A subatomic particle called the muon caused waves when its experimental behaviour didn't align with a prediction based on the standard model. A new calculation might resolve the discrepancy – but some particle physicists are sceptical.
The puzzling behaviour of a fundamental particle called a muon, which some scientists think could be a sign of new physics, might actually be explained by our current theories. That’s according to a new calculation that its proponents say is the most precise ever. But not all physicists are convinced.
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| In experiments, subatomic particles don’t always behave as theory would predict ATLAS Collaboration |
The muon is a tiny charged particle similar to the electron, but much heavier. Its electric charge makes it spin and wobble slightly when placed in a magnetic field, behaviour that can be measured as a quantity called the g-factor. Recent measurements of the muon’s g-factor, made by scientists at Fermilab in Illinois, show that it appears to be spinning slightly faster than a prediction from the standard model, our best picture of how all particles and forces affect one another.
This prediction is based on a common calculation that has accurately predicted the properties of many different particles. But for the muon, the significant gap between prediction and measurement had physicists excited at the prospect of new physics that might explain the discrepancy.
However, Zoltan Fodor at Pennsylvania State University and his colleagues suggest that discrepancy might not exist at all. Using a technique called lattice quantum chromodynamics (QCD), they calculated the most precise theoretical value for the g-factor so far – and found that it aligned with the Fermilab measurement. “If our calculation is correct, then it seems to be that this is a beautiful confirmation of the standard model,” says Fodor.
Their lattice QCD approach is newer and less tested than the theoretical technique that found a discrepancy. But it has had success in calculating certain quantities, such as the mass of the proton or neutron, that closely agree with experimental measurements. Those results suggest the technique is sound, says Fodor.
“It agrees with the measurements up to 12 digits,” he says. “It’s a fantastic triumph.”
But because the technique hasn’t been tested as much, there could still be problems with the calculations, says Alex Keshavarzi at the University of Manchester, UK. “I’m not saying that it shouldn’t be trusted, but the uncertainties that come from it are sizeable and they’re difficult to quantify well.”
Although the new result appears to line up with the Fermilab measurement, that doesn’t mean it is correct and the older calculation is wrong, says Fedor Ignatov at the University of Liverpool, UK. Researchers still need to understand why the results differ. “It’s nice that it’s consistent with the Fermilab measurement, but still, until we understand other pieces of this problem, we cannot claim that we’ve solved the problem.”
To confirm the new results, says Fodor, we will need to wait for other groups that are currently working on their own lattice QCD calculations, as well as further experimental data expected from the J-PARC experiment in Tokai, Japan. If they match, then it will strongly suggest that the standard model is right, he says.
“We should understand that, usually, it’s a more difficult task than to calculate a single measurement,” says Ignatov.
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