which studied a subatomic particle called the muon, do not match the predictions of the Standard Model, on which all particle physics is based, and instead reconfirm a discrepancy that had been detected in an experiment 20 years previously.

In other words, the physics we know cannot alone explain the results measured.


The muon g-2 experiment:

Fermilab, which houses the American particle accelerator, has released the first results from its ‘muon g-2’ experiment.

These results spotlight the anomalous behaviour of the elementary particle called the muon. The muon, a heavier cousin of the electron, is expected to have a value of 2 for its magnetic moment, labelled ‘g’.

However, the muon exists not in isolation but 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.


What is the Standard Model?

The Standard Model is a rigorous theory that predicts the behaviour of the building blocks of the universe.

It lays out the rules for six types of quarks, six leptons, the Higgs boson, three fundamental forces, and how the subatomic particles behave under the influence of electromagnetic forces.

The muon is one of the leptons. It is similar to the electron, but 200 times larger, and much more unstable, surviving for a fraction of a second.

The experiment, called Muon g–2 (g minus two), was conducted at the US Department of Energy’s Fermi National Accelerator Laboratory (Fermilab).


Deviations from Standard Model to the Experimental results:

  1. The Standard Model of particle physics calculates this correction, called the anomalous magnetic moment, very accurately.
  2. The muon g-2 experiment measured the extent of the anomaly, Fermilab announced that the measured ‘g’ deviated from the amount predicted by the Standard Model.
  3. That is, while the calculated value in the Standard Model is 2.00233183620 approximately, the experimental results show a value of 2.00233184122.


The g factor:

  1. The muon is also known as the ‘fat electron’. It is produced copiously in the Fermilab experiments and occurs naturally in cosmic ray showers.
  2. Like the electron, the muon has a magnetic moment because of which, when it is 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.
  3. As the muon spins, it also interacts with the surrounding environment, which consists of short-lived particles popping in and out of a vacuum.
  4. 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.
  5. So, a contradiction of the Standard Model would imply that there exist new particles, and their interactions with known particles would enlarge the canvas of particle physics.
  6. These new particles could be the dark matter particles which people have been looking out for, in a long time.
  7. These interactions make corrections to the g factor, and this affects the precession of the muon.



The results from Brookhaven, and now Fermilab, hint at the existence of unknown interactions between the muon and the magnetic field interactions that could involve new particles or forces. It is, however, not the last the word in opening up the road to new physics.

Thus, 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 is strong evidence that the muon is sensitive to something that is not in our best theory.

This observation together with the recently observed anomaly in B decays at CERN indicates that the effects of new yet unobserved particles and forces is being seen as quantum effects.