Faster-than-light neutrino claim bolstered

Representatives from the OPERA collaboration spoke in a seminar at CERN today, supporting their astonishing claim that neutrinos can travel faster than the speed of light.

The result is conceptually simple: neutrinos travelling from a particle accelerator at CERN in Switzerland arrived 60 nanoseconds too early at a detector in the Gran Sasso cavern in Italy. And it relies on three conceptually simple measurements, explained Dario Autiero of the Institute of Nuclear Physics in Lyon: the distance between the labs, the time the neutrinos left Switzerland, and the time they arrived in Italy.

But actually measuring those times and distances to the accuracy needed to detect differences of billionths of a second (1 nanosecond = 1 billionth of a second) is no easy task.

Details, details

"These are experiments where the devil is in the details ? the details of how each piece of equipment works, and how it all goes together," said Rob Plunkett of Fermilab in Batavia, Illinois.

The detector in the Gran Sasso cavern is located 1400 metres underground. At that depth Earth's crust shields OPERA (which stands for Oscillation Project with Emulsion-tRacking Apparatus) from noise-inducing cosmic rays, but also obscures its exact latitude and longitude. To pinpoint its position precisely, the researchers stopped traffic in one lane of a 10-kilometre long highway tunnel for a week to place GPS receivers on either side.

The GPS measurements, which were so accurate they could detect the crawling drift of the planet's tectonic plates, gave precise benchmarks for each side of the tunnel, allowing the researchers to triangulate the underground detector's position in the planet. Combining that with the known position of the neutrino source at CERN gave a distance of 730,534.61 metres, plus or minus 20 centimetres.

To determine exactly when the neutrinos left CERN and arrived at Gran Sasso, the team hooked both detectors to caesium clocks, which can measure time to an accuracy of one second in about 30 million years. That linked the labs' timekeepers to within one nanosecond.

"These kinds of techniques that we have been using are maybe unusual in high energy physics, but they are quite standard in metrology," Autiero said. Just to be sure, the collaboration had two independent metrology teams from Switzerland and Germany check their work. It all checked out.

The researchers also accounted for an odd feature of general relativity in which clocks at different heights keep different times.

A ?beautiful experiment'

Other physicists are impressed."This is certainly very precise timing, more than you need to record for normal accelerator operations," Plunkett told New Scientist. His project, the MINOS experiment at Fermilab, has already requested an upgrade to their timing system so they can replicate the results, perhaps as soon as 2014.

"I want to congratulate you on this extremely beautiful experiment," said Nobel laureate Samuel Ting of the Massachusetts Institute of Technology in Cambridge during the question and answer session that followed Autiero's talk. "The experiment is very carefully done, and the systematic error carefully checked."

But only time will tell whether the result holds up to additional scrutiny, and whether it can be reproduced . There is still room for uncertainty in the neutrinos' departure time, Plunkett says, because there is no neutrino detector on CERN's end of the line. The only way to know when the neutrinos left is to extrapolate from data on the blob of protons used to produce them.

"Of course we need to approach it sceptically," he says. "I believe everyone will be pulling together to figure this out."

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