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Muon Results Challenge Physics
8 February 2001 7:00 pm
The Standard Model of particle physics may have suffered a serious blow today. Physicists at Brookhaven National Laboratory in Upton, New York, have announced results of a long-awaited experiment--results that contradict what the model predicts. Barring a statistical fluke or undetected systematic error--both real possibilities--the new observations appear to be the best evidence yet that the Standard Model is just a province of a larger, shadowy realm called supersymmetry.
The Brookhaven physicists measured the so-called magnetic moment of the muon, a heavier cousin of the electron. The magnetic moment describes how strongly an object twists in a magnetic field--and in the subatomic realm, calculating that quantity gets very complicated. A muon might be surrounded by a zoo of photons, Z particles, and W particles, and it would interact with each member in the menagerie. Those interactions mess up the classical calculations of the muon's effective magnetic moment, throwing the value off by a tiny amount. The Standard Model of particle physics has done a superb job of accounting for that anomaly--until now.
For the past 3 years, physicists have been smashing beams of protons into targets, creating particles that eventually decay into muons. They feed the muons into the field of a 14-meter-wide superconducting magnet. The intense field forces the particles to race in a circle and causes them to twist. By analyzing the products when the muons decay, the physicists can infer how much the muon twisted in the magnetic field, revealing its magnetic moment with an unprecedented precision of about 1.3 parts per million. Unexpectedly, the theoretical value and the experimental value disagree by about 4 parts per million.
The finding is suggestive, but not definitive, says Lee Roberts, a Boston University physicist working on the experiment. "But I would claim that it is very interesting and provocative."
If the measurement holds up, it reveals a major flaw in the Standard Model. Physicists might respond by adopting an expanded model that includes new particles whose interactions affect the muon's magnetic moment. The best candidate thus far is supersymmetry, a theory that links the particles that make up matter with those that carry forces by providing every known particle with a still-undiscovered twin. The new measurements "imply that there must be physics beyond the Standard Model," says physicist Thomas Kirk of Brookhaven.