Superluminal neutrinos are slowing fast. Last September, a preliminary but electrifying result suggested these ghostly subatomic particles might travel faster than light. Now a second experiment, ICARUS - based in the same lab in the Gran Sasso mountain, Italy, as the original one, OPERA - has measured the particles' flight time and found that they seem to fly at light speed after all.
The neutrinos that seemed to fly faster than light travelled to Gran Sasso from a particle accelerator at CERN near Geneva, Switzerland. A re-run of the experiment with an improved neutrino beam from CERN gave the same result a few months later.
The result flew in the face of Einstein's well-tested theory of special relativity, which holds that nothing can accelerate past the speed of light. The announcement triggered a torrent of speculation about what exotic physics could account for the result: extra dimensions? Tachyons? Extra subatomic particles not yet discovered?
Most physicists remained sceptical, though. Theoretical problems with the result piled up quickly, including a suggestion from Andrew Cohen and Nobel laureate Sheldon Glashow, both at Boston University, that neutrinos flying faster than light should emit radiation analogous to an electromagnetic sonic boom.
In October ICARUS searched for that radiation and saw nothing. Now ICARUS has measured the neutrinos' time of flight directly - again travelling from CERN to Gran Sasso. The verdict: neutrinos obey Einstein's speed limit.
"ICARUS measures the neutrino's velocity to be no faster than the speed of light," said ICARUS spokesperson Carlo Rubbia, according to a press release. The result "has provided an important cross-check of the anomalous result reports from OPERA last year".
Because it was running for only a few months, ICARUS based this conclusion on detections of just seven neutrinos. OPERA's measurement with the improved beam line was based on 35 neutrinos.
The curtain seems to be falling on OPERA, though. Last month, the collaboration announced that it had found two flaws in the experimental set-up: a leaky fibre-optic cable and a malfunctioning clock. Neither can entirely explain the neutrinos' early arrival, but with so much doubt swirling around the measurement, any error is bad news.
All four of the experiments in Gran Sasso - OPERA, ICARUS, BOREXINO and LVD - will be making new measurements with short beams from CERN in May. The MINOS experiment at Fermilab in Batavia, Illinois, may also weigh in, if there is any doubt left about the neutrinos' speedster status.
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