One of the most prevalent subatomic particles in the universe, the muon, appears to misbehave. Or at least it isn’t behaving the way physicists expect. In fact, muons deviate so much from the laws of physics that scientists have begun to believe that our knowledge of it is incomplete, or that there is a force in the universe that we do not yet know.
Muons are like large electrons: they have negative charges but are 207 times heavier than electrons. Thanks to their charge and a property called “spin”, they act like tiny magnets. So when muons are immersed in another magnetic field, they undergo an infinitesimal oscillation. But in a study Physicists in Fermilab, Illinois, the post this week published a discrepancy between the number of muons that must oscillate and the number of muons that must actually oscillate during a laboratory experiment.
The difference is large enough that many scientists are convinced that particles or forces we have yet to discover must be involved. In other words, this discovery is further evidence that something mysterious has played a role in shaping our universe – something that is absent from the rules of current physics.
“In this respect, the new scale may be the beginning of a revolution in our understanding of nature,” Thomas Tuebner, a theoretical physicist at the University of Liverpool and co-author of the new study, told Insider. It is possible that this unknown phenomenon is also related to dark matter, the mysterious cousin of matter that arose immediately after the Big Bang and makes up a quarter of the universe.
Shoot muons into a circle at the speed of light
When cosmic rays enter Earth’s atmosphere, they create muons. Several hundred muons hit your head every second. They can penetrate things like x-rays – a few years ago scientists used muons to discover a hidden room in the Great Pyramid of Egypt – but the particles only last two millionth of a second. Then, they disintegrate into groups of lighter particles.
During its short life, each muon stays directed around a single point, just as a compass always points north. But when it encounters a magnetic field, the muon direction deviates slightly from that point. This is the critical oscillation, known as the g factor, that the Fermilab trial is examining.
Fermilab is a US Department of Energy project, associated with the University of Chicago, dedicated to the study of particle physics. Scientists can produce muons there to study by passing a beam of protons very quickly through the metal with a particle accelerator.
Therefore, the researchers behind the new study took these muons and directed them toward a circular electromagnet with a diameter of 15 meters. Then the muons traveled at close to the speed of light, and orbited around the circle more than 1,000 times. As muons in the device decay, the ultra-sensitive detectors can measure the direction in which the resulting small particles are moving. Physicists can then use this information to calculate the location of the fixed point per muon.
It should be possible to calculate the exact amount of muons that will oscillate using the Standard Model of Physics, which includes everything we know about the behavior of particles. But the Fermilab team found that their muon oscillation did not meet these expectations. On the contrary, it is offset by a third of a millionth of a hundredth.
This difference may sound incredibly small, but Thomas Teubner said it was, in fact, a “milestone” for particle physics. And it’s not likely to be the result of an error: the team found that there was only one chance in 40,000 that the difference in their measurements was due to chance. “This is strong evidence that muons are sensitive to something not in our best theories,” René Fatemi, one of the leaders of the Fermilab Muon experiment, said in a report. Press release.
A 20-year-old puzzle
This is not the first time that muons have not behaved as the best scientific theories have predicted. In 2001, Brookhaven National Laboratory in New York conducted a similar experiment using the same giant electromagnet. These results also showed that the muon oscillation in vitro was deviating from what it should have been. But these results were less statistically significant than those for Fermilab: there was a 1 in 1,000 chance that it was a coincidence.
Today, the Fermilab results confirm what the Brookhaven physicists discovered 20 years ago – and that “made the difference already observed with the ancient finding even more interesting,” Thomas Teubner said. Fermilab is expected to publish data from two similar trials over the next two years. A fourth trial is already underway and a fifth under preparation.
What affects muons could have something to do with dark matter
According to Thomas Tuebner, it is possible that a force not part of the Standard Model of physics could explain the muon jitter. He says that this force could also explain the existence of dark matter, and perhaps even dark energy, which plays a major role in accelerating the expansion of the universe. “Theorists will find it interesting to solve more than one problem at a time,” says Thomas Teubner.
One hypothesis that could apply to both muons and dark matter, he added, is that muons and all other particles have nearly identical partner particles that interact weakly with them. This concept is known as supersymmetry.
But current Fermilab technologies are not sensitive enough to test this idea. Moreover, Thomas Teopner adds, the mysterious effect on muons may not be related to dark matter at all, which means that the rules of physics are inadequate in more ways than one.
An original copy : Eileen Woodward / Insider
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