Muon’s (g-2): the obstinate deviation from the Standard Model

It’s been 50 years since a small group at CERN measured the muon (g-2) for the first time. Several other experiments have followed over the years. The latest measurement at Brookhaven (2004) gave a value that obstinately remains about 3 standard deviations away from the prediction of the Standard Model. Francis Farley, one of the fathers of the (g-2) experiments, argues that a statement such as “everything we observe is accounted for by the Standard Model” is not acceptable.


Francis J. M. Farley.

Francis J. M. Farley, Fellow of the Royal Society since 1972 and the 1980 winner of the Hughes Medal "for his ultra-precise measurements of the muon magnetic moment, a severe test of quantum electrodynamics and of the nature of the muon", is among the scientists who still look at the (g-2) anomaly as one of the first proofs of the existence of new physics. “Although it seems to be generally believed that all experiments agree with the Standard Model, theory cannot explain the muon (g-2) result,” says Francis Farley. “The most recent experimental results were published by a Brookhaven National Laboratory experiment in 2004. Since then, many theorists have laboured to nudge theory into agreement with experimental results, but none have succeeded. The discrepancy has remained obstinately at 3 standard deviations or more. The measurements are very clear: three virtually independent runs including μ+ and μ-, with blind analyses agreeing with each other, are all well above the value provided by theory. Although theorists are increasingly confident in their Standard Model value, no common experimental mistake has been suggested so far.”

“g” is a dimensionless quantity related to the magnetic properties of a particle. Assuming that the muon obeys the simplest equations of quantum mechanics, then g should equal exactly 2. However, this is modified by the quantum fluctuations in the electromagnetic field around the muon, specified by the rules of the well-established Quantum Electrodynamics theory within the Standard Model, making g larger by about 1 part in 800. The quantum effects include rare fluctuations, which involve virtual pion states, strongly interacting vector mesons, bosons of the weak interaction, and perhaps other particles as yet unknown. “The main motivation for measuring the (g−2) of the muon is to see whether the known particles play their predicted roles or whether there is something more to be discovered,” observes Francis Farley. “In this way, the Brookhaven muon (g-2) measurement is the first indication of physics beyond the Standard Model. However, it does not tell us which of the many speculations is correct.”

The conundrum remains on the table. "Eventually", Francis Farley hopes, “a new theory will evolve and our result will be explained.”

by Antonella Del Rosso