Faster than light? Maybe not

While theorists have been sceptical about the faster than light neutrinos for a while, there is now a new experiment which casts further doubt on the results of the OPERA collaboration. The ICARUS experiment offers a rebuttal of the earlier claims of neutrinos travelling faster than light.

The ICARUS(Imaging Cosmic and Rare Underground Signals) experiment also collects neutrinos that travel from CERN to Gran Sasso, but measures the energy spectrum rather than the time of flight, as is done by the OPERA experiment, which is also located at Gran Sasso and reported superluminal (faster than light) speeds. ICARUS has shown that the energy spectrum does not show the signature of the Cohen-Glashow effect which is the analogue of the Cerenkov radiation emitted by charged particles. Charged particles such as electrons that travel in media with velocities greater than the velocity of light in that medium, emit radiation known as Cerenkov radiation, and lose energy in the process. If the neutrinos of the Opera experiment did travel with velocities greater than light, they would emit particles (electron, positron pairs and photons, mediated by a Z0 boson, as per Cohen and Glashow), and lose energy themselves in the process. There is a straightforward relation between the rate at which the neutrinos lose energy and the speed at which they travel. The average energy of the neutrinos that leave CERN is 28.2 GeV. If they actually travelled at superluminal speeds, they would reach Gran Sasso, where both OPERA, and ICARUS are located, with an average energy of 12.1 GeV. Instead, ICARUS reports that the neutrinos detected by them, have an average energy of 26 GeV, about what the neutrinos would have, if they travelled at the boring old speed of light!

By the way, ICARUS only has about 100 reliable neutrino events, whereas OPERA has about 16,000. However, the results of the ICARUS experiment rely on a straightforward measurement of the distribution of energies, and hence do not get mired down by issues like the synchronisation and slowing down of clocks that the time of flight experiments of OPERA do, and are being taken very seriously. As in all the other issues like the Tevatron bump, and the missing Higgs, which have come up recently, only more measurements, and further experiments by independent collaborations, can resolve the question. We look forward to more exciting results.

Tailpiece: Does E still equal mc squared? (The Corrigan brothers). We don't know yet, but that's the way to bet.

Happy Diwali, everyone.

This blog post is by Neelima Gupte and Sumathi Rao.