Muon Experiment Suggests a Hint
Scientists at the Brookhaven National Laboratory in Upton, Long Island, New York, have just announced a surprising measurement of one of Nature's tiniest particles, the muon (pronounced "MEW-on"). Its magnetic properties as determined by a precision experiment are in disagreement with theory.
of New Fundamental Physics
by Jupiter Scientific Publishing
If the findings are confirmed by further analysis, then modifications will be necessary to the Standard Model of particle physics. This theory, which has enjoyed tremendous success in explaining accelerator experiments for three decades, describes the basic microscopic constituents and forces of the universe.
Since the Standard Model has a few aesthetically unattractive features, theoretical physicists have been hoping for such a development for years. Not surprisingly, there is excitement in the particle physics community. "This work could open up a whole new world of exploration," says Boston University physicist Lee Roberts, a co-spokesperson for the experiment.
In an article submitted to Physical Review Letters on February 8, the measured and theoretical values of the muon's magnetic moment are reported to differ by 2.6 standard deviations, meaning that there is only a 1% chance that the experimental result is a statistical fluke.
However, because of the extraordinary nature of the findings, scientists cannot yet claim that the Standard Model is flawed or that there is new fundamental physics. For this, the statistical error must be reduced further. Additional data, which should be available within a year, will decrease the statistical uncertainty to a sufficiently low level.
Muons are subatomic particles known as leptons ('light particles'). By definition, such particles do not feel the strong force (the force that holds the nucleus together). The familiar electron is another example of a lepton.
In fact, the muon can be viewed as just a heavier version of the electron, the former weighing about 200 times the latter. The muon is a better probe of new physics at high energy than the electron because of its larger mass. It was discovered in 1936 in cosmic rays streaming down on Earth from outer space.
At the time, the existence of the muon surprised physicists so much that the Nobel laureate Isidor I. Rabi exclaimed, "Who ordered that?"
To create muons, which do not naturally exist, a team of approximately 70 researchers from 11 institutions in the United States, Russia, Japan, and Germany use an accelerator known as the Alternating Gradient Synchrotron to smash protons into a slab of nickel. From the debris, muons are created and extracted and then inserted into a strong-magnetic field device. About a billion muons have been analyzed.
A spinning charged particle such as an electron or muon creates a magnetic moment, which can be thought of as a little magnet. By subjecting the muon to powerful magnetic fields, experimentalists cause it to wobble, in much the same way as a tilted top spinning on a floor. Using the wobble rate, physicists can measure the magnetism of the muon and deduce its magnetic moment.
Due to quantum mechanics, leptons only spin by a fixed amount, like a top that is compelled to rotate at a certain number of revolutions per second. As a consequence, the magnetic moment assumes a particular value, which in the absence of interactions is twice a fundamental unit. This value, known as g, is 2 for a "free" muon, that is, a muon that does not experience any forces.
However, "real" muons feel the weak subnuclear and electromagnetic forces. This creates tiny corrections to the magnetic moment causing g to differ slightly from 2. For this reason, the magnetic moment experiments are known as g-2 measurements (pronounced "gee-minus-two" experiments). The Brookhaven experimental group has measured g-2 to an amazing accuracy of one part in a million.
If the anomalous result persists, then the simplest way to rectify the discrepancy between theory and experiment would be to introduce new, very heavy particles that interact with the leptons (and muons, in particular). Such particles are predicted by many theoretical extensions of the Standard Model including supersymmetry. Supersymmetry is an essential feature of one of the most popular unification schemes: superstring theory.
One of the goals of the upgraded Tevatron accelerator at the Fermi National Accelerator in Batavia, Illinois will be to look for supersymmetric particles. The machine will spend much of the next few years searching for such exotic physics.
Dr. Gerald Gabrielse, chairman of the physics department at Harvard University, called the muon magnetic moment results "tremendously exciting" because of the possible hint of new physics. Dr. William Marciano, a theorist at the Brookhaven laboratory, even thinks that supersymmetry would be the most likely explanation. Other proponents of supersymmetry and superstrings are equally enthusiastic.
Dr. Vernon Hughes, a leading physicist of the muon experimental group and who ironically studied under Professor Rabi, now jokingly thinks he knows the answer to Rabi's question: "The theorists who do supersymmetry ordered it."