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<\/p>\nA new high-precision calculation of a key component underpinning the magnetic moment of the muon, a heavier cousin of the electron, brings theory and experiment into rare alignment, reinforcing the Standard Model and dimming hopes of new physics.<\/p>\n
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A muon particle passing through lead in a cloud chamber. Image credit: Jino John 1996 \/ CC BY-SA 4.0.<\/p>\n
The muon is a subatomic particle similar to an electron but around 200 times heavier.<\/p>\n
Muons are produced when cosmic rays hit Earth\u2019s atmosphere. Roughly 50 of these muons pass through the human body every second.<\/p>\n
Like the electron, the muon behaves as a tiny magnet. The strength of this magnetism (its magnetic moment) has long served as a powerful test of the Standard Model, the theory describing the fundamental particles and forces of nature.<\/p>\n
\u201cThe muon is a short-lived elementary particle with spin 1\/2 and a mass 207\u2009times larger than that of the electron,\u201d said Adelaide University physicist Finn Stokes and his colleagues.<\/p>\n
\u201cBoth particles create a magnetic field around them, characterized by a magnetic dipole moment.\u201d<\/p>\n
\u201cThis moment is proportional to the spin and charge of the particle and inversely proportional to twice its mass.\u201d<\/p>\n
For years, the strength of the muon\u2019s magnetism has exhibited a persistent discrepancy between theory and experiment, hinting at the possibility of undiscovered physics beyond the Standard Model.<\/p>\n
However, the team\u2019s new study finally resolves this discrepancy, reinforcing this model, rather than breaking it.<\/p>\n
\u201cOur research focuses on the most uncertain part of the theoretical prediction: the hadronic vacuum polarization contribution, which arises from the complex interactions of quarks and gluons governed by quantum chromodynamics (QCD),\u201d Dr. Stokes said.<\/p>\n
\u201cThese strong-force effects are really difficult to calculate with high precision.\u201d<\/p>\n
\u201cTo overcome this challenge, we used a novel hybrid approach that combines large-scale computer simulations with experimental data.\u201d<\/p>\n
Using some of the world\u2019s most powerful supercomputers and a technique known as lattice QCD, the researchers performed calculations at a higher resolution than ever before, allowing them to significantly reduce uncertainties.<\/p>\n
The result is almost twice as precise as the previous worldwide consensus.<\/p>\n
They determined the hadronic vacuum polarization contribution with unprecedented accuracy, leading to a new Standard Model prediction for the muon\u2019s magnetic moment.<\/p>\n
This updated prediction agrees with the latest experimental measurements to within just 0.5 standard deviations.<\/p>\n
\u201cThe work demonstrates the power of combining theoretical and experimental techniques to tackle some of the most challenging problems in physics,\u201d Dr. Stokes said.<\/p>\n
\u201cThis is a major step forward in our ability to test the Standard Model. With this reduction in uncertainties, we can now compare theory and experiment with unprecedented precision, providing a remarkable validation of the Standard Model to 11 decimal places.\u201d<\/p>\n