HEADLINE: Tiny neutrinos accelerate a physics debate

BYLINE: Dan Vergano

Physicists expressed caution over new experimental results pointing to flaws in a long-standing physics theory.

A multi-university team working at the Energy Department's Fermi National Accelerator Laboratory in Batavia, Ill., last week reported an "inconsistency that makes the hair stand up" in measures of tiny physics particles called neutrinos.

Almost 100,000 times smaller than the tiny electrons inside atoms, neutrinos were confirmed last year as possessing a minuscule mass by Japan's "K2K" detector experiment, a finding at odds with one of the tenets of the venerable "standard model" of physics.

The standard model defines the subatomic particles that make up the universe and explains how they hold together. While the theory describes much of what goes on inside nuclear bombs and stars, it fails to explain gravity. By finding some new particle that explains gravity, physicists hope to discover a "theory of everything" that explains every force in the universe.

Seeking to further explore the standard model's weaknesses using neutrinos, the Fermilab "Neutrinos at the Tevatron" (NuTeV) experiment team fired a neutrino beam at steel plates to see what would come out the other side.

Surprisingly, the team found a 1% shortfall in neutrinos, compared with the predictions of the standard model, a big gap by physics standards, says team member Kevin McFarland of the University of Rochester (N.Y.). The shortfall suggests that a yet-undiscovered particle may be hijacking the missing neutrinos, he says. The team reports a 99.75% probability that their finding isn't a statistical fluke or error.

Outside the NuTeV team, however, physicists reacted with caution to the announcement. While the neutrino mass finding threatened to tweak the standard model, which has delivered very accurate predictions for decades, last week's neutrino discrepancy would throw a serious wrench in its works.

"You have to pay attention when something challenges the standard model," says physicist Jonathan Rosner of the University of Chicago. "But I'd be reluctant to call this new physics until others confirm the experiment."

Dramatic claims require dramatic evidence, adds physicist John Learned of the University of Hawaii in Honolulu, a member of the K2K team. "We'd really like to see even more certainty before they bother us with this one."

New, more powerful particle accelerator experiments should settle the issue in coming years, McFarland says. Both Fermilab and a European accelerator facility have such experiments planned. "What we've shown is a signpost for the future," McFarland says.

His team has submitted its finding to the journal Physical Review Letters.