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 paper itself, 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, suggests that previous theoretical calculations of the magnetic moment may have been inaccurate. See : Nature.com
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