▲ Director Yannis SEMERTZIDIS standing in the center of the muon storage ring at Brookhaven.

The US Fermi National Accelerator Laboratory (Fermilab) has unveiled the newest results from the muon g-2 experiment. The goal of this ambitious experiment, which had a major input and impact from the Institute for Science Center for Axion and Precision Physics Research (IBS-CAPP), was to achieve the most precise measurement of a muon’s magnetic moment. By comparing the experimentally obtained value against theoretical prediction, the physicists sought to put the 'Standard Model', the cornerstone of modern particle physics, to the test.

Muons, a class of particles similar to electrons but 207 times heavier, are produced during energetic particle collision events. The muons were first discovered in 1936 during the study of cosmic rays that bombard the Earth’s atmosphere. According to the Standard Model, muons are considered one of the 17 fundamental particles that make up the ordinary matter in the universe.

The theoretical value of a muon’s magnetic moment should be ‘2’, according to the Standard Model. However, the model also predicts that the experimentally measured value will be slightly higher due to the emergence of ‘virtual particles’ that arise spontaneously in the vacuum. This deviation between these two values is called g-2, which is predicted to be 0.00233183620 according to the Standard Model. The precise measurement of this value is the goal of Fermilab’s experiment.

The latest Muon g-2 experiment obtained the most accurate results up to date by encompassing five times more muon particles (400 billion) than last year’s announcement. The result so far showed a g-2 factor of 0.00233184110, which may or may not exceed the value predicted by the Standard Model depending on the final conclusion regarding the theoretical approach in estimating the same number.

▲ Value and error range of the muon g-2, obtained by Brookhaven and Fermilab experiments

After 6 years of data collection, the Fermilab finally shut down the muon beam, gathering 21 times more data than the previous Brookhaven experiment. However, Fermilab has not definitively concluded whether this experiment's results necessitate the revision of the current Standard Model, citing there are still additional steps needed to curate the data. It is expected that the refinement of the muon's g-factor will be continued until 2025, which will mark the final showdown between the experiment and the Standard Model.

Director Yannis SEMERTZIDIS of the IBS-CAPP has served as a researcher overseeing the electrostatic focusing system of the muon storage ring in the Brookhaven Experiment in 2001. Even after being appointed as the Director of IBS-CAPP, he continued to participate in the Fermilab experiment by providing crucial expertise in designing the experimental apparatus.

IBS-CAPP members did an extensive study and developed hardware that was used in runs 5 and 6 (the last run that ended this year) at Fermilab. This IBS-CAPP system eliminated two important systematic error sources for the experiment and IBS-CAPP is regarded very highly within the collaboration for this ingenious application.