HIGH-ENERGY PHYSICS:
Neutrino Oddity Sends News of the Weak

 Physicists are excited, once again, about a potential conflict with the Standard Model of Particle Physics. Measurements of the behavior of neutrinos, made by a team at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, suggest that the Standard Model may misgauge the strength of one of the fundamental forces of nature. Although not conclusive, the results might signify an undiscovered particle--or an experimental fluke.

CREDIT: FNAL

The Fermilab experiment measured q_W ("theta-sub-w"), a quantity called the weak mixing angle. Although not an angle in the ordinary sense, q_W smells like one to a mathematician. Roughly speaking, it measures the relation between the electromagnetic and weak forces: Different values of q_W yield different pictures about the relative strengths of the forces at different energies. Unlike a similar-sounding quantity called the neutrino mixing angle, which determines the properties of neutrinos (Science, 2 November, p. 987), q_W measures a fundamental force of nature, something that is fully accounted for in the Standard Model.

 So when the Fermilab researchers measured q_W using neutrinos produced by the Tevatron accelerator, they didn't expect to see anything unusual. The Tevatron produced powerful protons, then slammed them into a beryllium-oxide target, producing kaons and pions with various charges. Using magnets, the scientists sifted these particles, picking out varieties that would decay and produce either neutrinos or antineutrinos. They then compared how the resulting neutrinos and antineutrinos interacted with a 700-ton steel detector. The neutrinos and antineutrinos have different spin states and thus are affected differently by the weak force--and q_W. By comparing the neutrinos' behavior with that of the antineutrinos, the team figured out the size of q_W.

 The result surprised them. The measured value of q_W 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 inconclusive result in atomic physics, says Erler, it is "almost crying out for Z'."

 But doubts will remain until new experiments can shed more light on the situation. "Three sigma can easily be a fluke," says Erler. "But we take it seriously enough to have a really close look." 
 

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