An experiment at Fermilab points to undiscovered forms of matter and energy lurking in the universe
There might be dozens of undiscovered subatomic particles in the universe, and new physics might be lurking around the corner, if the results of a recent scientific experiment, conducted at Fermilab in the US, are correct. An international team of 200 particle physicists spread among seven countries made the announcement earlier this month, immediately setting the world of physics abuzz.
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Graziano Venanzoni, one of the leaders of the experiment and a physicist at the Italian National Institute for Nuclear Physics, underscored the importance of the experiment in a suitably grandiose manner, calling the day of the announcement, “An extraordinary day, long awaited not only by us but by the whole international physics community.”
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The team had observed that a particle called a muon – a heavier cousin of the electron that carries electricity – was not behaving as predicted in the presence of a magnetic field. Muons occur in nature when cosmic rays strike Earth’s atmosphere, and particle accelerators at Fermilab produce them in large numbers. Like electrons, muons act as if they have a tiny internal magnet.
In a strong magnetic field, the direction of the muon’s magnet wobbles, much like the axis of a spinning top or gyroscope. At the Fermilab experiment, the muon was wobbling too much – in a manner inconsistent with the so-called Standard Model, the highly successful theory of physics that describes the subatomic world.
An experiment at Brookhaven National Lab had seen something similar 20 years ago, but physicists hadn’t been sure that the results were statistically valid. So they had decided to repeat it with more precision.
And the muons seemed to be wobbling too much again, implying some unknown particles and forces were giving them an extra push. The Fermilab team calculated that their measurements have about one chance in 40,000 of being wrong. The Standard Model has held sway for 50 years, even though everyone agrees it is incomplete. Is it goodbye to all that?
“Something is missing in the Standard Model,” said William Morse, an experimental particle physicist at Brookhaven National Laboratory who had been part of the team that had seen the muons misbehave 20 years ago. “We wouldn’t be here if the Standard Model were right and complete.”
He was hinting at the fact that, while we see observational evidence of the Big Bang that created the universe, the Standard Model cannot account for the presence of stars and galaxies in the universe.
Another discrepancy is the presence of dark matter, which our telescopes cannot see but which we know is there from its gravitational footprint. The Standard Model doesn’t account for the large discrepancy of matter over antimatter in the universe – matter and antimatter are supposed to annihilate when they come together; antimatter is one of the predictions of the Standard Model.
So, if the Standard Model is incomplete, what could be going on? An explanation of the increased muon wobbling could be a theory called supersymmetry, says Morse, which, if true, would double the number of elementary particles. “Every known particle would have a supersymmetric partner … Supersymmetry could also give you a universe you could live in.”
There are a few different versions of supersymmetry, but what they share in common is that these partner particles would be heavier than the ones in the Standard Model. The Large Hadron Collider, the best particle smasher in the world that was built a few years ago in Geneva by CERN, was supposed to have found supersymmetric particles but hasn’t seen any so far.
This leads some to question if it is the right explanation. William Marciano, a theorist at Brookhaven, put it this way, “Supersymmetry has a good theoretical framework, and has the potential to explain dark matter, but its star has dimmed.”
Many theorists urged caution before throwing out the Standard Model. Marciano suggested that if there is indeed an excess wobble in the muon due to new physics, it may affect other things such as the magnetic moment of an electron. “If the muon is showing an effect due to new physics, the electron may also show a small effect,” he said. An experiment to determine the magnetic moment of the electron more precisely is being pursued.
Zoltan Fodor at Penn State is another theorist who disagrees with the implications of the Fermilab paper. He has done a theoretical simulation of how much the muon’s wobble should be using a different technique that called for hundreds of hours of supercomputer time. “There is no new physics,” he stressed.
The story is far from over. The Fermilab team is continuing its experiments with muons, having only analysed 6% of the data the experiment will eventually collect. The team is hoping that it might find more evidence that will lead to the overthrow of the Standard Model and herald a universe teeming with more elementary particles. If that happens, the Standard Model will follow many other previous theories that have fallen to the wayside in the inexorable march of science.