A new precision measurement of the muon’s magnetism during an experiment at Brookhaven has shown a tiny unexplained discrepancy.
The experiment is one of the few in particle physics that does not study particle scattering. A team of physicists from Germany, Japan, Russia and the US injects 3.09 GeV polarized (spin-oriented) positively charged muons from Brookhaven’s Alternating Gradient Synchrotron into a superconducting storage ring with a circumference of 14.2 m. As they circulate round the ring, the stored muons decay into positrons, which can be detected, and neutrinos, which cannot, over periods measured in microseconds.
A muon spins round an internal axis. A spinning charged particle acts like a tiny magnet, and the positrons emitted as the muons decay inside the ring reflect the magnetic behaviour of the parent particle.
Classical Dirac quantum theory of spin 1/2 particles shows that the “gyromagnetic ratio” (g) of the magnetic moment of a charged particle, such as the muon, to its spin angular momentum is exactly two. However, additional small effects can creep in to change this value, so that g-2 is not zero. Such precision magnetism measurements are collectively known as “g-2” experiments.
The additional effects mean that the muon magnets do not line up inexactly the direction of the magnetic field in the storage ring. Instead, each muon wobbles (precesses) as it circulates round the ring, and the observed positron pattern reflects these wobbles.
What are these additional magnetic effects? First, the muon’s magnetism is affected by its attendant electromagnetic cloud. The muon behaves like a heavy cousin of the electron, and the discovery in 1947 by Polycarp Kusch and Henry Foley that the electron’s g-2 is not zero provided some of the first experimental evidence for the then new theory of quantum electrodynamics. This describes the way in which charged particles like electrons and muons are surrounded by tiny clouds of additional electromagnetic effects. Quantum electrodynamics predicted exactly what the electron’s g-2 should be, and the agreement with experimental results was an impressive confirmation of the new theory.
In the 1960s and 1970s a series of precision experiments at CERN measured g-2, this time for muons, to a few parts per million. These were among the most precise particle physics results ever obtained at that time. This pioneered the idea of a storage ring in which the muons could decay.
Unlike the earlier experiments at CERN, the Brookhaven g-2 experiment injects muons into the ring. The CERN studies injected pions, which then decayed in orbit. Muon injection was suggested by the late g-2 pioneer Fred Combley.
As well as interacting electromagnetically, the muon is also affected by weak interactions. In addition, the photon – the carrier of the electromagnetic force – has a minute quark-gluon component, which is affected by the strong nuclear force. This has a further effect on the muon’s g-2.
Taking all of these effects into account, the experimental measurement at Brookhaven (to a precision of about one part per million) and the theoretical prediction differ by 2.6 times the estimated error of the measurement.
This result from Brookhaven is based on 2.9 billion muon decays carefully accumulated during 1999. Analysis of the experiment’s 2000 data sample has not yet been completed.
With such a precise result apparently differing from the theoretical prediction, those involved in the experiment may indulge in the luxury of speculation. Is additional physics being seen for the first time? Only with more g-2 information will we know.
