End of the Line for Superluminal Neutrinos?

By Stephen Sadler

Physicists were shocked last September when a paper published by the OPERA collaboration suggested that neutrinos in their detector in the Gran Sasso underground lab in Italy had been caught travelling faster than the speed of light. If correct, the result calls into question Einstein’s Theory of Relativity, and open the door to all sorts of weird and wonderful effects such as the reversal of causality and time travel. In fact, so perturbed were the researchers at the implications of their results, that they delayed publishing their findings for 5 months while they meticulously checked the experiment for errors, before finally concluding that they could do no more without the help of the wider particle physics community.

Unsurprisingly, interest has been huge, and as of today (February 26th 2012) a search of the keywords ‘superluminal neutrino’ on arXiv.org yields 163 papers on the subject. However, despite the focused attention of some of the world’s top minds, until recently no mistakes in the experimental method or data analysis had come to light. Indeed, the OPERA collaboration repeated their experiment with a neutrino beam configuration that allows for more precise timing, and found the same effect. Many scientists still doubted the result though, and last December Ramanath Cowsik, professor of physics in Arts & Sciences and director of the McDonnell Center for the Space Sciences at Washington University in St. Louis, and his team of collaborators pointed out a glaring problem which had been overlooked.

Neutrino beams for particle physics experiments are produced in a three-step process that begins by accelerating protons to 99.9999991% the speed of light, in an accelerator such as the Large Hadron Collider at CERN in Switzerland. These ultra-relativistic protons are then smashed into a graphite target producing, amongst other debris, secondary particles called pions. Finally, these short-lived pions are focussed into a tight beam by magnetic horns before they quickly decay into a beam of neutrinos and charged sister particles of the electron called muons, each of which carries off some fraction of the total pion momentum. Finally a ‘beam dump’ at the end of the decay pipe stops all particles other than the neutrinos, leaving a pure neutrino beam.

The trouble is that in order to produce the high energy neutrinos observed at OPERA, the fraction of momentum carried off by the neutrinos needs to be less than about 0.05. This, in turn, implies that the decaying pions must have an extremely high momentum, and Einstein’s theory of relativity tells us that this very high momentum would extend the pions’ lifetime so much that they would not have time to decay in the beam pipe at CERN before smashing into the concrete beam dump.

“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. “So we are saying that in the present framework of physics, superluminal neutrinos would be difficult to produce.”

Now, it seems as though Cowsik was right to be skeptical, as an email from CERN Director General Rolf Heuer to CERN staff last week announced that the OPERA collaboration had identified two possible sources of error in their neutrino velocity measurement. The first has to do with an oscillator used in the timing system of the experiment, and could only increase the size of the faster-than-light effect. The second, though, concerns a potentially faulty optical fibre connection that sends an external GPS signal to the OPERA master clock, and could serve to bring the velocity of the neutrinos back down to the sub-light-speeds physicists are used to.

The OPERA collaboration have fixed the problems and are now in the process of determining the effect they may have had on the results. New data taken with the repaired detector is expected in May, but for now scientists around the world are applauding the OPERA team for the open and transparent way in which they have reported their surprising result. In an interview for BBC News Sergio Bertolucci, director of research at CERN, said “One has to realise that the collaboration has never stopped to try to ‘kill’ the measurement (proving that it was erroneous)”. Even if the result turns out to be a false alarm due to loose wiring, the story has been a textbook example of good scientific practice.

The paper announcing the superluminal measurement can be found at:  http://arxiv.org/abs/1109.4897, whilst a preprint of Cowsik’s work detailing the problems raised by pion decay kinematics appears here: http://arxiv.org/abs/1110.0241v2.