The odd behaviour of a fundamental particle hints at new particles and forces

Laura Burn discusses recent findings on the strange behaviour of a fundamental particle in physics and what the implications of this could be.

The discovery of the Higgs Boson particle in 2012 seemed like the final piece of the puzzle in our understanding about the universe, fitting easily into the Standard Model, which is used to describe the building blocks of matter. Despite the model being able to make exceptionally precise predictions about particle interactions, there are still some things it cannot explain. One of these big experimental mysteries is the weird and wonderful muon.

The muon is a fundamental particle, meaning it cannot be broken down into any smaller particles. It is similar to the electron, but with much greater mass. The muon is an unstable particle which has a lifetime of approximately 2.2 μs (0.0000022 s). To put that into perspective, a honeybee’s wing flap takes 0.0005 s. As the muon is unstable, it cannot make up the matter in the universe. Instead, it is usually found as a product of cosmic rays colliding with the Earth’s atmosphere.

The muon is an unstable particle which has a lifetime of approximately 2.2 μs

The muon is electrically charged, meaning that when in a magnetic field they start to spin. The speed of this spinning is predicted by the Standard Model, which has had success in predicting this very precisely for other fundamental particles. However, in 2006, the Brookhaven National Laboratory found, in an experiment named muon g-2, that the muons were spinning faster than was predicted. This posed a large problem for the validity of the Standard Model; however, the results were not statistically significant enough to confirm the theory was wrong.

In April 2021, Fermilab, continuing with thee experiment but using more precise instruments, found the same anomaly: the muons were spinning faster than predicted. Despite the experiment producing more significant results than Brookhaven, they are still not significant enough for scientists to claim that a discovery has been made. Nonetheless, these results are still very compelling, especially since 2 experiments have found the same anomaly. Moreover, there is still hope in proving this anomaly because Fermilab hasn’t finished analysing all the data.

So, what do these results mean for the Standard Model? It means it may not be the current picture of the universe and this model may not be the best fit. Scientists have proposed that these results may indicate the existence of a new force and an associated particle. Currently there are 4 recognised forces in the world: gravity, electromagnetism, and the strong and weak forces. These are the fundamental forces that help scienctist describe why the world works the way it does. The recognition of a new force may help to solve other mysteries, such as dark energy, the name given to the unknown energy that is speeding up the expansion of the universe.

The current picture of the universe and the Standard Model may not be the best fit

These results have brought particle physics back to the forefront of science. Despite it raising more questions, it is exciting to know there are more puzzles to solve about the make-up of the universe, and that the mysterious muon may pave the way to answering these.