Yesterday, I couldn't restrain the physicist inside me. I just had to watch the seminar from CERN where Dario Autiero presented the result from CERN and CNGS of a measurement of the neutrino's velocity. The measurement is a tour de force of modern experimental physics, which harnesses amazing technologies such as the global positioning system, large scale data processing, picosecond lasers, ultrafast digital electronics, billion-volt particle beams, and highway tunnels a mile beneath a mountain in Italy. The scientific communication system is also state-of-the-art. The preprint (with 174 authors) appeared in ArXiv.org on thursday night: "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam", arXiv:1109.4897v1 [hep-ex]. And CERN webcasted the presentation live around the world, with not even a hiccup in the video feed.
What the news reports have failed to convey is that despite the impressive effort, the accuracy of the velocity measurement is teased out of the data with statistical procedures that are sure to come under intense scrutiny.
The way the experiment works is this. Pulses of neutrinos lasting 10.5 microseconds are generated by the accelerator at CERN, pointed at neutrino detectors 730km away under the mountain in Italy. Neutrinos are incredibly hard to detect (they have no difficulty traveling through 450 miles of rock), so only a tiny fraction of them are detected. Over 3 years, 16,111 of the CERN neutrinos were detected in Italy. The shape and timing of each generated pulse is measured and stored to be compared later with the timing of the detected neutrinos. The nub of the matter is shown in this graph, which shows only the leading and trailing edges of the accumulated data:
As you can see, the leading edge of the neutrino pulse is about 500 nsec wide. The red line is the cumulative shape of the generated pulse, the data show the counts and relative timing of the detected neutrinos.
The evidence for superluminal neutrinos is that the red curves at on the bottom, shifted by 60.7 nsec faster than the speed of light, are a better fit to the data than the red curves at the top. The claim is made that the bottom fit is 6 sigma away from the fit at the top. What do you think? Isn't physics fun?
What the news reports have failed to convey is that despite the impressive effort, the accuracy of the velocity measurement is teased out of the data with statistical procedures that are sure to come under intense scrutiny.
The way the experiment works is this. Pulses of neutrinos lasting 10.5 microseconds are generated by the accelerator at CERN, pointed at neutrino detectors 730km away under the mountain in Italy. Neutrinos are incredibly hard to detect (they have no difficulty traveling through 450 miles of rock), so only a tiny fraction of them are detected. Over 3 years, 16,111 of the CERN neutrinos were detected in Italy. The shape and timing of each generated pulse is measured and stored to be compared later with the timing of the detected neutrinos. The nub of the matter is shown in this graph, which shows only the leading and trailing edges of the accumulated data:
The evidence for superluminal neutrinos is that the red curves at on the bottom, shifted by 60.7 nsec faster than the speed of light, are a better fit to the data than the red curves at the top. The claim is made that the bottom fit is 6 sigma away from the fit at the top. What do you think? Isn't physics fun?
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