Closing in on the speed of light (Image: Volker Steger/ Science Photo Library)

The faster-than-light neutrino saga is officially over. Today, at the Neutrino 2012 conference in Kyoto, Japan, the OPERA collaboration announced that according to their latest measurements, neutrinos travel at almost exactly the speed of light.

“Although this result isn’t as exciting as some would have liked, it is what we all expected deep down,” said CERN research director Sergio Bertolucci in a statement.

With the dust settling, OPERA is getting back to its real job: finding tau neutrinos. This week the team also announced that they have found the second-ever instance of a muon neutrino morphing into a tau neutrino, strengthening the case that neutrinos have mass.


Close to light speed

OPERA shocked the world in September 2011 when it announced that neutrinos zipping from CERN in Switzerland to detectors beneath the Gran Sasso mountains in Italy were outpacing the speed of light, a feat that violated Einstein’s rules of relativity and opened the door to exotic physics. But over the next few months, two errors – a leaky fibre-optic cable and a malfunctioning clock – emerged, which slowed the neutrinos back down, dashing post-Einsteinian dreams and causing chaos within the OPERA collaboration.

The measurement wasn’t a waste of time, says OPERA team member Dario Autiero. The new, preliminary result shows that neutrinos arrived at OPERA 1.6 nanoseconds slower than light would have, with an error of 6.2 nanoseconds. That error should shrink with further analysis, says Giovanni De Lellis of the Italian National Nuclear Physics Institute (INFN) and co-spokesman for OPERA.

“This is the first time that velocity was measured with that level of accuracy,” Autiero says.

But all of that was a sidebar to the experiment’s real goal: catching shape-shifting neutrinos in the act. Neutrinos come in three flavours: electron, muon and tau. Several experiments had seen evidence for neutrinos spontaneously switching, or oscillating, from one type to another. Those oscillations proved, to many physicists’ surprise, that the supposed massless particles must have some infinitesimal mass, and offered a route to explaining why there is more matter than anti-matter in the universe.

Oscillate wildly

Before OPERA, all the evidence for neutrino oscillations came from disappearances: detectors would end up with less of a certain type of neutrino than they started with, suggesting some had morphed into other flavours. Then in 2010, OPERA found the first tau neutrino in a beam of billions of muon neutrinos streaming to the Gran Sasso detectors from CERN. The discovery was a big deal at the time, but the team said they needed more tau neutrinos to make it statistically significant.

Now, a second tau neutrino has shown up in the detectors, they report.

“This result shows that the collaboration is definitely and effectively back to its original goal of discovering neutrino oscillations in appearance mode,” De Lellis says.

OPERA will need at least six tau neutrinos to definitively claim they’re seeing the oscillation effect, so they’re not there yet. And when they do, they may find they’ve been scooped: in another experiment, the team behind the T2K detector in Japan announced this week that they have seen 10 muon neutrinos shifting into electron neutrinos.

This story has been updated to reflect more accurate figures for the neutrinos’ speed.