Chemistry and Physics

Muon Magnetism Mystery: New Physics?

A tube protrudes from a complicated mass of wires and scientific eqipment
Part of the LHCb experiment
Christina Giallombardo

Physics is all about challenging our current understanding of the universe and searching for answers to many unresolved issues. Recently, the muon has become the star of the show with the release of evidence from the LHCb at CERN and Fermilab’s Muon g-2 experiment, both of which hint at tantalising, new physics.

The Standard Model of Particle Physics is a compilation of theories and discoveries by thousands of physicists since the 1930s. The known fundamental building blocks of matter and the governing forces of the subatomic world are encapsulated in the Standard Model. The muon, first discovered in 1936, is a particle in the model and is 207 times heavier than the electron.

The main issue with the muon is one concerning its wobble. The initial experiment alluding to the muon’s behaviour disagreeing with the standard model was conducted in 2001 at the Department of Energy’s Brookhaven National Laboratory.

Fermilab’s Muon g-2 experiment aimed to continue the DOE’s research using more sophisticated equipment and began operations in 2018. The experiment involved more than 200 scientists from 35 institutions in 7 different countries.

The muon wobbled more than it should have, hinting at new physics

The experiment consisted of high energy muons circulating a 15 metre ring thousands of times whilst travelling at nearly the speed of light. Whilst the muons circulated the ring, they were subjected to a very strong magnetic field. Muons act like tiny magnets which, when placed in a magnetic field, causes them to spin and wobble (combined, this is known as precession) like a spinning top.

The amount the muon wobbled was then measured using detectors lining the ring. The problem was, however, that the so-called g-factor which described the muons wobble, was not the expected value theorised by the standard model! In fact, the muon g-2 experiment suggested that the muon wobbled more than it should have, hinting at new physics.

Fermilab scientist Chris Polly said “less than 6%” of the data collected from the experiment has been analysed thus far. Five runs of data analysis will be combined to calculate a value for this g-factor.

Despite the excitement stirred by this result, caution should be exercised as the accuracy of the experiment is currently not high enough to warrant claiming it as a discovery.

Whatever may be correct, the results found at CERNs LHCb regarding the muon adds fuel to the fire, suggesting there’s new physics we haven’t discovered yet. The LHCb is a high precision detector which looks at the decay of particles and aims to test the standard model.

In this experiment, they were looking at the decay of a particular meson, another type of subatomic particle made up of fundamental particles in the Standard Model of Particle Physics.

Perhaps it’s a new particle or maybe a new force

When the meson decays, it can decay into either an electron or a muon. The LCHb experiment aimed to measure how often the meson decayed into these two particles. From the standard model, it should be equally likely that an electron is produced compared to a muon, however, the experiment suggested that the meson favoured its decay to an electron!

The clear favouritism of decay to the electron compared to the muon, and assuming the experimental results are correct, strongly suggests there is unknown physics at play. Perhaps it’s a new particle or maybe a new force.

News of the muon has caused great excitement in the science world and as further research is conducted in the field, who knows what physics will be uncovered?

Christina Giallombardo

Featured Photo by Rosa Menkman from Flickr. Image license found here. No changes were made to this image.

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