Undiscovered Particle or Fluke?

Neutrino hunter. This giant detector at Fermilab gathered some puzzling data on neutrinos.

New neutrino news has scientists excited about a potential conflict with the standard model of particle physics. Measurements from the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, have yielded evidence that the standard model may misgauge the strength of one of the fundamental forces. Although not conclusive, the results might signify an undiscovered particle--or an experimental fluke.

The Fermilab experiment measured qW (pronounced "theta-sub-w"), a quantity called the weak mixing angle. Roughly speaking, qW measures the relation between the electromagnetic and weak forces. Unlike the neutrino's mixing angle, which determines the properties of neutrinos (Science, 2 November 2001, p. 987), qW is a measure of a fundamental force of nature, something that is fully accounted for in the standard model of particle physics. So when Fermilab researchers calculated qW by comparing the behavior of neutrinos and antineutrinos produced by the Tevatron accelerator, they didn't expect to see anything unusual.

The result surprised them. The measured value of qW disagreed with what the standard model predicts by three standard deviations--"three sigma." "A three-sigma result is interesting--it gets people's attention," says Kevin McFarland, a physicist at the University of Rochester in New York state and member of the Fermilab team. In particle physics, such a result is usually considered provocative but not ironclad. But McFarland is sanguine."I spent the last 8 years of my career making one measurement," he says, and after thorough checking and rechecking, the conflict with the standard model remained.

If real, the anomaly might be caused by an undiscovered particle such as a hypothetical new carrier of the weak force called Z' ("Z-prime"), says Jens Erler, a physicist at the University of Pennsylvania in Philadelphia. "The [Fermilab] experiment is not explained by Z', but helped," he says. When combined with another recent intriguing but statistically inconclusive result in atomic physics, says Erler, it is "almost crying out for Z'." But doubts remain. "Three sigma can easily be a fluke," says Erler. "But we take it seriously enough to have a really close look."

Related sites

Neutrino experiment home page
The Neutrino Oscillation Industry

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