Muon Mutiny

The mu's screws are loose, and that's good news for physicists hoping for a glimpse of new physics. At a seminar at Brookhaven National Laboratory in Upton, New York, today, physicists announced that Brookhaven's so-called muon g-2 experiment strengthens indications that muons flout the Standard Model of particle physics.

The Standard Model describes, to a few parts in a billion or so, how a muon twists in a magnetic field. But in 2001, when experimentalists at Brookhaven first measured that twisting--its magnetic moment--with comparable precision, the results were higher than the Model predicted (ScienceNOW, 8 February 2001). Despite an embarrassing round of backpedaling after theorists made an arithmetic error that exaggerated the clash between observation and theory (ScienceNOW, 13 November 2001), the apparent discrepancy persisted as the experiment continued.

Now, similar measurements of negatively charged muons, known as mu-minus particles, have supplied independent evidence that something odd is afoot. In order to allow the equipment to handle mu-minuses rather than the mu-pluses of the previous data sets, the lab had to reverse every magnet in the experiment, says Boston University physicist Lee Roberts, co-spokesperson for the g-2 (pronounced "g minus two") experiment. The resulting mu-minus data bear out earlier measurements but disagree even more sharply with the Standard Model, Roberts says. Taken all together, the numbers show that the muon's twistiness is nearly 3 standard deviations, or sigma, away from what theory predicts--a serious, although not conclusive, suggestion that the Standard Model has failed.

"Getting close to 3 sigma in this kind of experiment is really quite strong," says Gordon Kane, a theorist at the University of Michigan, Ann Arbor, who suspects that the discrepancy is due to the subtle influence of as-yet-undetected particles required by an extension of the Standard Model known as supersymmetry. The g-2 experiment finished its last run in 2001, but Roberts is hoping that Japanese labs will make even more precise measurements of the muon's magnetic moment later in the decade. Kane, meanwhile, hopes that experiments with other particles will finally propel physics beyond the Standard Model.

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The Brookhaven g-2 experiment

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