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.
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 the measured ‘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.
When the results are combined with those from a 20-year-old experiment at Brookhaven National Laboratory in the U.S., they show an accuracy of about 4.2 sigma. This 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 significant enough to constitute a discovery, for which a significance of 5 sigma is needed.
It is interesting that while the Brookhaven experiment had to be closed down in 2001, the main component of the experimental set up, a large superconducting magnetic storage ring that measures over 15 metres in diameter, was transported a distance of over 5,000 kilometres, in 2013, to be reused in the Fermilab experiment.
Rahul Sinha of The Institute of Mathematical Sciences, Chennai, who specialises in studying physics beyond the Standard Model, is very excited by the Fermilab result. According to him, even though the experiment measures a very elementary property related to the magnetic moment of the muon it is by no means a simple experiment. “Kudos to the experimental team for such a detailed treatment of the mindboggling systematic effects that makes this measurement so special. The muon g-2 experiment at Fermilab promises to be a torch bearer in the search for new physics,” he says.
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 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.
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 the Standard 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.
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 SM does not account for. “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,” says Prof. Sinha.
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 could have large errors and need to be substantiated further.
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