One of the smallest things in the universe could have just changed everything we know about it.
On Wednesday, the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) in Illinois revealed much-anticipated results from a major physics experiment known as Muon g-2. The bizarre results, which showed something quite different from what standard theories projected, shocked physicists around the world – and if confirmed, suggest that fundamental physical theories may be wrong.
“This is our Mars rover landing moment,” Fermilab physicist Chris Polly told the New York Times of the findings.
The data, published in the journal Physical Review Letters, showed that fundamental particles called muons behave in a way not predicted by the standard model of particle physics. The standard model is a gold standard theory that explains the four known forces in the universe and all fundamental particles. The standard model even had the existence of the Higgs Boson decades before it was experimentally detected in 2012.
“This is strong evidence that the muon is sensitive to something that is not according to our best theory,” said Renee Fatemi, a physicist at the University of Kentucky and the simulation manager of the Muon g-2 experiment. a press release said.
The above particles, known as muons, acted strangely when exposed to a strong magnetic field at Fermilab. That strange result could be the result of a new, as yet undiscovered fundamental particle – which could possibly throw a wrench into everything people know about physics.
But not all physicists buy the results. The reason has to do in part with a number called sigma.
Sigma seeks
In physics, as in most sciences involving experiments, the experimental results are characterized by a number, sigma, which reflects how likely it is that the result is a random chance.
Say you have come up with a theory that says coins will always come upside down, and then perform an experiment in which you turn a coin over 100 times and see your coin stick out head up every time. It is in fact possible that this could happen – in fact about one in a thousand times, but your results, although initially shocking, would not reconsider the theory of coin flip. This is because 100 times is not enough trials to justify a sigma number that would mean ‘true without a doubt’. This requires a so-called 5 sigma result, which corresponds to the probability of one in every 30 million that your experiment has had a lucky shot.
The Fermilab experiment with muons was a follow-up to an experiment at Brookhaven National Laboratory in 2001, which had a significance of about 3.7 sigma. Combined with the results of the Fermilab, the sigma value rose to a 4.2 sigma; 5 is the gold standard for scientists to claim a new discovery.
In other words, the Muon g-2 experiment did not reach that gold standard five-sigma bar.
Once in a blue muon
Although it is one of twelve fundamental particles in the universe, muons are rarely seen; they have properties similar to everyday electrons, in that they have a charge, yet their mass is much larger than their electron cousins. Muons are also very short-lived: after being created in high-energy collisions, such as when cosmic rays hit the Earth’s atmosphere, they decay an average of 1.56 microseconds later. It is one of the great mysteries of physics that some of the fundamental particles of the universe would be so incapable of surviving in this universe.
Similar to its cousin the electron, muons have an internal magnetism; like any magnet, it can be manipulated and redirected in the presence of magnetic fields. Particle accelerators at Fermilab can produce muons in large quantities. This is what researchers from Fermilab did for the Muon g-2 experiment – tracked how muons interact in a particle accelerator in the presence of a strong magnetic field.
In such a magnetic field, the muon oscillates in a way determined by an intrinsic number known as the g-factor. This number changes depending on the muon’s environment and interaction with other particles. Muon g-2 is designed to measure the g-factor of the muon very high.
What happened during the Muon g-2 experiment is simply that the expected result is different from what the theory determines. The difference on paper seems small. According to the standard model, the accepted g-factor for the muon is 2.00233183620. But the new experiment yielded results at 2.00233184122 – a difference of 0.00000000502.
It may seem small. But for a theory that accurately predicts the properties of particles more than numbers, this difference is very large.
“This amount we measure reflects the muon’s interactions with everyone else in the universe,” Fatemi said. “But if the theorists calculate the same amount, using all the known forces and particles in the Standard Model, we do not get the same answer.”
A cloud of muons, a grain of salt
But despite the excitement of the Fermilab team, some physicists are wary of the findings.
“There’s a bit of skepticism about it,” Bruce Schumm, a professor of physics at the University of California – Santa Cruz and author of a popular book on the Standard Model, told Salon. Schumm has highlighted the success of the Standard Model so far. “If you make a measurement and you compare the expectation based on everything we know – the standard model – there is a bit of concern that the calculation may not be entirely correct,” he notes.
Avi Loeb, former chair of the astronomy department at Harvard University, was more optimistic about the findings – but, as he noted, cautious.
“The measurement is intriguing, but its statistical significance of 4.2 standard deviations did not reach the gold standard in particle physics data of 5,” Loeb told Salon in an email. “Also, it is unclear whether the deviation represents new physics or a theoretical miscalculation; about half a dozen groups of theorists calculate the expected value and theoretical uncertainties obscure the importance of the difference.”
Loeb added: “Over the years, many deviations have emerged only to disappear, leaving the standard model of particle physics unchanged.”
This is indeed true and speaks to the efficiency of the standard model. One previous anomaly occurred in 2018, an experiment related to the binding of muons to protons and then the measurement of the proton’s radius, yielded a peculiar result of the proton. The observed width of the proton, when bound to a muon, appears to be about 4 percent shorter than expected. Some physicists speculate that the result can be explained by ‘new physics’ – non-spatial dimensions, new fundamental particles or something similar. Future studies have found closer to the expected values for the proton’s width; yet these did not have the sigma values to be definitive. As of 2020, the jury is not yet available, but new physics seems less likely.
As for the Muon g-2 results, Schumm said that physicists do know that there is currently a “new effect” on the muons. But that does not mean that new particles have been discovered.
“If there is a new effect, we just know that there is probably a new particle that can be discovered, related to it,” Schumm said.
Could it be that the standard model is wrong?
“It’s definitely an over-dramatization to say that the standard model is threatened,” Schumm said. The standard model has always been known as an ‘effective theory’ since the invention. Schumm compares the standard model to the ‘tip of an iceberg’, in which the point is observed and well understood, even if we do not know exactly what is underwater. I will bet any amount of money [the Standard Model] will never be overthrown, as a representation of the tip of the iceberg, ‘he said.
Schumm compares this scenario to the connection between Newton’s laws and Einstein’s theory of relativity – and notes that Albert Einstein does not throw away Newton’s laws, but builds on them. In other words, if there is a new particle, it is unlikely that the standard model will be thrown to the side, but rather will be built on.
Note that if the difference is confirmed by future experiments, it can not only change the physics, but can promote our understanding of the universe – and may also explain unexplained phenomena such as dark matter, which are related to undiscovered particles.
“If the contradiction is shown by future improvements in experimental data and theoretical calculations, the new particles implied may be related to the dark matter in the universe,” Loeb said. “At the moment we do not know the nature of most of matter in the universe. If we know it, it can help us understand how galaxies like the Milky Way are composed over cosmic history.”
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