The field of particle physics has a new and unlikely star: the muon, sometimes referred to as a large electron. For the second time, this unpretentious resident of the subatomic world appeared to flout the known laws of nature. For some experts, this suggests that it could reveal entirely unknown rules.
Muons caught the attention of physicists around the world after an experiment at the Fermi National Accelerator Laboratory, or Fermilab, in Ill., Showed they are much more magnetic than expected. The results, published in April by the Muon g-2 collaboration (pronounced “g minus two”), goes against the predictions of the best theory available in particle physics. This leaves three possibilities: the theory itself is wrong, the experience was flawed, or, most enticingly, it could also mean that we are about to discover new forms of matter and energy which are essential to order of the cosmos but not detected.
Many scientists adopt the latter explanation. If they’re right, this represents the first credible challenge to the Standard Model of particle physics, which has dominated for half a century as the best description of the basic elements of the universe and how they interact. Wherever he made a prediction, it has proven to be true – until now.
Marcela Carena, head of theoretical physics at Fermilab, thinks these results could prove to be more significant than even the historic 2012 discovery of the the Higgs boson, which permeates all other mass particles. âIt can really shake up the way we think about everything we currently know about particle physics,â she says. “It’s a huge discovery, and it’s a huge discovery that we weren’t expecting.”
A magical and magnetic moment
The possibility of unexplained muon phenomena first surfaced in 2001, when similar experiments at Brookhaven National Laboratory in New York City found something wrong: when it was thrown through a magnetic field in an accelerator 15 meters in diameter, the tiny particle did not act as expected. In technical terms, its “magnetic moment” – a property that essentially causes muons to spin, or wobble, like magnetic bars – was surprisingly large. He wavered more than anyone expected.
The magnetic moment is affected by a menagerie of virtual particles that continually appear and disappear. Chris Polly, another physicist from Fermilab, explains in a blog post that no particle is ever really alone. Even in a vacuum, it is surrounded by an “entourage” of these virtual particles, each of which partly determines its behavior. Calculations of the muon’s magnetic moment, called “g-2” (hence the name of the collaboration), carefully take these external influences into account.
However, a significant difference exists between the measured and predicted values. If known virtual particles cannot sufficiently explain this discrepancy – assuming the measurements are correct – researchers deduce that other unidentified particles must make up the difference.
Lee Roberts, a physicist at Boston University, worked on both the Brookhaven and Fermilab experiments. âIt was a big deal,â he says of the reveal two decades ago, âbecause everyone was desperate for physics beyond the Standard Model.â After the loss of program funding, this data point remained alone and anomalous for two decades. Now, says Roberts, âWe have confirmed the Brookhaven experience. The two results are perfectly consistent.
Beyond the standard model
As thrilling as it may be, the new experience presents a puzzle with no idea of ââa solution. âWe know something new has to exist,â Carena says, âbut we don’t know what it is. Theorists have developed a constellation of explanations, and none particularly stands out.
Given the choice, Carena’s preferred theory is supersymmetry, a popular extension of the standard model. He postulates that every known particle has an undiscovered partner. These stealthy counterparts could be the virtual particles contributing to the muon’s magnetic moment, as well as dark matter ingredients believed to make up about 27% of the universe – solving two mysteries with a single eureka.
Still, she admits that this is wishful thinking: âI think it would be a lot sleeker if we got clues from this experience to other things in the universe, but nature may not work. in this way.” Other hypothetical particles – mainly leptoquarks and Z ‘bosons – could fill the void just as easily, without further expanding our understanding of reality. If these do exist, however, the Large Hadron Collider should eventually detect them.
The question is very complex, in part because scientists do not know where to look for the answer. The Fermilab experiment offers only indirect evidence, alluding to new laws of physics across the muon. Overall, he clearly does not identify any particular suspect. “In order to distinguish which of these is right,” Carena says, “we’ll have to look elsewhere.”
As to why muons behave badly, it is too early to draw firm conclusions. It is also, perhaps, early enough to say with confidence that they are doing it in the first place. So far, Muon g-2 members have only released about 5% of their data. Given how thoroughly they analyzed the first set, Roberts is optimistic that the rest will reinforce the same conclusion that has been released over the next several years. But for now, the sparse evidence does not justify updating the Standard Model.
Some physicists have hesitated to accept a redesign of the most successful scientific theory of all time. In another study Posted in Nature the same day, Zoltan Fodor of Pennsylvania State University and his colleagues recalculated g-2 to check for errors in theoretical work from the past two decades. Their calculations yielded a new theoretical value which differs from that on which physicists have generally agreed, but which corresponds to the experimental results of Brookhaven and Fermilab.
This suggests that the Standard Model can adapt to experiences after all. “While it may have been exciting to discover clues to new physics,” Fodor writes, “our new theory seems to say that this time the Standard Model is holding up.”
As with any scientific discovery, time and subsequent research will tell. For now, one thing is certain: the standard model remains incomplete. It doesn’t say anything about dark matter and dark energy, or why our universe is made up of matter rather than antimatter. But physicists may have recently stumbled upon a clue, and for many, that in itself deserves celebration. âFinding something outside of this Standard Model is sort of the goal of all particle physics,â says Roberts. “There are all these very deep fundamental questions, so any insight into that is very exciting.”