September 28 2011

Interesting Neutrinos from CERN

The recent CERN paper on neutrinos traveling (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns faster than the speed of light in a vacuum is interesting from a number of perspectives. First, the original purpose and layout of the experiment is pretty interesting all by itself. The idea is basically to create muon neutrinos (νμ) using a proton beam fired at a graphite target. This produces a number of particles that are focused via magnetic lenses towards a detector at the Gran Sasso laboratory OPERA detector, 730 km away. Various shields and 730 km of rock prevent any particle other than the neutrinos from reaching the detector. Here’s a nice picture of this:

As the νμ are merrily ghosting through the rock, some of them oscillate to become tau neutrinos (ντ). The first point of the experiment is to detect the oscillation: νμ→ντ. While they were doing this, they also realized they could try to measure the neutrino velocity from CERN to OPERA. The paper is largely concerned with the velocity measurement.
In order to measure the speed of the neutrinos they need a couple of pieces of information. They need the time of flight (TOF) of the neutrinos between emission and the detection and the distance traveled. They measured the TOF in a fairly clever fashion. Here is a picture from the paper of the TOF measurement:


There are a couple of clever things here. They are using a common GPS as the clock for the experiment rather than trying to messily synchronize diverse clocks on the ground (this would lead to all sorts of problems). Then, rather than using the GPS clock as a stopwatch, they are using it to timestamp the profile of the time of neutrino emission and the time of neutrino detection. These profiles then can be compared and the difference compared to the travel time of light. Chad Orzel has a nice post here on how this is done.
So, they do an experiment, try to nail down all the uncertainty and find out that the neutrinos are getting there before a photon would travel that distance. This is where a couple of more interesting things happen. Being scientists, they write a paper but then, rather than sending it off for peer review (although there are a bunch of them listed as authors) they send it off to the internet for comment. This is actually kind of cool if we now think of the whole internet as the peer review process. This is actually kind of similar to the process that got followed for Deolalikar’s P vs NP paper. As a review process it actually works quite well and gets a lot of eyes looking at interesting results.
If it turns out that there is a mistake somewhere in the set-up or calculations, this will be interesting and help future experimental set-ups. If there isn’t a mistake and the neutrino’s are really arriving faster than a photon then all sorts of interesting possibilities arise (and no, it doesn’t necessarily mean that relativity is wrong). We’ll have to stay tuned to see just what all is learned from this. Science at work–cool stuff.


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Posted September 28, 2011 by user in category "Interesting stuff

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