Checking the radar: neutrinos don’t deserve speeding tickets just yet

There have been some interesting developments concerning these nasty FTL neutrinos from the OPERA experiment. The story has already broke out all over the internet and some people feel like this adds credibility to the initial result. I have already discussed the experiment quite a bit ( part I and II), so I won’t come back to the details.

Can we now believe just a little bit more that neutrinos go faster than light? Not yet. What we can believe however is that the complicated statistical analysis that was necessary to compare the starting and arriving time of neutrinos was not the culprit.

This complicated analysis was necessary due to the following. The start time is measured by looking at the protons before they are converted to neutrinos by being slammed into a target. The proton bunches originally lasted about 10 microseconds and only occasionally would one of these protons result in a neutrino signal at OPERA. You had to collect a very large amount of  neutrinos to make sure that you got neutrinos from protons everywhere in the bunches. The more neutrinos you collected, the more precise the timing became.

The new result from OPERA used proton bunches of 3 nanoseconds, rendering the complicated statistical analysis unnecessary. With proton bunches of 10 microseconds, they needed 15,000 events to get a timing measurement precise enough. With 3 nanoseconds bunches, they only needed 20 events to attain the same precision. That was a pretty quick check to perform, given that the SPS at CERN is capable of making these on demand.

OPERA managed to get rid of the complicated statistical analysis in their experiment, and the neutrinos still appear to travel faster than light. The only thing this means is that the statistical analysis was not the culprit. No more, no less.

The more I think about this experiment, the more it bugs me. One of the profs of my high energy physics group suspected that there could be timing issues with the pickup coils that measure the start time as the proton bunches pass through them. At first glance, it makes sense. If your start time gets an additional delay, it will shorten the difference between arrival and start times. It’s just like going from Montréal to Québec, which should take between 3 and 4 hours. You get to Québec at 5PM, and you departed at 1:30PM. If you think you departed at 3:00 you will underestimate the time of your trip to only 2 hours.

But can a delay be introduced by a pickup coil? It certainly can. A pickup coil circuit can be (over)simplified as a resistance and an inductance in series. The inductance is the coil, and you measure what went through the coil by measuring the voltage at the resistance. However, any circuit with an inductance introduces funny effects if the current running through it is not constant. When the proton bunches pass nearby the pickup coil, they generate a voltage impulse at the inductance, thanks to Faraday’s law. This impulse however, is not instantly measured at the resistance, as it takes time to transfer out of the inductance. In the case of the pickup coil that was used in the FTL neutrino experiment, that delay is of the order of several microseconds. It is far too big to be the culprit and must surely be accounted for.

The next big controversial aspect to target would be the synchronization of the two clocks on both sites by GPS. I thought of a solution for that, but I am not sure how costly/practical it might be. The idea is to lay two optical lines of slightly different lengths going from CERN to the OPERA detector at Gran Sasso. The idea would be to have only one clock at CERN, and have the OPERA detector fire a signal back to CERN every time it detects a neutrino. This way, you have one clock generating both start and arrival time stamps, which means no synchronization to perform (and no complicated general relativity involved). All you have to do is to account for the delays in the optical lines, which can be done with a very elegant technique when you have two lines of different lengths. You could even have a setup clever enough to monitor the delay in the lines in real time, by sending regular calibration impulses.

Anyway, enough of my ramblings. I will echo what many others have already said about this new OPERA result: what we really need is independent verification. We need a neutrino time-of-flight measurement from another experiment, such as MINOS at Fermilab or T2K in Japan. There is no point getting excited until another experiment confirms these results. However, I think it is great that OPERA target potential culprits and find ways to improve their measurement.

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