Recent (2021) analysis of experiments carried out at the US Fermilab accelerator have found that the 'magnetic moment' (magnetic field strength) of the Muon is not in exact agreement with calculations derived from the Standard Model

The experiments are a confirmation of previous results from US Brookhaven National Laboratory (BNL) in 2001, which also found anomalies.

The measurements only differ by a few millionths of a percent from the calculated values - but nevertheless can't currently be explained.

A Fermilab press release says the the results "strengthen evidence of new physics". Other media reports say there is "Strong evidence" for a new force of Nature (in addition to the Standard Model's currently known four forces)

The research paperitself, however, makes no mention of those claims - simply stating instead that the discoveries will motivate the development of further investigations.

(Note: Another paper, published within days of the Fermilab report, and which used computer-based simulations of current Quantum ChromoDynamics (QCD) lattice theory, suggested that previous theoretical calculations of the magnetic moment may have been inaccurate. See : Nature.com)

Update Aug 2023 :

A new set of experiments performed at the Fermi National Accelerator Laboratory, US, have confirmed previous measurements to an accuracy of around one part in five million.

However, another (independent) experiment at CMD-3, based in Novosibirsk, Russia, found results that did not agree exactly with the Fermi results, or with the theoretical QCD calculations.

To sum up. There are as-yet-unexplained differences between calculated results and experimental measurements. There could be errors in one or more of the approaches, but researchers don't currently agree where the errors are occurring. Other groups suggest that perhaps there may not be any errors in the experimental results, and that the theoretical computational models might be missing the full picture.

For a roundup of the current (2023) viewpoints, see Scientific American August 10, 2023