18 November 2011: An updated version of this story is now available (see ‘Neutrino experiment replicates faster-than-light finding’).
It is a remarkable confirmation of a stunning result; but most physicists remain skeptical. That seems the most probable outcome of a release of new data expected on 17 November from researchers with the Italian OPERA collaboration, who say they have confirmed their controversial finding that flighty subatomic neutrinos can travel faster than light.
“It’s slightly better than the previous result,” says OPERA’s physics coordinator Dario Autiero of the Institut de Physique Nucleaire de Lyon in France (pictured). He adds that most of the members of the collaboration who declined to sign the original paper because they wanted more time to check the result have now come on board. One of these is Caren Hagner of the University of Hamburg in Germany. She says not only has the experiment’s precision been improved, the statistical analysis is more robust and has been replicated by different groups within OPERA not just the original team. “We gained much more confidence,” Hagner says.
OPERA (which stands for Oscillation Project with Emulsion-tracking Apparatus) made headlines in September with a claim to have clocked neutrinos traveling faster than light, a result at odds with Albert Einstein’s well-established Special Theory of Relativity, which sets light as the ultimate cosmic speed limit. The group used a pulsed beam of neutrinos produced by a particle accelerator at CERN near Geneva which traveled some 730 kilometers to Gran Sasso near L’Aquila, Italy where the particles were detected.
The result was highly statistically significant but following Carl Sagan’s well-known mantra that “extraordinary claims require extraordinary evidence”, most physicists expressed doubts. While OPERA appeared to have conducted its data-taking and analysis carefully, there was rampant speculation about possible sources of error and some made claims of mistakes that the collaboration brushed off.
One set of concerns centered on the relatively long timescale – 10.5 microseconds, or 10.5 millionths of a second – of the proton pulses produced at CERN that result in the neutrino pulses OPERA detects. OPERA did not know whether individual neutrinos received at Gran Sasso corresponded to protons early or late in the proton pulse, creating uncertainty around their detection of them. In October OPERA therefore asked CERN to generate shorter proton pulses lasting just 3 nanoseconds. They have now recorded 20 events in the new data run and say that they have reached a similar level of statistical significance to the first time around, with the neutrinos again reaching Gran Sasso 60 nanoseconds faster than a light beam would do.
OPERA expects the new result to rule out uncertainties due to the long timescale of the proton pulses. But concerns about the experiment’s use of the Global Positioning System to synchronize clocks at each end of the neutrino beam are unlikely to be as easily allayed, The use of GPS is novel in the field of high energy and particle physics and the same system was used for both the original experiment and the new run. Hagner also adds that she’d like to see the time measurement checked using another part of the detector, to increase confidence further.
For most physicists outside the collaboration, however, the key test will be replication by an independent experiment. The one best placed to independently confirm or refute OPERA’s result is MINOS (the Main Injector Neutrino Oscillation Search) at Fermilab in Batavia, Illinois. In response to the latest OPERA result, MINOS issued a statement saying it is upgrading its timing system to match OPERA’s precision and might have preliminary results obtained using the existing system that are relevant to assessing OPERA’s results as soon as early 2012.
“OPERA is to be congratulated for doing some important and sensitive checks but independent checks are the way to go,” says Rob Plunkett, co-spokesman for MINOS.