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Neutrino parents set speed limits: faster-than-light flight would violate conservation laws.

Physicists have found yet another reason to doubt recent reports of neutrinos traveling faster than light. The existence of such speedy particles would screw up not only Einstein's theory of special relativity, but also the laws of conservation of energy and momentum.

In September, the OPERA experiment reported clocking neutrinos traveling faster than the speed of light, arriving 60 nanoseconds early on their 730kilometer journey between the European laboratory CERN, near Geneva, and the Gran Sasso National Laboratory in Italy. To try to explain the result, two new studies examined the particles that give birth to neutrinos. Both found that these particles, called pions, could not possibly have had enough energy to give rise to the faster-than-light, or superluminal, speeds indicated by OPERA.

"We give a clear constraint on the superluminality of neutrinos," says Xiaojun Bi, a particle astrophysicist at the Chinese Academy of Sciences' Institute of High Energy Physics in Beijing. His team reported its findings in the Dec. 9 Physical Review Letters.

If neutrinos can travel faster than light, they should get heavier as their energy increases. So there's a limit to how fast the particles can zip along, dictated by the energy of their unstable pion parents.

OPERA's pions, made at CERN, have on average 3.5 times as much energy as their neutrino progeny. That sets a neutrino speed limit that's lower than the speed measured by OPERA, physicist Ramanath Cowsik of Washington University in St. Louis and colleagues reported in the Dec. 16 Physical Review Letters. Bi suggests that OPERA's highest-energy neutrinos push this speed limit even lower.

Achieving the mind-boggling velocities measured by OPERA would have required pions with energies 20 times greater than their offspring, Cowsik's team calculates. At such energies, though, the lifetimes of pions would be six times longer, which has been ruled out by measurements from OPERA and other experiments.

For Cowsik and other researchers, these problems and contradictions suggest that the laws of physics as currently understood are correct. But physicists will still be watching other neutrino experiments that can check OPERA's result, which may be clouded by some unknown source of error.
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Title Annotation:Atom & Cosmos
Author:Powell, Devin
Publication:Science News
Geographic Code:1USA
Date:Jan 28, 2012
Words:354
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