After a more careful reading of the paper, the listening to a seminar on the result, and some discussions, I can share with you a few more details on the Opera measurement.
- The jitter: as you can see from the distribution I posted in the other article earlier today, showing the "delta t" for the 20 new neutrino interactions obtained from internal and external charged current interactions detected by Opera during the fast pulse runs of the CNGS beam, the jitter of 50 nanoseconds is causing the neutrino events to occur at random according to a uniform distribution, a "box" of width equal to 50 nanoseconds. What is causing this jitter had escaped me during the first careless and quick reading I gave to the paper last night. It is due to the fact that Opera cannot actually measure times with better than 50 nanosecond resolution: their "clock" runs at 20 megahertz, so they have a 50-ns "granularity" in the measurement.
- This raises a rather annoying question: what would happen if this clock were read with just "one tick" of mismatch ? It might shift the delta t measurement to a more reasonable "8+-10 ns" - or to a unarguable 108+-10 ns. Which of the two is more probable, I let you decide.
- The dataset: the new analysis of the 3-year data includes 5% fewer events than the original one. I had overlooked this fact earlier, but I had some strange feeling when I wrote "15223" in the earlier post. So what happens is that Opera discarded, upon reanalyzing the data, some 800 events which were found to be unsuitable for analysis: due to noise or otherwise unusable. Carefully analyzing your data multiple times helps!
- An effect of the Earth rotation had not been taken into account in the original preprint: the trajectory of the neutrinos is longer by about 60 cm than what one measures on a fixed reference system. This, however, goes in the direction of making the measured speed even higher, albeit by just one thirtieth.
- The light fiber which introduces a 40,000+-1 ns offset (which is subtracted along with other factors when computing the time difference) will be remeasured soon. The refraction index, and thus the time it takes for a signal to propagate in it, depends of course on the temperature, as well as on its stability as a material. At CERN a similar fiber was monitored continuously, showing no funny behaviour - so this might not be a problem in the measurement after all.
- Now new measurements will be performed. One idea would be to unroll a light fiber from CERN to Gran Sasso, to bypass completely the GPS measurements. Another implies carrying physically an atomic clock from the point of creation to the point of detection of neutrinos... We will see what that brings.
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