By Eugenie Samuel Reich of Nature magazine

The claim that neutrinos can travel faster than light has been given a knock by an independent experiment.

On 17 October, the Imaging Cosmic and Rare Underground Signals (ICARUS) collaboration submitted a paper to the preprint server, in which it offered a rebuttal of claims to have clocked subatomic particles called neutrinos traveling faster than the speed of light. The original results were published on 22 September by the Oscillation Project with Emulsion-Tracking Apparatus (OPERA) experiment.

Both experiments are based at Gran Sasso National Laboratory near L'Aquila, Italy, and detect neutrinos coming in a beam from CERN, Europe's high-energy particle physics laboratory near Geneva in Switzerland, about 730 kilometers away. Unlike OPERA, ICARUS does not measure the neutrinos' speed directly. Instead, it has shown that the energy spectrum of the neutrinos does not exhibit an effect predicted last month by Andrew Cohen and Sheldon Glashow, theoretical physicists at Boston University in Massachusetts.

If the Cohen-Glashow effect is a valid prediction, "neutrinos are not superluminal," says Sandro Centro, a physicist at the University of Padua in Italy, deputy spokesman for ICARUS and a co-author of the latest paper.

Cohen says that an energy spectrum provided by OPERA showed the same inconsistency, and that the spectrum from ICARUS has added to the problem. "There's always value to having things checked independently," says Cohen. "I think it's great ICARUS has done this so quickly."

Too much momentum

The Cohen-Glashow effect is an extension of another phenomenon, well known to physicists. The speed of light traveling through materials such as water is lower than that in a vacuum, and charged particles such as electrons are able to exceed this lower speed when traveling through the medium. When they do, they have excess energy for their momentum and radiate some away in the form of photons, or 'erenkov radiation'.

Cohen and Glashow concluded that neutrinos traveling faster than light would behave similarly, although as neutral particles they would radiate pairs of electrons and positrons rather than photons. This would reduce the energy of neutrinos traveling long distances.

Such an energy reduction is not seen in the neutrinos from CERN at their destination in Gran Sasso. Indeed, Dario Autiero, a physicist at the Institute of Nuclear Physics in Lyons, France, and OPERA's physics coordinator, says that measurements of the neutrino energies by OPERA, reported in a February 2011 paper4, already failed to show signs of the effect later predicted by Cohen and Glashow. "It is very well known, and it has been presented in tens of OPERA talks at conferences," he says, "it is not something that we learn today because of ICARUS."

Autiero adds that the assumptions made by Cohen and Glashow may not be universally valid. Giacomo Cacciapaglia, a theoretical physicist at King's College London, agrees, saying that not all models of faster-than-light neutrinos have to respect the assumptions of Cohen and Glashow. For example, neutrinos might be able to travel faster than light by taking a shortcut through extra dimensions, in which case they might not radiate.

But Jorge Páramos, a theoretical physicist at the Higher Technical Institute in Lisbon, says that tinkering with the theory in this way is a dangerous game. "It requires you to choose from the available range of theoretical concepts, and could also lead to disagreement with other well established experimental results (not related to the speed of light)," he says.

More than 80 papers have been posted on arXiv discussing OPERA's result. Most try to explain it theoretically, but a small minority claim to find problems. Autiero thinks that despite the huge interest from the public and the media, the debate will have to play out at the normal pace of science, "which is necessarily slow". The experimental work that was the basis for OPERA's claim took almost six years. "Further developments will be quicker but cannot happen on a few days' timescale," he says.

Two experiments are planning to try to test OPERA's measurement of neutrino velocity: the Main Injector Neutrino Oscillation Search (MINOS) experiment based at Fermilab in Batavia, Illinois, and the Tokai to Kamioka (T2K) experiment in Japan. Neither is likely to have results for some months.

This article is reproduced with permission from the magazine Nature. The article was first published on October 20, 2011.