# Muon G–2 Experiment

Note4Students

From UPSC perspective, the following things are important :

Prelims level : Muan

Mains level : Particle physics and its various anomalies

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.

After genetics, AI and the blockchain, Particle Physics is making several headlines these days. This is something intuitive.

What is Muon?

• 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.

#### Main characteristic: Anomalous magnetic moment

• 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 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.
• 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 processes, 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.
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