Posted by If 3σ results are wrong half the time, does that mean 6σ results are wrong all the time?
The social networks are a-buzz over the claim of a significant detection by the OPERA experiment of a neutrino pulse propagating superluminally over a 750 km baseline from CERN to the Gran Sasso lab.
You heard the claim – neutrino pulse generated 400 GeV protons fro the old Super Proton Synchrotron.
Every 6 secs a kicker magnet bumps two 10.5μs wide proton pulses, separated by 10ms.
These crash into a 2m graphite target (that is 7 ns travel time through target);
the mesons (π and K predominantly) are focused into a 1 km vacuum tunnel where they decay, with decay products clearly in a narrow beam down the tunnel, aimed right at the heart of Italy (isn’t that an act of war or something?) and some of the resultant neutrinos blow through the Gran Sasso lab chamber, where a few of them scatter and give a signal.
The experiment is optimizes for 17GeV ν production, with primary science goal of looking at mu-tau oscillations.
The 10,000 nsec wide pulse of neutrinos is cleanly detected at Gran Sasso, that is not in question. The claim is that the neutrinos are, on average, 60 ns early – ie that they are traveling very slightly faster than light – light travel time over the distance is about 2.5 ms,
or 2.5 million nanosecs.
So, I thought I’d better read the bloomin’ thing…
The claim is based on a statistical analysis of the distribution of the proton beam, vs the distribution of the neutrino arrival times, and is sensitive to the leading and trailing edges of the signal.
The authors claim a combined random and systematic error budget of 10 ns, and therefore a tentative 6σ detection of faster than light transport of the neutrino pulse.
The decay tunnel is 3 μs long, or about 50 times longer than the anomalous early arrival, the mesons enter the tunnel at speeds extremely close to the speed of light.
There are several interesting points in the blog posts below, including the comments:
One is the obvious, that the claims strongly contradicts the speed limits on neutrinos from Kamiokande detections of neutrinos from SN 1987a.
Those were anti-electron neutrinos, not muon/tau neutrinos, and at about 1,000 times lower energy.
Note there were controversies about an earlier neutrino burst detected at Mont Blanc, but even if real, the time difference is inconsistent with the propagation speed claimed here.
Secondly, there is concern about the local clocks, which are used to synch back to the GPS master clocks.
Butterworth makes an interesting point, namely that the actual detectors are in a particular location in the neutrino beam, which is about 3 km wide by the time it reaches Gran Sasso, this could bias the arrival time, there is a tiny bit of curvature on the beam front.
Finally, Eric asks whether they really used the geodesic distance, rather than the chord distance, which would be embarrassing, but the distance difference is close to the claimed time difference.
The claimed distance is 730534.61 ± 0.20 m
60 ns corresponds to a distance error of 20 mm m – (oops, my bad, typo. The “100 times the 20cm” is correct of course)
or about 100 times the claimed distance precision.
Note the very impressive relative changes in the distances due to geology are irrelevant if the baseline distance is erroneous because of confounding of geodesic distance and l-o-s distance, unlikely as that may be.
The statistics are based on about 16,000 neutrino interactions generated from about 1020 proton events.
So, what do I make of it?
Well, along with 99.87% of physicists, I am very skeptical.
It is almost certainly due to either a “silly error” – like the question about geodesic vs direct distance, or it is due to a subtle systematic error that the team overlooked but will be beaten out by the hordes of people reading over the paper this week.
A very, very faint possibility is that either relativity is wrong; or, muon neutrinos are weakly tachyonic; or, the neutrino tunneling between flavours is evidence of some funky stringy higher dimensional tunneling, and the geometry is weakly non-3D.
All of those are interesting, very interesting, but unlikely.
If the effect is real, the the NuMI/MINOS experiment in the US at Fermilab and Soudan ought to be able to see something.
An even more elegant experiment could also be done, by coupling the neutrino detector at Gran Sasso to the kick magnet timer at the SPS in CERN.
After about 100,000 or so pulses, the OPERA detection ought to shut off the SPS injection before the beam is fired…
of course the world would then also probably vanish in a puff of logic, or the Sun goes unstable, or a huge earthquake destroys Gran Sasso just in time to save us and sustain the Causality Protection Conjecture.
In the mean time, I confidently predict that hundreds of theorists will come up with thousands of possible explanations, most or all of which will be completely wrong.
These will be comfortably outnumbered by the not-even-wrong speculations in social media.
All of these will be entertaining for a while, then become tedious, and then be quietly resolved.
The most interesting applications of this would of course be for signaling; however there private enterprise is way ahead of us, and clearly some very clever hedge fund with deep pockets has already co-located neutrino detectors at the exchanges to front run everyone else.
Its just engineering really.
Explains a lot…
Ethan at Starts with a Bang
Chad makes a dog’s breakfast of it at Uncertain Principles
Sean’s take at Cosmic Variance
Excellent perspective by Jon Butterworth at the Grauniad’s Life and Physics
PS: More Sean at CV
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