Getting to grips with . . .

The fifth force of nature

Unexpected behaviour of subatomic particle could change our understanding of the universe

A tiny subatomic particle appears to be disobeying the laws of physics in a way that scientists say could change our fundamental understanding of how the universe works. 

The “particle celebre” is the muon, like an electron but far heavier, and “is an integral element of the cosmos” reports The New York Times

Scientists have observed the particle behaving in a way that is significantly different to how it should under the accepted laws of physics, suggesting the particle is “sensitive to something” outside our current understanding of the universe.

Currently, we divide the forces that govern our lives into four categories: gravity; electromagnetism; the strong force, which holds particles together; and the weak force, which is involved in radioactive decay.

But the “standard model” – the currently accepted theory to explain how the building blocks of the universe behave – can’t account for gravity, leading scientists to believe that the model “cannot be the whole story”, and so they have been “looking to move beyond it”, says The Times

Now they may be able to. Scientists at Fermilab, a particle accelerator near Chicago in the United States, spun muons around a 15-metre magnetic ring at nearly the speed of light, and noted the speed at which they wobbled. The muons’ behaviour in the magnetic field was different from what would be expected using the standard model: they wobbled at a faster rate. 

This suggests, said Chris Polly from Fermilab, “there might be monsters we haven’t yet imagined, that are emerging from the vacuum interacting with our muons”. In other words: a new force or subatomic particle. 

But the physicists are celebrating cautiously for now, as the results have a one in 40,000 chance of being a statistical fluke. The statistical confidence level is 4.1 sigma, and a level of 5 sigma (one in 3.5 million chance of being a coincidence) is needed to hail the findings as a new discovery, says the BBC

But the results do chime with those of a previous experiment, Muon g2, run in Brookhaven National Laboratory, New York, in 2001.

“After 20 years of people wondering about this mystery from Brookhaven, the headline of any news here is that we confirmed the Brookhaven experimental results,” Dr Polly said.

The results could help explain some of the “big puzzles” of the universe, says the BBC. For example, the expansion of the universe, which has previously been attributed to a “mysterious” phenomenon called dark energy, could in fact be evidence of a fifth force. 

“I’m very excited,” said Marcela Carena, head of theoretical physics at Fermilab, but not part of the experiment. “I feel like this tiny wobble may shake the foundations of what we thought we knew.”

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