The peripatetic Muon g-2 experiment took data from the Brookhaven National Laboratory in New York from 1997 to 2001 and has been taking data from the Fermi National Accelerator Laboratory in Illinois since 2018.
Michael Linden CC BY-NC 2.0
By Adrian Cho
Talk about rain at the parade of your colleagues. On April 7, a collaboration of more than 200 experiments with great excitement announced that a particle called the muon is slightly more magnetic than the physical scientific standard model predicts, a difference that may indicate new particles waiting to be discovered. But on the same day, 14 theorists published a paper indicating that the consensus-theoretical prediction was incorrect. Its value is closer to the experimental result and makes the tantalizing difference almost disappear.
“According to our calculation, the standard model is just fine,” said Zoltan Fodor, a theorist at Pennsylvania State University, University Park, and the leader of the Budapest-Marseille-Wuppertal (BMW) collaboration, which yielded the new theoretical result. . Others say, however, that it is too early to throw away the previous calculation, which is the result of decades of careful effort. “We can not immediately ignore everything we know and switch to a single new result of a new method,” says Christoph Lehner, a theorist at the University of Regensburg.
The muon, which is a heavier, unstable cousin of the electron, acts like a small bar magnet, and magnetism is a way of looking for hints of new particles. Quantum mechanics and relativity require the muon to have a certain basic magnetism. Thanks to quantum uncertainty, particles and antiparticles are also constantly flashing in and out of existence around the muon. These “virtual” particles can not be directly observed, but they can affect the muon’s properties, including magnetism. Standard model particles should increase the magnetism by about 0.1%, and still unknown particles will give their own boost. Such particles can one day be broken up at an atomic breaker.
This is why physicists were so excited when the Muon g-2 experiment at the Fermi National Accelerator Laboratory confirmed a 20-year hint that the muon is about 2.5 parts per billion more magnetic than the standard model predicts, according to the consensus value, hammered. last year by the Muon g-2 Theory Initiative of 132 members.
To make the prediction, the theorists had to account for the thousands of ways in which standard models can flutter particles around the muon and influence its behavior. One family of processes, known as hadronic vacuum polarization, is particularly challenging and limits the accuracy of the entire calculation. In it, the muon emits particles known as hadrons, and consists of other particles called quarks. The quark theory and the strong nuclear force that bind them, quantum chromodynamics (QCD), are so awkward that theorists cannot calculate the effects through the usual series of smaller approaches. Instead, they have to rely on data from accelerators that create hadrons by colliding electrons and positrons.
Not noticing the gap?
If a new “grid” value for the magnetism of the muon is correct, a mysterious gap between other predictions and a recent measurement will all but disappear.
GRAPH: V. ALTOUNIAN /SCIENCE; DATA: ABI ET AL., PHYS. NOTE EASY., 2021; BORSANYI ET AL., EARTH DOI: 10.1038 / S41586-021-03418-1; BLUM ET AL., PHYS. NOTE EASY., 121, 2018; AOYAMA ET AL., PHYSICS REPORTS, 2020
However, there is another way. Theorists can perform QCD calculations on supercomputers on brute force as they model the continuum of space and time as a grid of discrete points occupied by quarks and particles called gluons, which transmit the strong force. Twelve years ago, theorists showed that this “grid QCD” technique could calculate the masses of the proton and the neutron, which are both hadrons. Several groups also applied the grid to the magnetism of the muon, albeit with great uncertainties.
Using hundreds of millions of processing hours at the Jülich Research Center in Germany, Fodor’s group made a grid calculation of the hadronic vacuum polarization and a value for the muon’s magnetism that accurately opposes the consensus standard model value. And the new result is just one part per billion below the experimental value, the team reports in Nature. Given the uncertainties, it is too close to indicate that it is contradictory, Fodor says.
He also asks questions about the consensus value. For key data, it mainly depends on the results of two collisions, and the two data sets do not differ to a worrying extent, says Fodor. Our team’s outcome is free of such uncertainties. “It’s the only calculation on the market, so some people are uncomfortable,” he says.
Yet some theorists say it is too early to place so much weight on a single grid calculation. Aida El-Khadra, a lattice theorist at the University of Illinois, Urbana-Champaign, and a leader of the Muon g-2 Theory Initiative, notes that the uncertainties in the consensus value mainly reflect the limited precision of the input data. In contrast, the uncertainty in the lattice value reflects the reliability of the method itself and is more difficult to quantify and interpret, says El-Khadra. “The meaning of the mistakes is very different,” she says.
In 2018, Lehner and colleagues also conducted an analysis that combines acceleration data and a lower-precision grid calculation. Their hybrid estimate of the muon’s magnetism agrees well with the consensus forecast, Lehner says.
“The BMW result needs to be confirmed by other independent grid calculations,” says Alexey Petrov, a theorist at Wayne State University. These high-precision calculations should appear within a year. But if the grid results agree, but not with the data-driven approach, theorists will still have to figure out why the two methods disagree, Petrov says.
Until then, it would be premature to say that the tantalizing mystery raised by the g-2 measurements has been explained, El-Khadra says. “The standard model calculation is solid,” she maintains. The experimental value is therefore also. And to the best of physicists’ knowledge, it’s different.