# Distance and time measurement in the famous Superluminal Neutrinos Experiment

I tried to understand the technical aspects of the OPERA/CERN experiment, but apparently it takes some professional experience. Therefore I would like to ask someone better acquainted with such experiments to give some details concerning the setup in plain English.

The paper says they conducted a "high-accuracy geodesy campaign that allowed measuring the 730 km CNGS baseline with a precision of 20 cm". They also describe the clock setting and synchronization, but I'm not sure I understand (there is something about "master clock"). So:

1. I figure that for some reason it wasn't possible - in order to obtain better accuracy than 20 cm - to run photons first along the track, and calculate the distance based on the velocity $c$ and time measured (or simply compare the times for photons and neutrinos), right? Was it because such "contraptions" do not allow for measuring photons or for some other reason? EDIT: I do realize now it's (practically) impossible.

2. How was the time measured? Were there two clocks at the beginning and at the end of the track, or was there just one clock hooked up (with the wretched fiber-optics cable) to both ends? If there were two clocks, were they synchronized through a GPS satellite or some other external device, or was there some sort of direct synchronization procedure? (What was it?)

To make sure: I looked at the paper before asking, but I am not really sure what they mean. I am not a professional particle physicist, and I have no knowledge whatsoever concerning the devices used, as well as procedures, technical jargon, etc. That's why I asked for help, and will be much obliged for information given in "plain English".

EDIT: In his answer dmckee said that: "Both the timing and the distance were established with the help of GPS." Does it mean that the margin of error is the same for both measurements - time and distance? If the the distance (of 730 km) was measured with 20 cm accuracy, would there be a margin of error for time (clock synchronization) of the same proportion? After all, time is distance and distance is time.

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The superluminal effect has (1) been refuted by another experiment in the same destination laboratory (Gran Sasso) and (2) retracted by OPERA. They found a problem with a fiber optic coupling in the timing chain. –  dmckee May 5 '14 at 13:55
I'm aware of that. Still, I would be much obliged for the details I asked about, if possible. –  bright magus May 5 '14 at 13:59
Note that v3 and v4 of the paper are much expanded over the original text. –  dmckee May 5 '14 at 14:18
see mi.infn.it/~psala/Icarus/nugsweb/cngs_und.jpg on your question 1 ('Ginevra' is where CERN is). The maximum depth of the path is 10km underground... Digging a tunnel of 700km length may technically be possible but is too expensive (not to mention the time it would take to complete...) –  Andre Holzner May 5 '14 at 15:50
@Andre: I wanted to be sure, so thanks. –  bright magus May 5 '14 at 16:45

You can't run a fiber or a laser along the track because most of the track is solid rock (it goes through a chord of the Earth, like all long-baseline beam-based neutrino experiments).

Both the timing and the distance were established with the help of GPS. If you use a fixed base-station you can integrate out the random variation in GPS computations and do much better than a typical handset. Add to that high precision, traditional (i.e. theodolite) surveys at either end and the GPS precision can be made the limiting factor along with the pion/kaon path timing at the horn and the signal propogation time in the detector itself.

The local timing of signals from GPS station to DAQ (i.e. the part of the distance that is surveyed) can be and is checked by reflection timing in a parallel fiber optic run.

Most of this is detailed in the preprint (see figures 3, 5 and 6 in version 4).1

1 If you are interested in the original report you can always look at v1 if you want

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So it means running a photon over exactly the same track is technically impossible, right? How many clocks were there? One or two? Were they synchronized directly (and how?) or through a third one? –  bright magus May 5 '14 at 14:05
This is all in the paper (see sections 4 & 6). There have to be two clocks because you are recording a data stream at each end. And the synchronization is managed (relativistically correctly) by using precision GPS. I'm not sure, right off, what frame of reference they work in, but the Earth centered frame would seem a reasonable choice; the choice of frame doesn't matter as long as the correct to a single frame. –  dmckee May 5 '14 at 14:18
I realize everything is there. It's not that I'm lazy. In the first sentence of my answer I gave the link to the paper (yes, I looked at it before asking) and said I am not really sure what they mean. I am not a professional particle physicist, and I have no knowledge whatsoever concerning the devices used, as well as procedures, technical jargon, etc. That's why I asked for help, and specifically (and kindly) requested it in "plain English". –  bright magus May 5 '14 at 14:37
@brightmagus: 20 cm / $c$ = 0.67 nanoseconds, about 5% of the claimed error bar of ±15 ns. –  rob May 5 '14 at 17:01
@brightmagus: Yes, it is a small contribution to the uncertainty of the time measurement. –  rob May 5 '14 at 17:38