A paper in the December 24 issue of Physical Review Letters goes to work on the finding of supposed faster-than-light neutrinos by the OPERA experiment. The FTL story has been popping up ever since OPERA — a collaboration between the Laboratori Nazionali del Gran Sasso (LNGS) in Gran Sasso, Italy and the CERN physics laboratory in Geneva — reported last September that neutrinos from CERN had arrived at Gran Sasso’s underground facilities 60 nanoseconds sooner than they would have been expected to arrive if travelling at the speed of light.

The resultant explosion of interest was understandable. Because neutrinos are now thought to have a non-zero mass, an FTL neutrino would be in direct violation of the theory of special relativity, which says that no object with mass can attain the speed of light. Now Ramanath Cowsik (Washington University, St. Louis) and collaborators have examined whether an FTL result was possible. Neutrinos in the experiment were produced by particle collisions that produced a stream of pions. The latter are unstable and decayed into muons and neutrinos.

What Cowsik and team wanted to know was whether pion decays could produce superluminal neutrinos, assuming the conservation of energy and momentum. The result:

“We’ve shown in this paper that if the neutrino that comes out of a pion decay were going faster than the speed of light, the pion lifetime would get longer, and the neutrino would carry a smaller fraction of the energy shared by the neutrino and the muon,” Cowsik says. “What’s more, these difficulties would only increase as the pion energy increases. So we are saying that in the present framework of physics, superluminal neutrinos would be difficult to produce.”

This news release from Washington University gives more details, pointing out that an important check on the OPERA results is the Antarctic neutrino observatory called IceCube, which detects neutrinos from a far different source than CERN. Cosmic rays striking the Earth’s atmosphere produce neutrinos with energies that IceCube has recorded that are in some cases 10,000 times higher than the neutrinos from the OPERA experiment. The IceCube results show that the high-energy pions from which the neutrinos decay generate neutrinos that come close to the speed of light but do not surpass it. This is backed up by conservation of energy and momentum calculations showing that the lifetimes of these pions would be too long for them to decay into superluminal neutrinos. The tantalizing OPERA results look more than ever in doubt.

Image: The IceCube experiment in Antarctica provides an experimental check on Cowsik’s theoretical calculations. According to Cowsik, neutrinos with extremely high energies should show up at IceCube only if superluminal neutrinos are an impossibility. Because IceCube is seeing high-energy neutrinos, there must be something wrong with the observation of superluminal neutrinos. Credit: ICE.WUSTL.EDU/Pete Guest.

As we continue to home in on what happened in the OPERA experiment, it’s heartening to see how many physicists are praising the OPERA team for their methods. Cowsik himself notes that the OPERA scientists worked for months searching for possible errors and, when they found none, published in an attempt to involve the physics community in solving the conundrum. Since then, Andrew Cohen and Sheldon Glashow have shown (in Physical Review Letters) that if superluminal neutrinos existed, they would radiate energy in the form of electron-positron pairs.

“We are saying that, given physics as we know it today, it should be hard to produce any neutrinos with superluminal velocities, and Cohen and Glashow are saying that even if you did, they’d quickly radiate away their energy and slow down,” Cowsik says.

The paper is Cowsik et al., “Superluminal Neutrinos at OPERA Confront Pion Decay Kinematics,” Physical Review Letters 107, 251801 (2011). Abstract available. The Cohen/Glashow paper is “Pair Creation Constrains Superluminal Neutrino Propagation,” Physical Review Letters 107, 181803 (2011), with abstract available here.