One of the puzzles of present-day physics is that of the existence of the muon. An experiment recently completed at CERN has done much to clarify the problem, although the puzzle still remains.

Early in 1961 a scientific communication from CERN announced that the "anomalous magnetic moment" of the muon had been measured directly for the first time, and had been found to confirm theoretical predictions to within 2% (Charpak et al. 1961). This result showed, contrary to what many had hoped, or even expected, that the muon was indeed very similar to the electron, in spite of being some 200 times heavier.

During the year, the experiments were refined and continued, and recently a new result has been published, confirming the similarity with even greater accuracy (Charpak et al. 1962). Treating the muon as a simple "Dirac particle", that is just as a heavy electron, its anomalous magnetic moment is calculated from the theory of "quantum electrodynamics" as 0.001165. This latest experimental result shows the value to be 0.001162 ± 0.000005. [The article continues for a time.]

Some implications

The first part of this number is the most probable "answer" from the experiments; from the second part it can be said that the odds are 20 to 1 against a true value larger than 0.001172 or smaller than 0.001152. Thus, it has been shown that the muon is best regarded simply as a "heavy electron", and not as some quite distinct particle. The closeness of the agreement with the calculated value shows in fact that, if the muon interacted with some unknown particle other than a photon, the "strength" of this interaction would be thousands of times weaker than the strong interaction of the proton.

The result is of great importance in itself, but also has a number of other implications of a more abstruse character. For example, although the theory of quantum electrodynamics has been remarkably successful in explaining very exactly many electromagnetic phenomena, it has often been suggested that it is really only valid down to a certain small, critical distance, in much the same way as classical dynamics is quite accurate enough for material bodies but breaks down for particles of nuclear size. If this were so, the value of the anomalous magnetic moment would be changed by a certain amount, depending on the value of this critical distance. Agreement between the experimental and theoretical values thus shows that there is no breakdown of quantum electrodynamics down to about 10-14 cm.

By combining the g - 2 results with the accurate value of the magnetic moment obtained by other experiments, the muon mass is now known with much higher precision than ever before. It is 206.768 ± 0.003 times the mass of the electron.

The successful completion of the g - 2 experiment has told us that the muon in itself has no unusual properties, and has extended the range over which the equations of quantum electrodynamics are certainly applicable. In doing so, it has highlighted the fundamental mystery of the muon. If these two particles, the electron and the muon, are not basically different, why should they both exist and what is the significance of their difference in mass? This remains a challenge for some future experiment.

Further reading

G Charpak et al. 1961 Phys. Rev. Lett. 6 128.
G Charpak et al. 1962 Physics Letters 1 16.

Editor's note

The CERN Courier came into being in August 1959, and in 1962 it became a regular monthly publication, appearing in something like its present form.

Following on from the selection of extracts published during 2004, CERN's 50th anniversary year, this regular archive feature will tell the story of particle physics through the pages of the CERN Courier from 1962 onwards.