Enigma of the muon

The g-2 experiments

In the 1950s the muon was still a complete enigma. Physicists could not yet say with certainty whether it was simply a much heavier electron (with 200 times the mass) or whether it belonged to another species of particle. Acting on an idea of a visiting American physicist, Leon Lederman, CERN launched the ‘g-2’ experiment in 1959, aimed at measuring one of the properties of this strange electron – its ‘magnetic moment’. The experiment was to test quantum electrodynamics, a theory elaborated in the 1940s to describe the effect of the electromagnetic force on charged subatomic particles such as electrons or muons. Among other things, the theory predicts an anomalously high value for the muon's magnetic moment ‘g’, which is a little more than 2, hence the name of the experiment.

Six physicists joined forces in 1959 to try and measure this value on CERN’s first accelerator, the Synchrocyclotron. Two years later, the team published the first direct measurement with a 2% precision with respect to the theoretical value. By 1962, the precison had been whittled down to just 0.4%. This was a great success since it validated the theory of quantum electrodynamics. As predicted, the muon turned out to be a heavy electron.

The team members then went their separate ways, but one of them stayed on and proposed a follow-up project at CERN’s newly commissioned accelerator, the Proton Synchrotron. A new group was set up and the second experiment, which started in 1966, pushed things even further by obtaining a result 25 times more precise than previous values. The greater sensitivity revealed a quantitative discrepancy between the experimental values and the theory, thus prompting theorists to re-calculate their models.

A third experiment was launched in 1969. The final results were published in 1979 and confirmed the theory to a precision of 0.0007%. From 1984, the United States took up the mantle of investigating the muon’s anomalous magnetic moment, putting the finishing touches to an edifice built at CERN.