Courtesy of Sean Carroll at Cosmic Variance (and many other sources, but this is the one I happen to like linking to), the ICARUS experiment has performed a direct measurement of the speed of neutrinos coming out of CERN, and they’ve found it to be exactly consistent with the speed of light. This is hot off the physics presses, so to speak; the paper was posted on arXiv just yesterday. CERN has updated their press release with the latest information.

The neat thing about the ICARUS result is that they use the same neutrino beam as the OPERA experiment. The same neutrinos can’t be traveling at two different speeds, so clearly one or the other of these results is wrong. Given that the OPERA team has already identified a couple of construction errors in their detector, and their result was the wacky one anyway, this pretty much settles the problem with the apparent faster-than-light neutrinos: they don’t exist. It was just a detector malfunction, not any sort of strange physics.

Of course, most physicists were pretty confident that this was the case all along, but you can never be sure that something is wrong with your experiment …

Among this week’s big news in physics is that the OPERA experiment, which reported a detection of faster-than-light neutrinos back in September, has repeated their experiment and is still finding the same results. What they have changed this time is the width of the neutrino pulses, down to \(\SI{50}{ns}\).

In the original measurement, neutrinos were generated in bunches more than \(\SI{3}{km}\) long, so that it took \(\SI{10.5}{\micro\second}\) for each bunch of neutrinos to pass through a detector. This spread is much larger than the \(\SI{60}{ns}\) or \(\SI{18}{m}\) discrepancy that the original OPERA paper reported. Some scientists were concerned that the neutrino bunches were getting distorted in flight, which could skew the apparent average time that it took for the neutrinos to travel the distance they did. I’m not sure how valid of a criticism this was, since if you look at the picture at the left, you can actually identify peaks and valleys of the experimentally detected shape of the neutrino bunch (the black dots, representing the rate at which neutrinos are detected as a function of time), which match up pretty well with corresponding features …