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Neutrino data hint at need for revised theories: nearly massless particles could turn physics on its ear.

Neutrinos are the big nothings of subatomic physics. Nearly massless and with no electric charge, these ghostly particles interact so weakly with other matter that more than 50 trillion of them pass through a person's body each second.

Yet two new experiments hint that neutrinos may be opening a window on a hidden world of subatomic particles and forces. The findings could end up being statistical flukes. But so far the results, announced June 14 at the Neutrino 2010 conference in Athens, indicate that neutrinos and their antiparticle counterparts, antineutrinos, are not the nearly exact mirror images of each other that current physics supposes them to be.

If confirmed, the results would "indicate a fundamentally new direction in our thinking" about subatomic particles and the origin of matter in the universe, says theorist Rabindra Mohapatra of the University of Maryland in College Park.

The findings may help explain how the universe, believed to have begun with matter and antimatter so perfectly balanced that they would have destroyed each other upon contact, became dominated by matter. The results "could even signal a tiny breakdown of Einstein's theory of special relativity," Mohapatra says.

Current theories of particle physics assume that known forces arise from interactions with neighboring particles and obey special relativity, which holds that the speed of light and the laws of physics are always the same regardless of a particle's speed or rotation. For that to hold true, particles and antiparticles--including neutrinos and their antipartners--must have the same mass, Mohapatra says. But new data from the MINOS (for Main Injector Neutrino Oscillation Search) experiment seem to contradict that notion.


The three known types of neutrinos (electron, muon and tan) transform from one type into another as they travel. During a 735-kilometer journey from Fermilab in Batavia, Ill., to the Soudan Underground Laboratory in Minnesota, about 37 percent of muon antineutrinos disappeared--presumably morphing into another neutrino type--compared with just 19 percent of muon neutrinos, reports MINOS spokesman Robert Plunkett of Fermilab. That difference suggests a difference in mass between antineutrinos and neutrinos--although more data will be needed to confirm the observation. Data collected so far leave a 5 percent chance that the particles weigh the same.

"If the masses are different for neutrinos and antineutrinos, then the most sacred symmetry of quantum field theory, CPT [for charge, parity and time], is broken in the neutrino sector," says Tom Weiler of Vanderbilt University in Nashville.

If particle interactions are thought of as a movie, CPT symmetry requires that whatever physics occurs during the show must be the same when the film is run forward or backward (time), viewed through a mirror (parity) and when replacing each particle by its antiparticle (charge).

If CPT is broken, then a cornerstone of Einstein's special relativity is also violated, Weiler notes.

In a smaller Fermilab study, an experiment called MiniBooNE found a different kind of asymmetry between particles and antiparticles. Over about half a kilometer, muon antineutrinos morphed into electron antineutrinos more often than muon neutrinos became electron neutrinos. That result also requires a mass difference between neutrinos and antineutrinos, Mohapatra says, though others disagree.

There's about a 3 percent chance the MiniBooNE finding is a fluke. But it does match results, later refuted, from an experiment at Los Alamos National Laboratory in New Mexico during the 1990s.

An asymmetry between particles and antiparticles in the standard model of particle physics isn't large enough to account for the MiniBooNE results, notes Fermilab's Boris Kayser. If confirmed, the findings may require a fourth, previously unknown neutrino type, dubbed sterile because it would interact with matter even more weakly than the other three.
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Title Annotation:Atom & Cosmos
Author:Cowen, Ron
Publication:Science News
Geographic Code:1USA
Date:Jul 17, 2010
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