The findings, published today in Nature, bring together experts from Europe, the United States and Australia, delivering the most precise calculation yet of a crucial factor behind the muon’s magnetic behaviour.
The muon is similar to an electron but roughly 200 times heavier. These particles are constantly produced when high-energy cosmic rays collide with Earth’s atmosphere, with around 50 passing through the human body every second.
Like electrons, muons act as tiny magnets. The strength of this magnetism, referred to as the magnetic moment, has long been used to test the accuracy of the Standard Model, the leading theory describing the fundamental particles and forces that make up the universe.
For years, scientists observed a persistent mismatch between theoretical predictions and experimental measurements of the muon’s magnetic moment. This gap raised hopes that it might point to previously unknown physics beyond the Standard Model.
However, the new research appears to close that gap, bringing theory and experiment into close alignment and strengthening confidence in the existing model.
Award-winning Adelaide-based physicist Dr Finn Stokes said the study focused on the most uncertain part of the theoretical calculation, known as the “hadronic vacuum polarisation” contribution. This component stems from complex interactions between quarks and gluons, governed by the theory of quantum chromodynamics.
“These strong-force effects are incredibly difficult to calculate with high precision,” Dr Stokes said.
“To tackle this, we developed a hybrid approach that combines large-scale computer simulations with real-world experimental data.”
Using some of the world’s most powerful supercomputers and a method called lattice quantum chromodynamics, the team achieved calculations at an unprecedented level of detail. The result is nearly twice as precise as previous global estimates.
The researchers were able to determine the hadronic vacuum polarisation contribution with unmatched accuracy, leading to an updated prediction for the muon’s magnetic moment. This revised figure aligns with the latest experimental results to within just 0.5 standard deviations, a remarkably close match in particle physics.
Dr Stokes said the breakthrough highlights the value of combining theoretical and experimental approaches to solve complex scientific problems.
“This is a significant step forward in testing the Standard Model,” he said.
“With uncertainties reduced, we can now compare theory and experiment with extraordinary precision, confirming the model to 11 decimal places.”
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