Beyond the Eighth Decimal: How a Muon's Magnetism is Cracking the Standard Model

Imagine a spinning particle that interacts with magnets. This property, called magnetic moment, is a fundamental feature of particles like the muon, a heavier cousin of the electron. 

Scientists precisely measure this property, but a tiny discrepancy between theory and experiment has them buzzing.

The difference is miniscule, only appearing at the eighth decimal place, yet it's been a puzzle since 1948. This tiny gap, called g-minus-2 (g-2), could reveal new forces or exotic particles like dark matter or extra Higgs bosons.

Theory vs. Experiment: A Magnetic Tug-of-War

Theory predicts a specific value for the muon's g-2, based on the Dirac equation. However, experiments show a slightly higher value. This difference hints at unseen factors influencing the muon's magnetism.

The challenge lies in calculating the contributions from other particles that flit in and out of existence around the muon. These virtual particles affect the measured g-2.

Two Methods, Two Answers: The Discrepancy Deepens

Scientists use two methods to estimate these virtual particle contributions. One relies on experimental data from electron-positron collisions, while the other uses powerful computer simulations called lattice QCD.

Here's the twist: these methods disagree! The electron-positron data doesn't match the total experimental value, while the lattice QCD simulations seem to align better. This mismatch needed solving.

“Precise determination of the muon’s magnetic moment has become a key issue in particle physics because investigation of this gap between the experimental data and the theoretical prediction can provide information that could lead to the discovery of some spectacular new effect,” physicist Diogo Boito, a professor at the University of São Paulo’s São Carlos Institute of Physics (IFSC-USP), told Agência FAPESP.

New Approach Shines Light on the Puzzle

Researchers developed a new method to compare the simulation results with the electron-positron data. They focused on specific particle interactions (connected Feynman diagrams) that simulations can calculate very precisely.

The results were striking. Eight independent simulations all agreed on the contribution of these specific interactions. However, these values differed from the electron-positron data.

A Suspect Emerges: The Two-Pion Channel

This discrepancy pointed to a potential culprit: the data used for two-pion interactions (a specific type of particle interaction) might be underestimated. New, unconfirmed data from Russia suggests this might be the case.

If the two-pion data is indeed underestimated, it could explain the discrepancy. Further investigation and confirmation of this new data are crucial.

This tiny magnetic difference in the muon holds the key to unlocking mysteries of the universe. By solving this puzzle, scientists may discover entirely new forces and particles, rewriting our understanding of the subatomic world.


Published 21 December 2023, Physical Review Letters “Data-Driven Determination of the Light-Quark Connected Component of the Intermediate-Window Contribution to the Muon g−2” 

DOI: 10.1103/PhysRevLett.131.251803


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