Sommaire:

Nearly twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon's anomalous magnetic moment, a_mu, with a remarkable precision of 0.54 parts per million. Since that time, the reference Standard Model prediction for a_mu has exhibited a discrepancy with experiment of over 3 standard deviations, raising the tantalizing possibility of elementary particles or fundamental forces as yet undiscovered. On April 7, 2021 the physicists of an ongoing experiment at Fermilab presented first results of a new measurement of a_mu, brilliantly confirming Brookhaven's measurement and bringing the discrepancy with the reference prediction to a near discovery level of 4.2 sigma. This discrepancy was further enhanced to 5.1 sigma this past with the publication of Fermilab’s new result that reduces the measurement uncertainty by a factor of 2. According to usual particle physics standards, such a discrepancy would mean that new fundamental physics has been uncovered.

In the meantime a very large-scale supercomputer calculation of the contribution that most limits the precision of the Standard Model prediction was performed by the Budapest-Marseille-Wuppertal collaboration. The results of this lattice quantum chromodynamics (QCD) calculation paint a very different picture, in particular reducing the difference between theory and experiment and suggesting that new physics may not be needed to explain the current, experimental, world-average value of a_mu. However, it does so at the expense of an untenable discrepancy with the data-driven determination of this most uncertain contribution.

After an introduction and a discussion of the current experimental and theoretical status of a_mu, I will present this precise lattice QCD calculation and partial confirmations by other teams. I will also present a framework that enables a comparison of the primary ingredients that are used in the lattice QCD and data-driven approaches and discuss the steps that are required to make a Standard Model prediction that will allow determine whether the final results of the Fermilab experiment, expected in 2025, indicate the presence of new fundamental physics.

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